DRM33395 Designer Reference Manual
33395 Evaluation Motor Board
40
Design Considerations
Design Considerations
Figure 5-2 Reconstructed Bus Current
5.5 Back-EMF Signals
Back-EMF and zero crossing signals are included to support sensorless
algorithms for brushless dc motors. Referring to
Figure 5-3
, which
shows the circuitry for phase C, the raw phase voltage is scaled down by
a voltage divider consisting of R216, R217, and R221. One output from
this divider, BEMF_sense_C produces Back-EMF sense voltage.
Resistor values are chosen such that a 16-volt maximum phase voltage
corresponds to a 3.3-volt maximum A/D input.
A zero crossing signal is obtained by comparing the motor phase voltage
with ½ the value of the motor bus voltage. Comparator U201A performs
this function, producing a zero crossing signal, Zero_cross_C.
Figure 5-3 Phase C Back-EMF Feedback
+
–
R305
390
!
+3.3V_A
+1.65V
I_reconst_DCB
R316
33 k-1%
2
3
1
U302A
MC33502D
R317 33 k-1%
I_sense_A
R318 33 k-1%
R319 33 k-1%
I_sense_B
I_sense_C
R132
100 k
R133
100 k
GNDA
R131
10 k
1
3
2
C108
100 nF
IS- (pin 17 of U103)
IS+ (pin 18 of U103)
GNDA
=
–
+
GNDA
GNDA
GNDA
10 k-1%
R216
R217
2.7 k-1%
R221
3
.3 k-1%
10 k
R218
C204
100 pF
R220
5.6 k
C205
22 pF
V_sense_DCB_half
R215
1 M
10 k
R219
+5V_D
U201A
LM339D
4
5
12
2
Zero_cross_C
BEMF_sense_C
Phase_C
