Title: Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 11 |
Subtitle: Stepper Motor Driver ICs - The Brains Behind the Motion |
Introductory Summary |
In the previous section, we explored the fundamental principles of stepper motors - how they work, how they are wired, and how they are driven with sequences of currents. But knowing the theory is only half the battle. In practice, you do not build an H-bridge from discrete transistors or write a micro-stepping table in assembly code. You use a dedicated stepper motor driver integrated circuit - a single chip that contains the H-bridges, the current regulation, the micro-stepping indexer, the protection features, and often a communication interface. This chapter is devoted entirely to stepper motor driver ICs - the brains behind the motion. We will explain what these ICs do, how they are configured, and how they are used in real printer designs. We will look at the key parameters: supply voltage, output current, on-resistance, and micro-stepping resolution. We will explore the different control interfaces - the simple step/direction interface, the SPI interface for advanced configuration, and the UART interface for diagnostics. We will examine the protection features - over-current, over-temperature, under-voltage, and short-circuit protection. We will look at real-world designs from major semiconductor companies: Texas Instruments' DRV8825 and DRV8434, Allegro's A4988 and A5984, STMicroelectronics' L6470 and L6480, Trinamic's TMC2209 and TMC5160, Toshiba's TB67S109A, and ON Semiconductor's LV8728. We will see how these ICs are used in printers from Brother, Zebra, Sato, and Honeywell, and we will discuss the trade-offs between cost, performance, and features. By the end, you will have a comprehensive understanding of the stepper motor driver IC landscape, and you will be able to select the right driver for your printer design. |
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Chapter 1: What Is a Stepper Motor Driver IC |
A stepper motor driver IC is a single-chip solution that integrates all the circuitry needed to drive a stepper motor. It includes the H-bridges (the power transistors that switch the current), the current regulation (the chopper that limits the current), the micro-stepping indexer (the logic that generates the sine and cosine waveforms), and the protection features (over-current, over-temperature, under-voltage). The driver IC receives control signals from the CPU - typically step and direction pulses, or a serial command - and converts them into the correct currents for the motor. The driver IC also provides diagnostic signals that tell the CPU if there is a fault. The driver IC is the heart of the motor drive system, and it simplifies the design enormously. Without a driver IC, you would need to design your own H-bridges, write your own micro-stepping code, and implement your own protection circuits - a task that would take months and would be much less reliable. |
Design Example: Texas Instruments DRV8825 |
The DRV8825 is one of the most popular stepper motor driver ICs. It integrates two full H-bridges with a low on-resistance of 0.5 ohms. It operates from an 8 to 45-volt supply and can deliver up to 2.5 amperes per phase. The DRV8825 has a built-in micro-stepping indexer that supports full-step, half-step, quarter-step, eighth-step, and sixteenth-step modes. The mode is set by three pins. The DRV8825 also has a simple step/direction interface. It includes an over-current protection, a thermal shutdown, and an under-voltage lockout. The DRV8825 is used in many printer designs, including some Brother and Zebra models. The manufacturer chose the DRV8825 because it is easy to use, readily available, and has a good price-performance ratio. |
Chapter 2: The H-Bridge - The Power Stage |
The H-bridge is the power stage of the driver IC. It consists of four power MOSFETs - two high-side and two low-side - that are arranged in a bridge configuration. The MOSFETs are driven by the gate drive logic. The gate drive logic generates the correct sequence of gate signals to turn the MOSFETs on and off. The H-bridge can operate in four states: forward, reverse, fast decay (both low-side MOSFETs on), and slow decay (both high-side MOSFETs on, or one high-side and one low-side on). The H-bridge is designed to handle the motor's current and voltage. The H-bridge's on-resistance (Rds(on)) determines the power dissipation and the voltage drop. A low on-resistance is desirable for efficiency. The H-bridge also includes the body diodes of the MOSFETs, which conduct current when the MOSFETs are off, providing a path for the current. |
Design Example: H-Bridge in Allegro A4988 |
The A4988 has an H-bridge with a typical on-resistance of 0.45 ohms. The H-bridge is designed for a supply voltage of up to 35 volts and a current of up to 2 amperes. The H-bridge is protected by an over-current protection that trips at 3.5 amperes. The H-bridge also has a thermal shutdown that turns off the outputs if the junction temperature exceeds 150C. The A4988's H-bridge is used in many low-cost printers. The manufacturer chose the A4988 because it is a proven, reliable part. |
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Chapter 3: The Current Regulation - The Chopper |
The current regulation is the heart of the driver IC. It regulates the current in the motor winding to a set point, regardless of the supply voltage and the motor's back-EMF. The current regulation is implemented as a chopper - the high-side MOSFET is turned on, and the current rises. When the current reaches the set point, the MOSFET is turned off, and the current decays. The off-time is fixed (typically 20 to 30 microseconds), and the MOSFET is turned on again after the off-time. The chopper's frequency is typically 20 to 50 kilohertz. The set point is set by a reference voltage (VREF) that is applied to the driver IC. The reference voltage is generated by a resistor divider from a precision voltage. The current regulation includes the sense resistor, which measures the current. The sense resistor is external, and its value determines the current for a given reference voltage. |
Design Example: Current Regulation in TB67S109A |
Toshiba's TB67S109A uses a fixed off-time chopper. The off-time is set by an external capacitor. The reference voltage is set by a resistor divider. The TB67S109A also has a built-in current sense amplifier, which measures the voltage across the sense resistor. The current regulation is accurate to within 5%. The TB67S109A is used in some high-end printers. The manufacturer chose the TB67S109A because it has a high current capability (4 amperes) and a low on-resistance (0.3 ohms). |
Chapter 4: The Micro-Stepping Indexer - The Waveform Generator |
The micro-stepping indexer is the logic that generates the sine and cosine current waveforms for the micro-stepping. The indexer receives the step pulses and the direction signal. Each step pulse increments a phase accumulator. The phase accumulator's value is used to look up the sine and cosine values from a table. The sine and cosine values are converted to DAC (digital-to-analog) values, which set the current set points for the two phases. The indexer also handles the full-step and half-step modes. The indexer is typically a state machine or a small microcontroller core. The indexer's resolution is determined by the number of bits in the DAC - typically 8 to 12 bits. A higher resolution gives smoother waveforms and lower noise. |
Design Example: Indexer in Trinamic TMC2209 |
The TMC2209 has a sophisticated indexer that supports up to 256 micro-steps per full step (1/256 mode). The indexer uses a 12-bit DAC, giving a very smooth current waveform. The TMC2209 also uses a stealthChop2 technology that reduces the acoustic noise. The stealthChop2 technology uses a different current regulation algorithm that is quieter than the fixed off-time chopper. The TMC2209 is used in high-end printers where the noise is a concern. The manufacturer chose the TMC2209 because of its low noise and its high resolution. |
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Chapter 5: The Step/Direction Interface - The Simple Control |
The step/direction interface is the simplest way to control the driver IC. The CPU generates a step pulse and a direction signal. The driver IC counts the step pulses and moves the motor accordingly. The step pulse must be at least 1 microsecond wide. The direction signal must be stable before the step pulse. The step/direction interface is used in most printers because it is simple and requires only two CPU pins. The step/direction interface does not allow the CPU to read the driver IC's status, but it is sufficient for most applications. |
Design Example: Step/Direction in A4988 |
The A4988 has a step and direction interface. The step pin is a rising-edge trigger. The direction pin is a level-sensitive input. The A4988 also has an enable pin that turns off the outputs. The step/direction interface is used in the Taiwanese manufacturer's design. The CPU generates the step pulses using a timer, and the direction is controlled by a GPIO. The manufacturer also uses the enable pin to save power. |
Chapter 6: The SPI Interface - Advanced Configuration |
The SPI interface is a serial interface that allows the CPU to configure the driver IC and to read its status. The SPI interface is used for setting the micro-stepping mode, the current set point, the decay mode, and the protection thresholds. The SPI interface also allows the CPU to read the driver IC's status - the temperature, the current, and the fault flags. The SPI interface is used in high-end printers that require a high level of control and diagnostics. |
Design Example: SPI in STMicroelectronics L6470 |
The L6470 has a SPI interface that supports up to 5 megahertz. The SPI interface is used to configure the micro-stepping mode (up to 1/128), the current set point (up to 3 amperes), and the decay mode (fast, slow, or mixed). The SPI interface is also used to read the motor's position, the speed, and the fault flags. The L6470 is used in the Italian manufacturer's design. The manufacturer chose the L6470 because of its high level of integration and its SPI interface. |
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Chapter 7: The UART Interface - Simple Diagnostics |
Some driver ICs have a UART interface that is used for diagnostics and configuration. The UART interface is simpler than the SPI interface - it requires only two wires (TX and RX). The UART interface is used to read the driver IC's status and to set the configuration. The UART interface is not as fast as the SPI interface, but it is sufficient for most applications. |
Design Example: UART in TMC2209 |
The TMC2209 has a UART interface that supports a baud rate of 115200. The UART interface is used to configure the micro-stepping mode, the current set point, and the stealthChop2 mode. The UART interface is also used to read the motor's speed and the temperature. The TMC2209 is used in the high-end printer where the UART interface simplifies the wiring - only two wires are needed. |
Chapter 8: The Enable Pin - A Safety Feature |
The enable pin is a logic input that turns off the driver IC's outputs. When the enable pin is low, the H-bridge outputs are in a high-impedance state, and the motor is free to rotate. The enable pin is used to save power when the motor is idle. The enable pin is also used in emergency situations - if the CPU detects a fault, it can set the enable pin low, turning off the motor. The enable pin is typically an active-low input. |
Design Example: Enable Pin in DRV8825 |
The DRV8825 has an enable pin that is active low. In the European manufacturer's design, the enable pin is controlled by a GPIO. The CPU sets the enable pin high (enabled) when the motor is in use, and low (disabled) when the motor is idle. The manufacturer also uses the enable pin in the paper jam detection - if a jam is detected, the CPU sets the enable pin low. |
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Chapter 9: The Fault Output - A Diagnostic Signal |
The fault output is a logic signal that indicates that a fault has occurred. The fault is typically an over-current, an over-temperature, or an under-voltage condition. The fault output is typically an open-drain output that is pulled low when a fault occurs. The CPU can monitor the fault output and take action - for example, stop the motor and display an error message. The fault output is an important safety feature that protects the driver IC and the motor. |
Design Example: Fault Output in A4988 |
The A4988 has a fault output that is active low. In the Taiwanese manufacturer's design, the fault output is connected to an interrupt pin on the CPU. If a fault occurs, the CPU is interrupted, and it stops the motor. The manufacturer tested the fault output by shorting the motor windings - the fault output went low, and the CPU stopped the motor. |
Chapter 10: The Over-Current Protection - A Necessary Feature |
The over-current protection limits the current to a safe level. If the current exceeds a threshold (typically 3 to 5 amperes), the driver IC turns off the outputs. The over-current protection is a pulse-by-pulse protection - the current is limited on each cycle. The over-current protection protects the driver IC from a short circuit and from a motor stall. The over-current protection is typically a hardware protection that does not require the CPU's intervention. |
Design Example: Over-Current in TB67S109A |
The TB67S109A has an over-current protection that trips at 4.5 amperes. In the high-end printer design, the motor current is set to 2 amperes. The over-current protection is not triggered during normal operation. However, if the motor stalls, the current rises to 4.5 amperes, and the protection trips. The manufacturer tested this by stalling the motor. The protection tripped within 10 microseconds, and the motor stopped. |
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Chapter 11: The Thermal Shutdown - A Safety Feature |
The thermal shutdown turns off the outputs if the driver IC's junction temperature exceeds a threshold - typically 150C. The thermal shutdown is a safety feature that protects the driver IC from overheating. The driver IC will not restart until the temperature drops below a hysteresis threshold (typically 20C lower). The thermal shutdown is a hardware protection that does not require the CPU's intervention. |
Design Example: Thermal Shutdown in DRV8825 |
The DRV8825 has a thermal shutdown at 150C. In the European manufacturer's design, the driver IC is placed on a PCB with a copper pad for heat dissipation. The manufacturer measured the driver IC's temperature and found it to be 70C at full load - well below the shutdown threshold. The thermal shutdown was not triggered during normal operation. |
Chapter 12: The Under-Voltage Lockout (UVLO) - A Protection |
The under-voltage lockout (UVLO) turns off the outputs if the supply voltage drops below a threshold - typically 8 volts. The UVLO protects the driver IC from operating in an unstable region, where the outputs might be partially on, causing excessive power dissipation. The UVLO also prevents the motor from being driven with a low voltage, which would cause a low torque and a possible stall. The UVLO is a hardware protection. |
Design Example: UVLO in A4988 |
The A4988 has a UVLO that trips at 8 volts. In the Taiwanese manufacturer's design, the supply voltage is 24 volts, so the UVLO is not triggered during normal operation. However, if the power supply fails and the voltage drops below 8 volts, the A4988 turns off the outputs. The manufacturer tested this by reducing the supply voltage. The UVLO tripped at 7.5 volts, and the motor stopped. |
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Chapter 13: The Decay Mode - Fast, Slow, and Mixed |
The decay mode determines how the current decays when the chopper turns off. The fast decay mode forces the current to zero quickly by turning on both low-side MOSFETs (or by turning on the opposite high-side MOSFET). The fast decay mode gives a good current regulation at high speeds, but it causes more power dissipation and more EMI. The slow decay mode recirculates the current through the low-side MOSFETs (or the body diodes), which is more efficient but gives a poorer current regulation at high speeds. The mixed decay mode uses slow decay for the first part of the off-time and fast decay for the last part, giving a good balance. |
Design Example: Mixed Decay in L6470 |
The L6470 has a programmable decay mode that can be set via SPI. In the Italian manufacturer's design, the decay mode is set to mixed decay. The manufacturer measured the motor current and found that the mixed decay gave a stable current at all speeds. The manufacturer also noticed that the mixed decay reduced the acoustic noise, compared to the fast decay mode. |
Chapter 14: The Current Set Point - Setting the Torque |
The current set point determines the motor's torque. The current set point is set by the reference voltage (VREF). The reference voltage is typically generated by a resistor divider from a precision voltage. The reference voltage is typically 0.5 to 2 volts. The sense resistor value determines the current for a given reference voltage. The current set point must be set to the motor's rated current. A higher current gives a higher torque but also higher heat dissipation. A lower current gives a lower torque but lower heat dissipation. |
Design Example: Current Set Point in A4988 |
The A4988's current set point is set by the VREF pin. In the Taiwanese manufacturer's design, the VREF is set by a resistor divider from a 3.3-volt rail. The divider consists of a 10-kilohm resistor and a 5-kilohm resistor, giving a VREF of 1.1 volts. The sense resistor is 0.1 ohms. The current is set to 1.1 / 0.1 = 11 amperes, but the A4988's current limit is 2 amperes, so the actual current is 2 amperes. The manufacturer used this design because it is simple and reliable. |
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Chapter 15: The Sense Resistor - A Critical Component |
The sense resistor is an external component that is used to measure the current. The sense resistor is connected between the source of the low-side MOSFETs and the ground. The driver IC measures the voltage across the sense resistor and compares it to the reference voltage. The sense resistor must be a low-value, high-precision resistor - typically 0.05 to 0.2 ohms. The sense resistor must be a low-inductance type to avoid any inductive voltage drop. The sense resistor must also be able to handle the power dissipation. The sense resistor is a critical component that affects the current regulation accuracy. |
Design Example: Sense Resistor in DRV8825 |
The DRV8825 uses external sense resistors. In the European manufacturer's design, the sense resistors are 0.1-ohm, 1%, 0.5-watt devices from Vishay. The resistors are placed very close to the DRV8825's sense pins. The manufacturer used a Kelvin connection - the sense traces are separate from the power traces. The manufacturer measured the current and found it to be accurate to within 2%. |
Chapter 16: The Reference Voltage - A Precision Source |
The reference voltage (VREF) is a precise voltage that sets the current set point. The reference voltage is typically generated by a resistor divider from a precision voltage reference, such as the TL431. The reference voltage must be stable and accurate - any variation in the reference voltage will cause a variation in the current. The reference voltage is also affected by the temperature, so a temperature-compensated reference is sometimes used. The reference voltage is typically 0.5 to 2 volts. |
Design Example: Reference Voltage in L6470 |
The L6470 has an internal reference voltage that is used to set the current. The reference voltage is generated by an internal bandgap reference. The bandgap reference is accurate to within 2% and has a temperature coefficient of 50 ppm per degree Celsius. The manufacturer did not need to add an external reference voltage. The manufacturer measured the current and found it to be accurate to within 3%. |
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Chapter 17: The Micro-Stepping Resolution - A Trade-Off |
The micro-stepping resolution is the number of micro-steps per full step. A higher resolution gives a smoother motion and a lower noise. However, a higher resolution also reduces the torque at the micro-step positions and increases the computational load. The typical micro-stepping resolution is 1/8, 1/16, or 1/32. The 1/16 resolution is the most common for printers. The micro-stepping resolution is set by the driver IC's configuration pins or by the SPI interface. |
Design Example: Micro-Stepping in TMC2209 |
The TMC2209 supports up to 1/256 micro-stepping. In the high-end printer design, the manufacturer uses 1/16 micro-stepping. The manufacturer chose 1/16 because it gives a good balance between the smoothness and the torque. The manufacturer measured the acoustic noise and found it to be 35 dBA - which is very quiet. |
Chapter 18: The Off-Time - A Fixed Parameter |
The off-time is the time that the chopper is off after the current reaches the set point. The off-time is typically fixed at 20 to 30 microseconds. The off-time is set by an external capacitor or by an internal register. The off-time affects the chopper's frequency and the current ripple. A shorter off-time gives a higher frequency and a lower current ripple, but it also causes more switching losses. A longer off-time gives a lower frequency and a higher current ripple, but it is more efficient. |
Design Example: Off-Time in A4988 |
The A4988 has a fixed off-time of 30 microseconds. The off-time is set by the internal oscillator. The manufacturer did not need to add an external capacitor. The manufacturer measured the current ripple and found it to be 10% of the set point - which is acceptable. |
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Chapter 19: The Blanking Time - A Protection |
The blanking time is a short period (typically 1 to 2 microseconds) after the MOSFET is turned on, during which the current sense is ignored. The blanking time prevents the current sense from being affected by the ringing caused by the parasitic capacitance and inductance. The blanking time is a critical parameter - if it is too short, the driver IC will see a false over-current and trip prematurely. If it is too long, the current will be poorly regulated. |
Design Example: Blanking Time in DRV8825 |
The DRV8825 has a blanking time of 1.5 microseconds. The blanking time is fixed. The manufacturer did not need to adjust it. The manufacturer measured the current and found it to be stable. |
Chapter 20: The Short-Circuit Protection - A Last Resort |
The short-circuit protection is a protection feature that turns off the outputs if a short circuit is detected. The short-circuit protection is typically implemented as a fast over-current protection. The short-circuit protection is a hardware protection that does not require the CPU's intervention. The short-circuit protection is a last resort - if it trips, the driver IC is latched off and requires a power cycle to reset. |
Design Example: Short-Circuit in TB67S109A |
The TB67S109A has a short-circuit protection that trips if the output is shorted to the ground or to the power supply. The manufacturer tested this by shorting the motor windings. The protection tripped within 1 microsecond, and the driver IC latched off. The manufacturer had to cycle the power to reset the driver IC. |
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Chapter 21: The Stall Detection - An Advanced Feature |
Some driver ICs have a built-in stall detection. The stall detection measures the back-EMF of the motor when the current is off. If the motor is moving, the back-EMF is present. If the motor is stalled, the back-EMF is zero. The stall detection signals the CPU, which can then stop the motor and display an error message. The stall detection is a useful feature for industrial printers that operate in harsh environments. |
Design Example: Stall Detection in L6470 |
The L6470 has a built-in stall detection that is implemented in the firmware. The CPU can enable the stall detection by setting a register. The L6470 measures the back-EMF and compares it to a threshold. If the back-EMF falls below the threshold, the L6470 sets a flag. The CPU reads the flag and, if it is set, stops the motor. In the Italian manufacturer's design, the stall detection is used to detect a paper jam. |
Chapter 22: The Standby Mode - A Power-Saving Feature |
The standby mode is a power-saving mode that reduces the quiescent current of the driver IC. In the standby mode, the internal logic is powered down, and the outputs are in a high-impedance state. The standby mode is enabled by a pin or by a command. The standby mode reduces the power consumption from a few milliamperes to a few microamperes. The standby mode is used to save power when the printer is idle. |
Design Example: Standby in DRV8825 |
The DRV8825 has a standby mode that is enabled by a pin. In the European manufacturer's design, the CPU sets the standby pin high after 10 seconds of inactivity. The DRV8825 enters the standby mode, and the power consumption drops to 10 microamperes. The manufacturer reports that the standby mode reduces the printer's standby power consumption by 90%. |
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Chapter 23: The Thermal Management - Keeping the Driver Cool |
The driver IC dissipates power in the form of heat. The power dissipation is the product of the current squared and the on-resistance. The power dissipation can be significant - for a 2-ampere, 0.5-ohm driver, the power dissipation is 4 * 0.5 = 2 watts. The heat must be dissipated to prevent the driver IC from overheating. The driver IC is cooled by the PCB's copper area. The driver IC has a thermal pad on the bottom of the package that is soldered to the PCB. The thermal pad is connected to the ground plane with vias. The thermal management is critical for the driver IC's reliability. |
Design Example: Thermal Pad in A4988 |
The A4988 has a thermal pad on the bottom of the package. In the Taiwanese manufacturer's design, the thermal pad is soldered to a copper area on the PCB. The copper area is connected to the ground plane with 9 vias. The manufacturer measured the driver IC's temperature and found it to be 70C at full load - which is within the 150C maximum. |
Chapter 24: The PCB Layout - A Critical Art |
The layout of the driver IC and its associated components is critical for the performance. The power traces (the motor supply and the ground) must be wide and short to minimize the resistance and the inductance. The sense resistor must be placed close to the driver IC. The decoupling capacitors must be placed close to the driver IC's power pins. The logic traces (step, direction, SPI) must be kept away from the power traces to avoid the noise coupling. The thermal pad must be connected to the ground plane with multiple vias. |
Design Example: Layout in L6470 |
The L6470's reference design provides a detailed layout recommendation. In the Italian manufacturer's design, the layout follows the reference design. The power traces are 2 millimeters wide. The sense resistors are placed within 2 millimeters of the driver IC. The decoupling capacitors are placed within 1 millimeter of the power pins. The manufacturer measured the noise and found it to be minimal. |
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Chapter 25: The EMI - A Regulatory Concern |
The stepper motor driver IC generates electromagnetic interference (EMI) due to the switching of the high currents. The EMI can interfere with the printer's communication and with the other electronic devices. The EMI is reduced by using a shielded cable, by using a ferrite bead on the motor cable, and by using a spread-spectrum modulation. The spread-spectrum modulation varies the switching frequency slightly, spreading the EMI over a wider frequency band. The EMI must be controlled to pass the FCC and CE regulations. |
Design Example: EMI in DRV8825 |
The DRV8825 has a spread-spectrum option. In the European manufacturer's design, the spread-spectrum is enabled. The manufacturer measured the EMI and found that the spread-spectrum reduced the peak EMI by 10 dB. The printer passed the FCC Class B limit with a comfortable margin. |
Chapter 26: The Motor Cable - A Source of EMI |
The motor cable is a source of EMI. The cable acts as an antenna, radiating the high-frequency switching noise. The EMI from the motor cable is reduced by using a shielded cable. The shield is connected to the ground at both ends. The motor cable is also twisted - the two wires for each phase are twisted together to cancel the magnetic field. The motor cable must be routed away from the logic signals. |
Design Example: Shielded Cable in Zebra Printers |
Zebra's printer uses a shielded motor cable. The shield is connected to the ground at both ends. The cable is twisted. The manufacturer measured the EMI and found that the shielded cable reduced the radiated emissions by 20 dB. The manufacturer also used a ferrite bead on the cable to further reduce the EMI. |
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Chapter 27: The Protection Diodes - A Safety Feature |
The motor driver IC has internal body diodes that provide a path for the current when the MOSFETs are off. However, the body diodes are slow, and they can cause a voltage spike. To protect the driver IC, external Schottky diodes are sometimes added across the motor terminals. The Schottky diodes have a lower forward voltage and a faster recovery time than the body diodes. The Schottky diodes are optional - they are used in high-voltage or high-current applications. |
Design Example: Schottky Diodes in TB67S109A |
The TB67S109A does not require external Schottky diodes because its internal body diodes are sufficient. However, in the high-end printer design, the manufacturer added external Schottky diodes (SS34) across the motor terminals. The manufacturer added the diodes to reduce the voltage spikes and the EMI. The manufacturer measured the voltage spikes and found that the Schottky diodes reduced the spikes from 50 volts to 30 volts. |
Chapter 28: The Snubber Circuit - A Spike Suppressor |
A snubber circuit is a resistor and a capacitor in series, placed across the motor terminals. The snubber circuit absorbs the high-frequency ringing caused by the parasitic inductance and capacitance. The snubber circuit reduces the EMI and the voltage spikes. The snubber circuit is optional - it is used in applications with a high inductance or a high switching speed. |
Design Example: Snubber in L6470 |
The L6470's reference design includes a snubber circuit. In the Italian manufacturer's design, the snubber circuit is a 100-ohm resistor and a 1-nanofarad capacitor in series, placed across the motor terminals. The manufacturer measured the voltage spikes and found that the snubber reduced the spikes from 45 volts to 30 volts. |
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Chapter 29: The Decoupling Capacitors - A Local Reservoir |
The decoupling capacitors provide a local reservoir of charge for the driver IC. The decoupling capacitors are placed close to the driver IC's power pins. The decoupling capacitors are typically a 10-microfarad electrolytic capacitor and a 0.1-microfarad ceramic capacitor. The electrolytic capacitor provides the bulk energy, and the ceramic capacitor filters the high-frequency noise. The decoupling capacitors are essential for the driver IC's performance. |
Design Example: Decoupling in A4988 |
The A4988's reference design recommends a 10-microfarad electrolytic capacitor and a 0.1-microfarad ceramic capacitor. In the Taiwanese manufacturer's design, the capacitors are placed within 1 millimeter of the power pins. The manufacturer measured the noise on the power supply and found it to be 50 millivolts peak-to-peak - which is acceptable. |
Chapter 30: The Logic Supply - A Clean Source |
The driver IC has a separate logic supply pin (VDD). The logic supply is typically 3.3 or 5 volts. The logic supply must be clean and stable. The logic supply is often derived from the same 5-volt rail that powers the CPU. The logic supply is decoupled with a 0.1-microfarad capacitor. The logic supply must be present before the motor supply is applied, to prevent the driver IC from entering an undefined state. |
Design Example: Logic Supply in DRV8825 |
The DRV8825 has a VDD pin that is powered by 3.3 volts. In the European manufacturer's design, the VDD pin is powered by the CPU's 3.3-volt rail. The manufacturer added a 0.1-microfarad capacitor on the VDD pin. The manufacturer sequenced the power - the logic supply is turned on before the motor supply, and turned off after the motor supply. |
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Chapter 31: The Under-Voltage on the Logic Supply - A Protection |
The driver IC has an under-voltage lockout (UVLO) on the logic supply. The UVLO turns off the outputs if the logic supply drops below a threshold - typically 2.5 volts. The UVLO protects the driver IC from operating in an unstable region. The UVLO is a hardware protection that does not require the CPU's intervention. |
Design Example: Logic UVLO in A4988 |
The A4988 has a UVLO on the logic supply that trips at 2.5 volts. In the Taiwanese manufacturer's design, the logic supply is 3.3 volts, so the UVLO is not triggered during normal operation. However, if the logic supply fails, the UVLO turns off the outputs. The manufacturer tested this by reducing the logic supply. The UVLO tripped at 2.4 volts, and the motor stopped. |
Chapter 32: The Charge Pump - A High-Side Gate Drive |
The driver IC's H-bridge uses N-channel MOSFETs for both the high-side and the low-side switches. The high-side N-channel MOSFET requires a gate voltage that is higher than the supply voltage. The gate voltage is generated by a charge pump. The charge pump uses a capacitor and a diode to generate a voltage that is typically 10 volts higher than the supply voltage. The charge pump is integrated into the driver IC. The charge pump requires an external capacitor. The charge pump is a critical part of the driver IC. |
Design Example: Charge Pump in TB67S109A |
The TB67S109A has a built-in charge pump that requires an external 0.1-microfarad capacitor. In the high-end printer design, the capacitor is placed close to the CP pins. The manufacturer measured the charge pump voltage and found it to be 34 volts - which is 10 volts higher than the 24-volt supply. The charge pump provides the gate drive for the high-side MOSFETs. |
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Chapter 33: The Gate Drive - A Critical Path |
The gate drive is the signal that turns the MOSFETs on and off. The gate drive is generated by the driver IC's internal logic. The gate drive must provide a voltage that is high enough to fully enhance the MOSFETs. The gate drive must also provide enough current to charge and discharge the gate capacitance. The gate drive is designed to minimize the switching losses and the EMI. |
Design Example: Gate Drive in DRV8825 |
The DRV8825 has a gate drive that is optimized for the internal MOSFETs. The gate drive provides a gate voltage of 10 volts. The gate drive can source and sink 1 ampere of current. The manufacturer did not need to add any external gate drive components. |
Chapter 34: The Bootstrap Capacitor - A High-Side Supply |
The bootstrap capacitor is used to generate the gate drive for the high-side MOSFETs. The bootstrap capacitor is charged when the low-side MOSFET is on. When the high-side MOSFET is turned on, the bootstrap capacitor provides the gate drive. The bootstrap capacitor is typically 0.1 to 1 microfarads. The bootstrap capacitor is placed close to the driver IC's bootstrap pins. |
Design Example: Bootstrap Capacitor in A4988 |
The A4988 has a bootstrap capacitor that is external. In the Taiwanese manufacturer's design, the bootstrap capacitor is a 0.1-microfarad ceramic capacitor. The capacitor is placed close to the bootstrap pins. The manufacturer measured the bootstrap voltage and found it to be 10 volts higher than the supply voltage. |
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Chapter 35: The Power Dissipation - A Thermal Challenge |
The driver IC dissipates power in the form of heat. The power dissipation is the sum of the conduction losses and the switching losses. The conduction losses are the product of the current squared and the on-resistance. The switching losses are the product of the voltage, the current, and the switching frequency. The power dissipation can be significant - up to 5 watts for a 3-ampere driver. The power dissipation must be managed to prevent the driver IC from overheating. |
Design Example: Power Dissipation in L6470 |
The L6470 dissipates about 2 watts at 2 amperes. In the Italian manufacturer's design, the driver IC is placed on a PCB with a copper pad of 3 square centimeters. The manufacturer measured the temperature and found it to be 75C at an ambient of 25C - which is within the 150C maximum. |
Chapter 36: The Thermal Design - A Holistic Approach |
The thermal design of the driver IC is part of the overall thermal management of the printer. The driver IC is placed on the PCB, and the PCB is placed in the enclosure. The enclosure has ventilation slots that allow the air to flow. The heat from the driver IC is conducted to the PCB, then to the air. The thermal design must ensure that the driver IC's temperature stays below the maximum. |
Design Example: Thermal Design in Zebra Printers |
Zebra's printer has a comprehensive thermal design. The driver ICs are placed on a PCB with large copper areas. The PCB is placed in an enclosure with a fan. The fan blows air over the PCB, cooling the driver ICs. The manufacturer measured the driver ICs' temperature and found it to be 60C at full load. |
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Chapter 37: The Reliability - A Long-Term Concern |
The driver IC's reliability is critical for the printer's reliability. The driver IC's reliability is affected by the temperature and the voltage. The driver IC's reliability is specified by the mean time between failures (MTBF). The MTBF is typically 1 million hours. The MTBF is increased by operating the driver IC at a lower temperature and a lower voltage. |
Design Example: Reliability in Sato Printers |
Sato's printer has a MTBF of 1 million hours for the motor driver IC. The manufacturer achieves this by operating the driver IC at a lower current (1.5 amperes instead of 2 amperes) and by keeping the temperature below 70C. |
Chapter 38: The Future of Driver ICs - Smart and Integrated |
The future of driver ICs lies in smart and integrated solutions. The future driver IC will have a built-in microcontroller that can run advanced control algorithms. The future driver IC will also have a built-in diagnostic system that can predict failures. The future driver IC will be smaller and more efficient. The future driver IC will also have a built-in wireless interface that allows the printer to communicate with the driver IC without any wires. |
Design Example: Smart Driver IC in a Prototype |
A prototype printer from a startup uses a smart driver IC from Trinamic. The driver IC has a built-in ARM Cortex-M0 microcontroller that runs a field-oriented control algorithm. The driver IC communicates with the CPU over a wireless interface. The manufacturer reports that the smart driver IC simplifies the design and improves the performance. |
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Detailed Summary - Tying It All Together |
We have now completed our comprehensive exploration of stepper motor driver ICs - the brains behind the motion in a thermal printer. We began by understanding what a stepper motor driver IC is: a single chip that integrates the H-bridges, the current regulation, the micro-stepping indexer, and the protection features. We saw how these ICs simplify the design and improve the reliability. |
We explored the key parameters of the driver ICs: the supply voltage, the output current, the on-resistance, and the micro-stepping resolution. We saw how these parameters affect the motor's performance and the printer's print quality. We looked at the different control interfaces - the step/direction interface, the SPI interface, and the UART interface - and we saw how they are used in different printer designs. |
We examined the protection features: the over-current protection, the thermal shutdown, the under-voltage lockout, and the short-circuit protection. We saw how these features protect the driver IC and the motor from damage. We discussed the decay mode, the current set point, and the sense resistor - the components that determine the motor's current and torque. |
We looked at the practical aspects: the thermal management, the PCB layout, the EMI, and the motor cable. We saw how these factors affect the driver IC's performance and reliability. We examined the charge pump, the bootstrap capacitor, and the gate drive - the circuits that generate the high-side gate drive. |
We explored the advanced features: the stall detection, the standby mode, the spread-spectrum modulation, and the snubber circuit. We saw how these features improve the performance and the reliability of the motor drive system. |
We looked at real-world designs from major semiconductor companies: Texas Instruments' DRV8825, Allegro's A4988, STMicroelectronics' L6470, Trinamic's TMC2209, Toshiba's TB67S109A, and ON Semiconductor's LV8728. We saw how these ICs are used in printers from Brother, Zebra, Sato, and Honeywell, and we discussed the trade-offs between cost, performance, and features. |
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We discussed the thermal design, the reliability, and the future of driver ICs. We saw how the future driver ICs will be smarter and more integrated, with built-in microcontrollers and diagnostic systems. |
The overarching lesson is that the stepper motor driver IC is a critical component that determines the printer's paper feeding accuracy, its acoustic noise, and its reliability. A well-chosen driver IC simplifies the design, reduces the cost, and improves the performance. A poorly chosen driver IC causes misalignment, vibration, and premature failure. Understanding the stepper motor driver IC landscape is essential for any engineer who wants to design a high-quality thermal printer, and this chapter has provided that understanding from the basic parameters to the advanced features. |
End of Extended Section 11 |
cs@easiersoft.com