H-bridge driver for small motors
(a)Concept
(b)Bipolar implementation
(c)MOSfet implementation

L9110 PCB

TB6612 PCB

Motor Drivers

Background

DC motors typically use an H-bridge drive circuit. This allows forward and backward operation by reversing the polarity of the voltage applied to the motor.

For the small motors in small robots, there are two basic ways to implement an H-bridge circuit:

• Bipolar transistors
• Field effect (MOS) transistors

When used as switches these two types have significantly different electrical characteristics.

In the on state, a bipolar transistor switch operates in what's called “saturation mode”. When in saturation, there is a relatively fixed voltage drop across the transistor. I say “relatively”, because it does vary somewhat with current. The saturation voltage can be a few tenths of a volt, up to one volt or more for power transistors handling large currents.

MOS transistor switches behave much like a resistor - the voltage across the transistor is proportional to the current it's carrying. The effective resistance value depends on both the physical size of the transistor and the applied gate voltage.

So the bottom line is that a MOS-based H-bridge typically has less voltage drop across the switch itself than a comparable bipolar implementation. That means less heating of the switch, and less need for a heat sink. Also, for low battery voltages (say, 5 volts and below) the difference in voltage applied to the motor itself can make a difference in performance.

Selection

These are some chips commonly used in small motor driver boards.

Chip	Input	Output	Voltage Drop
L9110	2.5-12V	800mA	2.85V@750mA,9V
HG7881	2.4-10V	0.6-1A	1.7V@800mA,6V
L298	≤46	2.0A	2.55V@1A 3.7@2A
TB6612	2.5-13.5	1.0A	0.5V@1A 0.15V@0.3A

Some comments:

• Values in the table are typical or recommended values from the data sheets. See the data sheets for specifics and limitations.
• The L9110 and HG7881 seem very similar. Differences in the table above may be differences in test conditions rather the differences in the chips themselves.
• The L298 can handle higher voltages and currents, but the high voltage drops mean it needs a good heat sink.
• The TB6612 is the only MOSfet-based unit in the list, and that's clearly visible in the voltage drops. When turned on, the chip basically looks like a 0.5 ohm resistor in series with the motor.

So as a selection guide:

• The L9110/HG7881's are the way to go for absolute lowest cost, typically well under $1.
• The TB6612 is a bit more expensive, typically $1-2. The better numbers are probably worth it in many cases.
• The L298 is an ok choice for higher currents and higher supply voltages, but I would consider some of the boards using a driver chip with external discrete MOSfet's instead.

Notes

1. L9110 data sheet
2. Data sheet for a board using the L9110
3. HG7881 Data Sheet
4. A data sheet for the L298 is available on the ST Microelectronics website.
5. www.rugged-circuits.com/the-motor-driver-myth About the L298 and voltage drops. Explains why you need a really big heat sink.
6. A data sheet for the TB6612 is available on the Toshiba website.