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BACK EMF DETECTION DURING PWM ON TIME BY ST7MC

Table 1. Table Resistor selection

It is preferred to use 1% resistors. But the performance will be acceptable using 5% resistors.

D16, D17, D18, R45, R51, and R55 in the starter kit can be removed if this new scheme is

used.

Figure 8 shows the comparison of back EMF detection during off time vs on time. If the back

EMF is detected during off, there is no attenuation. The signal is clamped at 5v. If the back

EMF is detected during PWM on time, the signal is attenuated.

Figure 8. (A) Waveforms for back EMF detection during PWM off time; (B) waveforms for

back EMF detection during PWM on time.

Figure 9 shows the detailed waveform of the zero crossing when the back EMF is detected

during PWM on time. From the waveform, we can see that the zero crossing is detected at the

end of PWM on time.

Vdc (V)

R4 (k)

R5(k)

R6(k)

R7(k)

R8(k)

R9(k)

Vref(V)

12

100

100

100

150

4.3

36

2.27

24

33

33

33

160

4.7

15

2.00

48

16

16

16

240

18

12

2.13

165

4.7

4.7

4.7

330

2.7

4.7

2.30

330

2.2

2.2

2.2

430

330

5.1

2.20

(A)

(B)