DRM33395 Datasheet, PDF (42/44 Pages) Motorola, Inc – ANALOG ICs 33395 EVALUATION MOTOR BOARD DESIGNER REFERENCE MANUALv

Freescale Semiconductor, Inc.
Design Considerations
I_sense_A
I_sense_B
I_sense_C
+3.3V_A
R131
10 k
GNDA
R317 33 k-1%
R318 33 k-1%
R319 33 k-1%
R305
390 Ω
+1.65V
=
3
GNDA
12
R316
33 k-1%
2
–₁
3₊
U302A
MC33502D
R132
100 k
R133
100 k
I_reconst_DCB
IS- (pin 17 of U103)
C108
100 nF
IS+ (pin 18 of U103)
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.
Phase_C
R216
10 k-1%
BEMF_sense_C
GNDA
R217
2.7 k-1%
R221
3.3 k-1%
C204
100 pF
R218
10 k
V_sense_DCB_half
GNDA
R220
5.6 k
GNDA
C205
22 pF
R215
1M
5
+₂
4 _{– U201A}
12 ^LM339D
+5V_D
R219
10 k
Zero_cross_C
Figure 5-3 Phase C Back-EMF Feedback
DRM33395 Designer Reference Manual
33395 Evaluation Motor Board
40
Design Considerations
MOTOROLA
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