3967
MICROSTEPPING DRIVER
WITH TRANSLATOR
Functional Description (cont’d)
RC Blanking.
In addition to the fixed off time of the
PWM control circuit, the C
T
component sets the compara-
tor blanking time. This function blanks the output of the
current-sense comparator when the outputs are switched
by the internal current-control circuitry. The comparator
output is blanked to prevent false over-current detection
due to reverse recovery currents of the clamp diodes, and/
or switching transients related to the capacitance of the
load. The blank time t
BLANK
can be approximated by:
t
BLANK
= 1400C
T
Enable Input (ENABLE).
This active-low input
enables all of the outputs. When logic high the outputs are
disabled. Inputs to the translator (STEP, DIRECTION,
MS
1
, MS
2
) are all active independent of the ENABLE
input state.
Shutdown.
In the event of a fault (excessive junction
temperature) the outputs of the device are disabled until
the fault condition is removed. At power up, and in the
event of low V
CC
, the under-voltage lockout (UVLO)
circuit disables the drivers and resets the translator to the
home state.
Sleep Mode (SLEEP).
An active-low control input
used to minimize power consumption when not in use.
This disables much of the internal circuitry including the
outputs. A logic high allows normal operation and startup
of the device in the home position.
Percent Fast Decay Input (PFD).
When a STEP
input signal commands a lower output current from the
previous step, it switches the output current decay to either
slow-, fast-, or mixed-decay depending on the voltage
level at the PFD input. If the voltage at the PFD input is
greater than 0.6V
CC
then slow-decay mode is selected. If
the voltage on the PFD input is less than 0.21V
CC
then
fast-decay mode is selected. Mixed decay is between
these two levels.
Mixed Decay Operation.
If the voltage on the PFD
input is between 0.6V
CC
and 0.21V
CC
, the bridge will
operate in mixed-decay mode depending on the step
sequence (see figures). As the trip point is reached, the
device will go into fast-decay mode until the voltage on
the RC terminal decays to the voltage applied to the PFD
terminal. The time that the device operates in fast decay is
approximated by:
t
FD
= R
T
C
T
In (0.6V
CC
/V
PFD
)
After this fast decay portion, t
FD
, the device will
switch to slow-decay mode for the remainder of the fixed
off-time period.
2.5
OUTPUT SATURATION VOLTAGE IN VOLTS
T
A
= +25°C
2.0
SOURCE DRIVER
1.5
Typical output saturation
voltages showing Satlington™
sink-driver operation.
1.0
0.5
SINK DRIVER
0
200
300
400
500
600
700
800
Dwg. GP-064-4
OUTPUT CURRENT IN MILLIAMPERES
6
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
