NJM37770
-
-
Ver.2006-04-11
Current level selection.
The status of I
0
and I
1
inputs determines the current level in the motor winding. Three fixed current levels can be
selected according to the table below.
Motor current I
0
I
1
High level 100% L L
Medium level 60% H L
Low level 20% L H
Zero current 0% H H
The specific values of the different current levels are determined by the reference voltage V
R
together with the value of
the sensing resistor R
S
.
The peak motor current can be calculated as follows:
i
m
= (V
R
• 0.080) / R
S
[A], at 100% level
The motor current can also be continuously varied by modulating the voltage reference input.
Current sensor
The current sensor contains a reference voltage divider and three comparators for measuring each of the selectable
current levels. The motor current is sensed as a voltage drop across the current sensing resistor, R
S
, and compared with
one of the voltage references from the divider. When the two voltages are equal, the comparator triggers the single-pulse
generator. Only one comparator at a time is activated by the input logic.
Single-pulse generator
The pulse generator is a monostable multivibrator triggered on the positive edge of the comparator output. The
multivibrator output is high during the pulse time, t
off
, which is determined by the timing components R
T
and C
T
.
t
off
= 0.69 • R
T
• C
T
The single pulse switches off the power feed to the motor winding, causing the winding to decrease during t
off
.
If a new trigger signal should occur during t
off
, it is ignored.
Output stage
The output stage contains four transistors and two diodes, connected in an H-bridge. Note that the upper recirculation
diodes are connected to the circuit externally. The two sinking transistors are used to switch the power supplied to the
motor winding, thus driving a constant current through the winding. See figures 3 and 4.
Overload protection
The circuit is equipped with a thermal shut-down function, which will limit the junction temperature. The output current
will be reduced if the maximum permissible junction temperature is exceeded. It should be noted, however, that it is not
short circuit protected.
Operation
When a voltage V
MM
is applied across the motor winding, the current rise follows the equation:
i
m
= (V
MM
/ R) • (1 - e
-(R • t ) / L
)
R = Winding resistance
L = Winding inductance
t = time
(see figure 3, arrow 1)
The motor current appears across the external sensing resistor, R
S
, as an analog voltage. This voltage is fed through a
low-pass filter, R
C
C
C
, to the voltage comparator input (DIP pin 10, EMP pin 12). At the moment the sensed voltage rises
above the comparator threshold voltage, the monostable is triggered and its output turns off the conducting sink
transistor.The polarity across the motor winding reverses and the current is forced to circulate through the appropriate
upper protection diode back through the source transistor (see figure 3, arrow 2).
After the monostable has timed out, the current has decayed and the analog voltage across the sensing resistor is
below the comparator threshold level.The sinking transistor then turns on and the motor current starts to increase again,
The cycle is repeated until the current is turned off via the logic inputs.When both I
1
and I
0
are high, all four transistors in
the output H-bridge are turned off, which means that inductive current recirculates through two opposite free-wheeling
diodes (see figure 3, arrow 3). this method of turning off the current results in a faster current decay than if only one
transistor was turned off and will therefore improve speed performance in half-stepping mode.
