AN2836 datasheet(18/57 Pages) STMICROELECTRONICS

18 / 57 page

Designing an application with L6225, L6226, L6227

AN2836

18/57

Doc ID 15084 Rev 1

Figure 11.

Obtaining a variable voltage through a PWM output of a µC

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

Equation 9

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

Assuming that the µC output impedance is lower than 1 k

Ω, with R

LP = 56 kΩ, RDIV = 15 kΩ,

CLP = 10 nF and a µC 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 Vref voltage is about 20 mV. Using

higher values for RLP, RDIV and CLP reduces the ripple, but the reference voltage takes more

time to vary after changing the duty cycle of the µC PWM, and too high values of RLP 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 the 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 the Vref and GND pins.

Note that the Vref pins cannot be left unconnected, while, if connected to GND, zero current

is not guaranteed due to the voltage offset in the sense comparator. The best way to cut

down the IC power consumption and clear the load current is to pull down the EN pins. With

very small reference voltage, PWM integrated circuitry can lose control of the current due to

the minimum allowed duration of tON (see Section 4.11).

4.9

Input logic pins

IN1A, IN2A, IN1B, IN2B are CMOS/TTL compatible logic input pins. The input comparator

has been configured with hysteresis to ensure the required noise immunity. Typical values

for turn-on and turn-off thresholds are Vth,ON = 1.8 V and Vth,OFF = 1.3 V. As shown in

Figure 12, these pins are ESD-protected (2 kV human-body electro-static discharge), and

can be directly connected to the logic outputs of a µC. A series resistor is generally not

recommended, as it could help inducted noise to disturb the inputs. All logic pins enforce a

specific behavior and cannot be left unconnected.

ref

5V DμC RDIV

⋅⋅

DIV

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

AN2836 Datasheet(PDF) 18 Page - STMicroelectronics