L6207 datasheet(12/53 Pages) STMICROELECTRONICS

12 / 53 page

AN1762 APPLICATION NOTE

12/53

3.8 Reference Voltage for PWM Current Control (L6207 ONLY)

The L6207 has two analog inputs, VrefA and VrefB, connected to the internal sense comparators, to control the

peak value of the motor current through the integrated PWM circuitry. In typical applications these pins are con-

nected together, in order to obtain the same current in the two motor windings. 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 application) 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 11).

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

where DµC is the duty-cycle of the PWM output of the µC.

Assuming that the µC output impedance is lower than 1k

Ω, with RLP = 56kΩ, RDIV = 15kΩ, CLP = 10nF and a

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

ripple on the Vref voltage will be about 20 mV. Using higher values for RLP, RDIV and CLP will reduce the ripple,

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

values of RLP will also increase the impedance of the Vref net at low frequencies, causing a poor noise immunity.

As sensing resistor values are typically kept small, a small noise on Vref input pins might cause a considerable

error in the output current. It's then recommended to decouple these pins with ceramic capacitors of some tens

of nF, placed very close to Vref and GND pins. Note that Vref pins 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 pins. With very small reference volt-

age, PWM integrated circuitry can loose control of the current due to the minimum allowed duration of tON (see

the

Programmable off-time Monostable section).

Figure 11. Obtaining a variable voltage through a PWM output of a µC.

re f

5V DµC RDIV

⋅⋅

DIV

-----------------------------------------

RLP

CLP

Vref

GND

PWM Output

of a µC

RDIV

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