User Manual

AND8399/D
http://onsemi.com
19
Sample The SLA Voltage
Figure 20 gives a typical application schematic.
Figure 20. Typical Application Schematic
HV20081114.1
DIR
NXT
DO
DI
CLK
CLR
SLA
AMIS−30521/
100 nF
100 nF 100 nF 100 nF
VDD VBB VBB
VCP
CPN
CPP
MOTXP
MOTXN
MOTYP
MOTYN
220 nF
R
3
R
2
C
4
C
2
C
3
C
6
C
7
M
C
5
TSTO
GND
C
8
R
1
+
C
1
CS
ERR
5
4
31
2
3
13
12
6
7
11
9
10
25,26
21,22
15,16
19,20
NCV70521
1,
8
17,
18
23,
24 30 28 29
32 14 27
D1
mC
220 nF
100 mF
V
BAT
V
DD
Sampling of the SLA voltage is done by the
microcontroller ADC. Because the BEMF can only be
sampled once per full step, it’s not needed to have a high
sampling rate. The ADC resolution depends on how
accurate one wants to measure the SLA voltage
3
.
As explained in Transparency Mode, the SLA voltage is
only updated after the coil current zero crossing is left (Not
Transparent Mode). Keep this in mind when sampling the
SLA voltage. Also, the low−pass filter (R1 and C8) will
introduce a delay in the measured signal.
Because the SLA voltage is updated at the PWM
Frequency rate in Transparent Mode, one could already
measure the BEMF during the coil current zero crossing. If
one knows how long the clamping and transient parts lasts
(defined in section Maximum operating Speed), the
microcontroller can sample the SLA voltage right after this
transient part. Or to make it even simpler, the
microcontroller could sample the SLA voltage at the
moment the next microstep is given by the microcontroller.
No need to introduce an additional delay
4
as would be the
case when working in Not Transparent mode (see
Figure 21).
When working in Transparent mode, the transient
behavior will be present on the SLA−pin. The
microcontroller should just ignore this.
3. The coil voltage is sampled by a Sample−and−Hold circuit. There is no digital conversion done by AMIS−305xx.
4. If the stepper motor is operated close to the maximum operating speed it’s advised to introduce a delay between the NXT pulse and
sampling of the SLA voltage.