3972
DUAL DMOS FULL-BRIDGE
MICROSTEPPING PWM MOTOR DRIVER
FUNCTIONAL DESCRIPTION (continued)
Word 1 Bit Assignments
Bit
D0
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
D12
D13
D14
D15
D16
D17
D18
Function
Word select = 1
Blank-time LSB
Blank-time MSB
Off-time LSB
Off-time bit 1
Off-time bit 2
Off-time bit 3
Off-time MSB
Fast-decay time LSB
Fast-decay time bit 1
Fast-decay time bit 2
Fast-decay time MSB
C0 oscillator control
C1 oscillator control
SR control bit 1
SR control bit 2
Reserved for testing
Reserved for testing
Idle mode
For example, with a master oscillator frequency of 4 MHz, the
fast-decay time will be adjustable from 1.75
µs
to 63.75
µs
in
increments of 2
µs.
D8 – D11 Fast Decay Time.
These four bits set the fast-
decay portion of fixed off-time for the internal PWM control
circuitry. The fast-decay portion is defined by:
t
fd
= [(1 + N) x 8/f
OSC
] - 1/f
OSC
where N = 0….15
For example, with an oscillator frequency of 4 MHz, the fast-
decay time will be adjustable from 1.75
µs
to 31.75
µs
in
increments of 2
µs.
For t
fd
> t
off
, the device will effectively
operate in fast-decay mode.
D12 – D13 Oscillator Control.
A 4 MHz internal oscillator
is used for the timing functions and charge-pump clock. If
more precise control is required, an external oscillator can be
input to the OSC terminal. To accommodate a wider range of
system clocks, an internal divider is provided to generate the
desired MO frequency according to the following table:
D13
0
0
1
1
D15
0
0
1
1
D12
0
1
0
1
D14
0
1
0
1
OSC
4 MHz internal clock
External clock
External clock/2
External clock/4
Synchronous Rectifier
Active
Disabled
Passive
Low side only
D1 – D2 Blank Time.
These two bits set the blank time for
the current-sense comparator.
When a source driver turns on, a
current spike occurs due to the reverse-recovery currents of the
clamp diodes and/or switching transients related to distributed
capacitance in the load. To prevent this current spike from
erroneously resetting the source-enable latch, the sense com-
parator is blanked. The blank timer runs after the off-time
counter to provide the programmable blanking function. The
blank timer is reset when PHASE is changed.
D2
0
0
1
1
D1
0
1
0
1
Time
4/f
OSC
6/f
OSC
8/f
OSC
12/f
OSC
D14 – D15 Synchronous Rectification.
The different modes of operation are in the synchronous
rectification section of the functional description.
D16, D17.
These bits are reserved for testing and should be
programmed to zero during normal operation.
D18 Idle Mode.
The device can be placed in a low power
“idle” mode by writing a “0” to D18. The outputs will be
disabled, the charge pump will be turned off, and the device
will draw a lower load supply currrent. The undervoltage
monitor circuit will remain active. D18 should be programmed
high for 1 ms before attempting to enable any output driver.
continued next page ...
D3 – D7 Fixed Off Time.
These five bits set the fixed off-
time for the internal PWM control circuitry. Fixed off-time is
defined by:
t
off
= [(1 + N) x 8/f
OSC
] - 1/f
OSC
where N = 0….31
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