Driving Brushed DC Motors Using PWM Output: H Bridge Constant-Current Driving

There are a number of methods for PWM driving of brushed DC motors. In this article, H-bridge constant-current driving is explained.

Driving Brushed DC Motors Using PWM Output:
H Bridge Constant-Current Driving

This is an example of a circuit for PWM constant-current driving. It is a general driver circuit, but an internal comparator is used for on/off control of the motor current, that is, for PWM control.

The resistor Rs connected to the GND side (RNF pin) of the power transistor (MOSFET) is used for current detection. The internal comparator compares the voltage occurring across this resistor Rs with the voltage at a reference voltage setting pin (Vref). If the RNF pin voltage exceeds the Vref voltage, the comparator output is L, and the power transistor supplying current on the power supply side is turned off. Whether current is supplied or the supply is stopped differs depending on the transistor on/off states corresponding to the operating mode–forward rotation, reverse rotation, and so on. In any case, Vref, Rs, and the comparator switch between supply and shutoff of motor current.

The off time (toff) is set by a method in which the frequency of the oscillator (OSC) is counted; during the off time, current is regenerated. When the preset off time has passed, the power transistor, which had been turned off, is again turned on, and current is supplied. Through repetition of this sequence, the motor can be turned by constant-current control with the Vref voltage divided by the Rs value as the peak current (Ipeak). The diagram shows the waveforms for the motor current, Rs current, and RNF voltage.

As a point to be noted, a current does not flow in Rs while a regeneration current is flowing, and so when current supply is restarted there is a large change in the current in Rs. Consequently, considerable voltage noise may occur due to parasitic inductances, and a current may flow that charges the parasitic capacitance in power transistors, so that the Vref voltage may be exceeded.

In order to keep power transistors from being turned off due to this voltage noise, a short blank time (tblnk) must be provided. The blank time must be provided to prevent malfunctions not only when current is turned on and off, but also when there is voltage noise during motor rotation state switching. The blank time is either optimized or can be adjusted, depending on the driver IC.