AN4243 Datasheet, PDF (13/29 Pages) STMicroelectronics – The L6230 DMOS driver for three-phase brushless DC motor

AN4243
Designing an application with the L6230
Figure 9. Obtaining a reference voltage through a PWM output of microcontroller
PWM Output
of a µC
R_CP1
CP-
R_CP2
C_CP-
GND
AM16571v1
Assuming that this output swings from 0 to 5V, the resulting average voltage is:
Equation 10
V_CP-
=
5_----V_------⋅_------D_{----μ---C-----}⋅_------R_{----C----P---2-}
R_CP1 + R_CP2
where DµC is the duty cycle of the PWM output of the microcontroller.
Assuming that the microcontroller output impedance is lower than 1 kΩ , with R_CP1 = 5.6 kΩ,
R_CP2 = 1.5 k, C_CP = 100 nF and a PWM switching from 0 to 5 V at 100 kHz, the low-pass
filter time constant is about 0.12 ms and the remaining ripple on the V_CP- voltage is about 20
mV. Higher values for R_CP1, R_CP2 and C_CP reduce the ripple, but the reference voltage takes
more time to vary after changing the duty cycle of the microcontroller PWM. Besides, too
high values of R_CP1 also increase the impedance of the CP- net at low frequencies, causing
a poor noise immunity.
As sensing resistor value is typically kept small, a small noise on CP- input pins might cause
a considerable error in the output current. It is then recommended to decouple this pin with a
ceramic capacitor of some tens of nF, placed very close to CP- and GND pins.
Note that CP- pin connected to GND cannot guarantee zero current due to voltage offset in
the internal comparator. The best way to cut down the IC power consumption and to clear
the load current is to pull down the DIAG/EN pin.
With very small reference voltage, PWM current control method can lose control of the
current due to the minimum allowed duration of on-time.
Doc ID 024199 Rev 1
13/29