AN1625 APPLICATION NOTE

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2.8 Reference Voltage for PWM Current Control

The device has an analog input, Vref, connected to the internal sense comparator, to control the peak value of

the motor current through the integrated PWM circuitry. A fixed reference voltage can be easily obtained through

a resistive divider from an available 5 V voltage rail (maybe the one supplying the µC or the rest of the applica-

tion) and GND.

A very simple way to obtain a variable voltage without using a DAC is to low-pass filter a PWM output of a µC

(see Figure 9).

Figure 9. Obtaining a variable voltage through a PWM output of a µC.

Assuming that the PWM output swings from 0 to 5V, the resulting average voltage will be

Assuming that the µC output impedance is lower than 1k

µC PWM switching from 0 to 5V at 100kHz, the low pass filter time constant is about 0.12 ms and the remaining

but the reference voltage will take more time to vary after changing the duty-cycle of the µC PWM, and too high

in the output current. It's then recommended to decouple this pin with a ceramic capacitor of some tens of nF,

placed very close to Vref and GND pins. Note that Vref pin cannot be left unconnected, while, if connected to

GND, zero current is not guaranteed due to voltage offset in the sense comparator. The best way to cut down

(IC) power consumption and clear the load current is pulling down the EN pin. With very small reference voltage,

Programmable off-time Monostable section).

GND

PWM Output

of a µC

V

ref

⋅⋅

R

LP

R

DIV

+

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=