PBL 3774/1
Functional Description
Each channel of the PBL 3774/1
consists of the following sections: an
output H-bridge with four transistors,
capable of driving up to 1000mA
continuous current to the motor winding;
a logic section that controls the output
transistors; an S-R flip-flop; and a
comparator. 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
resistor, R
S
, effectively connected in
series with the motor winding during the
turn-on period. As the current increases,
a voltage develops across the resistor,
and 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, turning
off the output transistors. The current
decreases until the clock oscillator
triggers the flip-flop, turning on the
output transistors, and the cycle is
repeated.
The current paths during turn-on, turn-
off and phase shift are shown in figure 5.
Note that the upper recirculation diodes
are connected to the circuit externally.
External recirculation diodes
1
V
MM
Applications Information
Current control
The output current to the motor is de-
termined by the voltage at the reference
input and value of sensing resistor, R
S
.
Chopping frequency, winding induc-
tance and supply voltage also affect the
current, but to much less extent. The
output current can be switched off com-
pletely by a
HIGH
input level at the Dis-
able input (Dis1 and Dis2 for respective
channels). When Disable goes
HIGH
, all
four transistors in the output stage are
switched off, and the output current
rapidly drops to zero (“fast current
decay” – see figure 5).
The peak motor current through the
sensing resistor and the motor winding
can be expressed as:
I
M,peak
= 0.18 • ( V
R
/ R
S
)
[A]
A 2.5 V reference voltage and a 0.47
ohm sensing resistor will produce an out-
put current level of approximately 960
mA.
To improve noise immunity at the V
R
input, the voltage control range can be
increased to 5 V if R
S
is correspondingly
changed (for example to 1ohm for 900
mA max output current).
+5 V
External components
For the device to function properly, four
external free-wheeling diodes must be
connected, as in figure 6. The diodes
should be of fast type with a reverse
recovery time of less than 100 ns. Com-
monly used types are UF4001 or
BYV27.
A low pass filter in series with the com-
parator input prevents erroneous switch-
ing 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 lowpass filtering action in-
troduces 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 what is defined by the
comparator threshold, V
CH
, set by the
reference input V
R
(V
CH
= 450 mV at V
R
=
2.5 V).
The time constant of the low-pass fil-
ter may therefore be reduced to
minimize the delay and optimize low-
current performance. Increasing the time
constant may result in unstable switch-
ing. The time constant should be
adjusted by changing the C
C
value.
V
MM
+
0.1 m
F
D1
2
10 m
F
D2
22
V
19
18
20
Phase
1
Dis
1
V
R1
CC
11
V
MM1
5
V
MM2
M
A1
12
M
B1
8
4
26
3
PBL 3774/1
Phase
2
Dis
2
V
R2
RC GND
23
1, 2,
3, 9,
13, 14,
15, 16,
17, 28.
M
C
1
21
1 kW
B2
M
A2
27
25
R
Motor Current
7
E
1
10
C
2
24
1 kW
E
2
6
D3
D4
S
STEPPER
MOTOR
V
MM
+5 V
15 kW
3300 pF
1
2
3
820 pF
RS
0.68 W
820 pF
RS
0.68 W
D1 - D4 are UF 4001 or
BYV 27, t
rr
£ 100 ns.
Pin numbers refer to
PLCC package.
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 3774/1.
5
ERICSSON [ ERICSSON ]
