A3958
DMOS Full-Bridge PWM Motor Driver
FUNCTIONAL DESCRIPTION
D7 – D10 Fast Decay Time. A four-bit word sets the
fast-decay portion of the fixed-off time for the internal
PWM control circuitry. This will only have impact if the
mixed-decay mode is selected (via bit D17 and the MODE
input terminal). For tfd > toff, the device will effectively
operate in the fast-decay mode. The fast decay portion is
defined by
Serial Interface. The A3958 is controlled via a 3-wire
(clock, data, strobe) serial port. The programmable
functions allow maximum flexibility in configuring the
PWM to the motor drive requirements. The serial data is
clocked in starting with D19.
Bit Function
D0 Blank Time LSB
D1 Blank Time MSB
D2 Off Time LSB
tfd = (8[1 + N]/fosc) - 1/fosc
where N = 0 … 15
D3 Off Time Bit 1
D4 Off Time Bit 2
D5 Off Time Bit 3
D6 Off Time MSB
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.
D11 Synchronous Rectification Mode. The active
mode prevents reversal of load current by turning off
synchronous rectification when a zero current level is
detected. The passive mode will allow reversal of current
but will turn off the synchronous rectifier circuit if the load
current inversion ramps up to the current limit set by
VREF/RS.
D7 Fast Decay Time LSB
D8 Fast Decay Time Bit 1
D9 Fast Decay Time Bit 2
D10 Fast Decay Time MSB
D11 Sync. Rect. Mode
D12 Sync. Rect. Enable
D13 External PWM Mode
D14 Enable
D11
Mode
D15 Phase
0
1
Active
Passive
D16 Reference Range Select
D17 Internal PWM Mode
D18 Test Use Only
D12 Synchronous Rectification Enable.
D19 Sleep Mode
D12 Synchronous Rect.
D0 – D1 Blank Time. The current-sense comparator is
blanked when any output driver is switched on, according
to the table below. fosc is the oscillator input frequency.
0
1
Disabled
Enabled
D13 External PWM Decay Mode. Bit D13 determines
the current-decay mode when using ENABLE chopping
for external PWM current control.
D1
D0
Blank Time
0
0
1
1
0
1
0
1
4/fosc
6/fosc
12/fosc
24/fosc
D13
0
1
Mode
Fast
Slow
D2 – D6 Fixed-Off Time. A five-bit word sets the fixed-
off time for internal PWM current control. The off time is
defined by
D14 Enable Logic. Bit D14, in conjunction with
ENABLE, determines if the output drivers are in the
chopped (OFF)(ENABLE = D14) or ON (ENABLE ≠
D14) state.
t
off = (8[1 + N]/fosc) - 1/fosc
where N = 0 … 31
ENABLE D14
Mode
Chopped
On
On
Chopped
For example, with an oscillator frequency of 4 MHz, the
off time will be adjustable from 1.75 μs to 63.75 μs in
increments of 2 μs.
0
1
0
1
0
0
1
1
Allegro MicroSystems, Inc.
115 Northeast Cutoff
5
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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