PBL 3771/1
External components
The voltage across the sensing resistor
is fed back to the comparator via a low-
pass filter section, to prevent erroneous
switching due to switching transients.
The recommended filter component
values, 1 kohm and 820 pF, are suitable
for a wide range of motors and
operational conditions.
Since the low-pass filtering action
introduces a small delay of the signal to
the comparator, peak voltage across the
sensing resistor, and hence the peak
motor current, will reach a slightly higher
level than the threshold, V
C
, set by the
reference voltage
(V
C
= 450 mV @V
R
= 2.5 V).
The time constant of the low-pass filter
may therefore be reduced to minimize
the delay and optimize low-current
performance, especially if a low (12 V)
supply voltage is used. Increasing the
time constant may result in unstable
switching.
The frequency of the clock oscillator is
set by the R-C combination at pin RC.
The recommended values give a
nominal frequency of 26.5 kHz. A lower
frequency will result in higher current
ripple and may cause audible noise from
the motor, while increasing the
frequency results in higher switching
losses and possibly increased iron
losses in the motor.
The sensing resistor, R
S
, should be
selected for maximum motor current.
The relationship between peak motor
current, reference voltage and the value
of R
S
is described under “Current
control” above. Be sure not to exceed
the maximum output current which is
650 mA per channel (or 500 mA per
channel, both channels fully on, see
“Recommended Operating Conditions”).
Motor selection
The PBL 3771/1 is designed for bipolar
motors, i.e., motors that have only one
winding per phase. A unipolar motor,
having windings with a center tap, can
also be used, see figure 14.
The chopping principle in the
PBL 3771/1 is based on a constant
frequency and a varying duty cycle. This
scheme imposes certain restrictions on
motor selection. Unstable chopping can
occur if the chopping duty cycle exceeds
approximately 50%. To avoid this, it is
necessary to choose a motor with a low
winding resistance. Low winding
resistance means less inductance and
will therefore enable higher stepping
rates, however it also means less torque
capability. A compromise has to be
made.
Choose a motor with the lowest
possible winding resistance that still
gives the required torque and use as
high supply voltage as possible without
exceeding the maximum recommended
40 V. Check that the chopping duty cycle
does not exceed 50% at maximum
current.
Since the PBL 3771/1 produces a
regulated, constant output current it is
not necessary to use a motor that is
rated at the same voltage as the actual
supply voltage. Only rated current needs
to be considered. Typical motors to be
used together with the PBL 3771/1 have
voltage ratings of 5 to 12 V, while the
supply voltage usually ranges from 24 to
40 V.
V
CC
(+5 V)
+
0.1
µF
0.1
µF
V
MM
10
µF
5
14
D0
V DD
Sign1
CD1
7
To
µP
15
16
6
17
22
1
D7
V
3
4
2
7
8
9
Phase
1
CD
1
V
R1
Phase
2
CD
2
V
R2
RC GND
12
+5 V
15 kΩ
3 300 pF
11
CC
3
V
MM1
20
V
MM2
M
A1
4
PBM 3960/1
A0
A1
WR
CS
RESET
V Ref
DA1
Sign2
CD2
M
B1
1
19
PBL 3771/1
M
A2
20
19
21
16
15
14
+2.5V
V SS
18
DA2
M
B2
C
1
10
1 kΩ
22
E
1
2
C
2
13
1 kΩ
E
2
21
5, 6,
17, 18
STEPPER
MOTOR
820 pF
1.0
Ω
RS
820 pF
1.0
Ω
RS
Pin numbers refer
to DIL package.
GND (V
MM
)
GND
(V
CC
)
Figure 7. Microstepping system with PBM 3960 /1 and PBL 3771/1.
6
ERICSSON [ ERICSSON ]
