Choosing The Right H-Bridge Driver For Low-Voltage Motors
What is an H-Bridge?
An H-bridge is an electronic circuit that enables voltage to be applied across a load, such as a motor, in either direction. It contains four switching elements, such as transistors or relays, that are arranged in a bridge configuration to allow current to flow through the motor in both directions, allowing bidirectional control of the motor.
Definition and Basic Functioning of an H-Bridge Driver
More specifically, an H-bridge motor driver circuit uses four switches to control a brushed DC motor. By activating the switches in certain sequences, the motor can be made to turn clockwise or counterclockwise, as well as turn off or brake. This allows precise control of low-power motors without the need for complex mechanical drivetrains.
An H-bridge driver can control the speed of a motor using pulse-width modulation (PWM). By quickly cycling the motor voltage on and off, the average power delivered to the motor over time can be modulated. The longer the “on” part of the cycle, the faster the motor turns. This allows variable speed control without efficiency losses from resistance dropping. H-bridge motor drivers are commonly found controlling motors in toys, robots, actuators, and other low-power mechanical devices.
Applications of H-Bridge Drivers
Motor Control
The most common application of H-bridge drivers is motor control. By using a microcontroller or other logic device to switch the H-bridge transistors on and off in specific orders, bidirectional control of a brushed DC motor is achieved. The H-bridge allows current flow through the motor to be reversed, thereby reversing motor direction.
PWM and Motor Speed Regulation
Nearly all modern H-bridge drivers have the ability to use pulse width modulation (PWM) to regulate motor speed. By cycling the power to the motor on and off very quickly, the average voltage applied to the motor can be controlled. The longer the “on” part of the PWM cycle, the faster the motor turns. PWM allows efficient and responsive speed regulation without the power losses of resistive solutions.
Reversing Motor Direction
By activating different pairings of the bridge transistors, an H-Bridge circuit permits current to flow through the motor in either direction. This allows reversible control of motor direction without any physical change to the motor or mechanical system.
Considerations for Low-Voltage Motors
Lower Voltage Motors Require Lower Rds(on)
Low-voltage brushed DC motors typically operate in the 5V to 16V range. Because of the low operating voltages, the on-state resistance (Rds(on)) of the H-bridge transistors can significantly impact motor performance. A high Rds(on) will cause undesirable voltage drops during operation. As such, it is preferred to select H-bridge driver ICs with the lowest possible Rds(on) specifications.
Importance of Low Saturation Voltage
At motor stall or low speeds, most H-bridge driver circuits transition to a linear operating mode. In this mode, the voltage rating and saturation characteristics become very important. A good low-voltage driver IC will have saturated-load voltage drops of under 500mV to maintain good motor torque through the full speed range.
Thermal Design Concerns
While low-voltage motors require less absolute power, the currents typically run higher for a given power load. Combined with small driver package sizes required in many applications, heat dissipation deserves special consideration. Proper heat sinking, PCB layout, and airflow should be ensured so that the driver can operate within safe thermal limits.
Recommended Driver Features
Built-in Current Regulation
Overcurrent events can damage both the H-bridge and motor. Some newer drivers incorporate current sensing and regulation circuitry directly on the IC. This can prevent catastrophic damage from a short circuit or other marginal operating conditions.
Overcurrent and Overtemperature Protection
At a minimum an H-bridge driver should have overcurrent shutdown thresholds to reduce damage in an event. Overtemperature shutdown is also recommended to prevent damage during long overload events. The driver should recover automatically once fault conditions are removed.
Enable/Disable Input
Drivers with enable/disable inputs allow the controller to turn off the output signals safely whenever the motor does not need to operate. This both saves power and ensures the motors behave predictably on startup.
Example Driver ICs
DRV8837 Low-Voltage Brushed DC Gate Driver
The DRV8837 from Texas Instruments is an excellent low-voltage brushed DC motor driver. It supports motor supply voltages from 2.5V to 11V, with a Rds(on) rating down to just 55 milliohms. This allows for excellent motor performance even close to stall.
The DRV8837 has adjustable current limiting up to +/- 2.5 amps to prevent damage from shorts. Overcurrent, undervoltage, and thermal shutdown protection keep the driver safe under a variety of conditions. With specialized PWM inputs, this driver is easy to control from low-end microcontrollers.
STMicro L99PM60 4A Integrated Motor Driver
For low-voltage brushed DC motors up to about 30 Watts of mechanical power, the L99PM60 from STMicroelectronics makes an excellent integrated motor drive solution. The on-board low Rds(on) drivers can supply up to 4 amps continuously to the motor.
By integrating the controller, gate drivers, and protection circuits in one package, the L99PM60 reduces PCB size and simplifies design. The wide 6V to 30V operational voltage range covers most low-voltage motor use cases. L99PM60 also has 4A current limiting as well as the full suite of overcurrent and thermal protections.
Example Code for DRV8837
Initialize Driver
void drv8837_init() {
// Set CS pins as outputs
DDRB |= (1<
Control Motor Speed and Direction
// Motor A forward at 50% duty cycle
set_drv8837_dutyA(128);
// Motor B reverse at 75% duty cycle
set_drv8837_dutyB(192);
void set_drv8837_dutyA(uint8_t duty) {
// Duty value from 0-255
OCR1A = duty;
// Set direction
if (duty > 0) {
PORTB |= (1<
Conclusion
Key Takeaways on Selecting Right H-Bridge Driver
The most important specs for a low-voltage brushed DC motor driver are low Rds(on) to minimize resistance losses and low saturation voltage to maintain torque through the full speed range. Features like adjustable current limiting, overcurrent shutdown, and thermal protection also prevent damage. Driver ICs like the DRV8837, L99PM60 optimize these parameters while packing robust feature sets into simple, tiny packages.
