AN1625 STMicroelectronics, AN1625 Datasheet - Page 10

• Manufacturer Part Number: AN1625
• Description: L6235 THREE PHASE BRUSHLESS DC MOTOR DRIVER
• Manufacturer: STMicroelectronics

AN1625 APPLICATION NOTE
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
where D
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
ripple on the V
but the reference voltage will take more time to vary after changing the duty-cycle of the µC PWM, and too high
values of R
As sensing resistor value is typically kept small, a small noise on V
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,
PWM integrated circuitry can loose control of the current due to the minimum allowed duration of t
Programmable off-time Monostable section).
10/39
µC
LP
is the duty-cycle of the PWM output of the µC.
will also increase the impedance of the V
ref
voltage will be about 20 mV. Using higher values for R
PWM Output
of a µC
V
ref
R
LP
=
5V D
---------------------------------------- -
R
R
DIV
ref
L P
net at low frequencies, causing a poor noise immunity.
+
C
R
with R
DIV
R
DIV
GND
ref
LP
input pins might cause a considerable error
C
LP
= 56k , R
LP
V
ref
, R
DIV
DIV
and C
= 15k , C
LP
will reduce the ripple,
LP
= 10nF and a
ON
(see the