2-Phase Stepper Motor Bipolar Driver ICs (2-Phase/1-2 Phase/W1-2 Phase Excitation)
UDN2916B/LB
Application Notes
qPWM
CURRENT CONTROL
The UDN2916B/LB dual bridges are designed to drive both wind-
ings of a bipolar stepper motor. Output current is sensed and
controlled independently in each bridge by an external sense
resistor (R
S
), internal comparator, and monostable multivibrator.
PWM OUTPUT CURRENT WAVE FORM
V
PHASE
Load-Current Paths
V
BB
Load
+
I
OUT
0
−
I
TRIP
R
SENSE
BRIDGE ON
SOURCE OFF
ALL OFF
t
d
t
off
When the bridge is turned ON, current increases in the motor
winding and it is sensed by the external sense resistor until the
sense voltage (V
SENSE
) reaches the level set at the comparator’s
input:
qLOGIC
CONTROL OF OUTPUT CURRENT
Two logic level inptus (I
0
and I
1
) allow digital selection of the
motor winding current at 100%, 67%, 33%, or 0% of the maxi-
mum level per the table. The 0% output current condition turns
OFF all drivers in the bridge and can be used as an OUTPUT
ENABLE function.
These logic level inputs greatly enhance the implementation of
I
TRIP
=V
REF
/ 10R
S
The comparator then triggers the monostable which turns OFF
the source driver of the bridge. The actual load current peak
will be slightly higher than the trip point (especially for low-in-
ductance loads) because of the internal logic and switching de-
lays. This delay (t
d
) is typically 2
µ
s. After turn-off, the motor
current decays, circulating through the ground-clamp diode and
sink transistor. The source driver’s OFF time (and therefore the
magnitude of the current decrease) is determined by the
monostable’s external RC timing components, where t
off
=R
T
C
T
wihtin the range of 20kΩ to 100kΩ and 100pF to 1000 pF.
When the source driver is re-enabled, the winding current (the
sense voltage) is again allowed to rise to the comparator ’s
threshold. This cycle repeats itself, maintaining the average
motor winding current at the desired level.
Loads with high distributed capacitances may result in high turn-
ON current peaks. This peak (appearing across R
S
) will attempt
to trip the comparator, resulting in erroneous current control or
high-frequency oscillations. An external R
C
C
C
time delay should
be used to further delay the action of the comparator. Depend-
ing on load type, many applications will not require these exter-
nal components (SENSE connected to E.)
µ
P-controlled drive formats.
During half-step operations, the I
0
and I
1
allow the
µ
P to control
the motor at a constant torque between all positions in an eight-
step sequence. This is accomplished by digitally selecting 100%
drive current when only one phase is ON and 67% drive current
when two phases are ON. Logic highs on both I
0
and I
1
turn
OFF all drivers to allow rapid current decay when switching
phases. This helps to ensure proper motor operation at high
step rates.
The logic control inputs can also be used to select a reduced
current level (and reduced power dissipation) for ‘hold’ condi-
tions and/or increased current (and available torque) for start-
up conditions.
qSWITCHING
THE EXCITATION CURRENT DIRECTION
The PHASE input to each bridge determines the direction moter
winding current flows. An internally generated deadtime (ap-
proximately 2
µ
s) prevents crossover currents that can occur
when switching the PHASE input.
80
UDN2916B/LB