PBL 3775/1
Functional Description
Each channel of the PBL 3775/1
consists of the following sections: an
output H-bridge with four transistors and
four recirculation diodes, capable of
driving up to 750 mA continuous current
to the motor winding,
a logic section that controls the output
transistors, an S-R flip-flop, and a com-
parator. The clock-oscillator is common
to both channels.
Constant current control is achieved
by switching the output current to the
windings. This is done by sensing the
peak current through the winding via a
current-sensing resistor R
S
, effectively
connected in series with the motor
winding. As the current increases, a
voltage develops across the sensing
resistor, which is fed back to the
comparator. At the predetermined level,
defined by the voltage at the reference
input V
R
, the comparator resets the flip-
flop, which turns off the upper output
transistor. The turn-off of one channel is
independent of the other channel. The
current decreases until the clock
oscillator triggers the flip-flops of both
channels simultaneously, which turns on
the output transistors again, and the
cycle is repeated.
To prevent erroneous switching due to
switching transients at turn-on, the
PBL 3775/1 includes a digital filter. The
clock oscillator provides a blanking
pulse which is used for digital filtering of
the voltage transient across the current
sensing resistor during turn-on.
The current paths during turn-on, turn-
off and phase shift are shown in figure 5.
reference voltage will produce an output
current of approximately 500 mA. R
S
should be selected for maximum motor
current. Be sure not to exceed the
absolute maximum output current which
is 850 mA. Chopping frequency, winding
inductance and supply voltage also
affect the current, but to much less
extent.
For accurate current regulation, the
sensing resistor should be a 0.5 - 1.0 W
precision resistor, i. e. less than 1%
tolerance and low temperature
coefficient.
Current sense filtering
At turn-on a current spike occurs, due to
the recovery of the recirculation diodes
and the capacitance of the motor
winding. To prevent this spike from
reseting the flip-flops through the
current sensing comparators, the clock
oscillator generates a blanking pulse at
turn-on. The blanking pulse pulse
disables the comparators for a short
time. Thereby any voltage transient
across the sensing resistor will be
ignored during the blanking time.
Applications Information
Current control
The regulated output current level to the
motor winding is determined by the
voltage at the reference input and the
value of the sensing resistor, R
S
. The
peak current through the sensing
resistor (and the motor winding) can be
expressed as:
I
M,peak
= 0.1•V
R
/ R
S
[A]
With a recommended value of 0.5 ohm
for the sensing resistor R
S
, a 2.5 V
V
MM
1
+5 V
+
V
MM
0.1
µF
12
V
9
2
0.1
µF
4
V
MM1
10
µF
19
V
MM2
CC
Phase
1
Dis
1
V
R1
Phase
2
Dis
2
V
R2
RC GND
11
C
1
8
E
1
2
C
2
15
5, 6,
17, 18
M
A1
3
10
7
M
B1
1
20
3
PBL 3775/1
M
A2
14
R
S
13
16
M
B2
E
2
21
22
STEPPER
MOTOR
Motor Current
+5 V
12 kΩ
4 700 pF
1
2
3
RS
0.47
Ω
RS
0.47
Ω
Pin numbers refer
to DIL package.
GND (V
MM
)
GND (V
CC
)
Fast Current Decay
Slow Current Decay
Time
Figure 5. Output stage with current paths
during turn-on, turn-off and phase shift.
Figure 6. Typical stepper motor driver application with PBL 3775/1.
5
ERICSSON [ ERICSSON ]
