PBL 3774/1
The frequency of the clock oscillator
is set by the R
T
-C
T
timing components at
the RC pin. The recommended values
result in a clock frequency (= switching
frequency) of 26.5 kHz. A lower
frequency will result in higher current
ripple, but may improve low-current
level linearity. A higher clock frequency
reduces current ripple, but increases the
switching losses in the IC and possibly
increased iron losses in the motor. If the
clock frequency needs to be changed,
the C
T
capacitor value should be
adjusted. The recommended R
T
resistor
value is 15 kohm.
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
1200mA peak when only one channel is
activated. Or recommended output
current, which is 1000mA peak, when
both channels is activated.
Motor selection
The PBL 3774/1 is designed for two-
phase bipolar stepper motors, i.e.
motors that have only one winding per
phase.
The chopping principle of the PBL
3774/1 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%. See figure
3 for definitions. To avoid this, it is
necessary to choose a motor with a low
winding resistance and inductance, i.e.
windings with a few turns.
It is not possible to use a motor that is
rated for the same voltage as the actual
supply voltage. Only rated current
needs to be considered. Typical motors
to be used together with the PBL 3774/1
have a voltage rating of 1 to 6 V, while
the supply voltage usually ranges from
12 to 40 V.
Low inductance, especially in
combination with a high supply voltage,
enables high stepping rates. However,
to give the same torque capability at low
speed, the reduced number of turns in
the winding of the low resistive, low
inductive motor must be compensated
by a higher current. A compromise has
to be made. Choose a motor with the
lowest possible winding resistance and
P
D
(W)
V
Ref
3.0
+5 V
1.2 kΩ
V
R1
and
V
R2
on
PBL 3774
10 kΩ 1.2 kΩ
70 % current
level
GND
2.2 kΩ
10 nF
2.0
els
nn
ha
on
1.0
Tw
o
c
0
0
O
ha
ne c
l
nne
on
0.20
0.40
0.60
0.80
I
M
(A)
Figure 7. Reduction of reference voltage
at the V
R
pin of PBL 3774/1.
Maximum allowable power dissipation [W]
6
Figure 8. Power dissipation vs. motor
current,T
A
= 25
°
C.
V
CE Sat
(V)
5
1.2
Am
1.0
0.8
0.6
0.4
erat
Batw
4
bie
nt
t
ing
em
pin
pe
3
ra
t
p
tem
ur
e
ure
2
1
0.2
0
-25
0
25
50
75
100
125
150
0
0
0.20
0.40
0.60
0.80
Temperature [°C]
PLCC package
DIP package
All ground pins soldered onto a
20 cm
2
PCB copper area with
free air convection.
I
M
(A)
Figure 9. Maximum allowable continuous
power dissipation vs. temperature.
Figure 10. Typical lower transistor
saturation voltage vs. output current.
V
d, ld
(V)
V
CE Sat
(V)
1.2
1.0
0.8
0.6
0.4
0.2
1.2
1.0
0.8
0.6
0.4
0.2
0
0
0.20
0.40
0.60
0.80
0
0
0.20
0.40
0.60
0.80
I
M
(A)
I
M
(A)
Figure 11. Typical lower diode voltage
drop vs. recirculating current.
6
Figure 12. Typical upper transistor
saturation voltage vs. output current.
ERICSSON [ ERICSSON ]
