3974 データシートの表示（PDF） - Allegro MicroSystems
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3974 Datasheet PDF : 12 Pages
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3974
DMOS DUAL FULL-BRIDGE
PWM MOTOR DRIVER
FUNCTIONAL DESCRIPTION
Serial Interface. The A3974SED 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 written as two 20-
bit words: 1 bit to select the word and 19 bits of data. The data
is clocked in starting with D19.
Word 0 Bit Assignments
Select Word 0 (D18 = 0)
Bit
Function
D0
Bridge 1 blank time LSB
D1
Bridge 1 blank time MSB
D2
Bridge 1 off-time LSB
D3
Bridge 1 off-time bit 1
D4
Bridge 1 off-time bit 2
D5
Bridge 1 off-time bit 3
D6
Bridge 1 off-time MSB
D7
Bridge 1 fast-decay time bit LSB
D8
Bridge 1 fast-decay time bit 1
D9
Bridge 1 fast-decay time bit 2
D10
Bridge 1 fast-decay time MSB
D11
Bridge 1 sync. rect. control
D12
Bridge 1 sync. rect. control
D13
Bridge 1 external PWM mode
D14
Bridge 1 enable
D15
Bridge 1 phase
D16
Bridge 1 reference range select
D17
Bridge 1 internal PWM mode
D18
Word select = 0
D19
Test mode
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.
D1 D0
0
0
0
1
1
0
1
1
Blank Time
4/fOSC
6/fOSC
12/fOSC
24/fOSC
D2 – D6 Fixed Off Time. This five-bit word sets the fixed
off-time for the internal PWM control circuitry. The off-time is
defined by
toff =(8 [1 + N]/fOSC) - 1/fOSC
where N = 0 .... 31
For example, with an oscillator frequency of 4 MHz, the
fixed off-time will be adjustable from 1.75 µs to 63.75 µs in
increments of 2 µs.
D7 – D10 Fast Decay Time. This 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 mixed-decay mode is
selected (via bit D17). For tfd > toff, the device will effectively
operate in fast-decay mode. The fast-decay portion is defined
by
tfd = (8[1 + N]/fOSC] - 1/fOSC
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.
D11 – D12 Synchronous Rectification.
D12 D11
0
0
0
1
1
0
1
1
Synchronous Rectifier
Disabled
Low side only
Active
Passive
The different modes of operation are described in the synchro-
nous rectification section of the functional description.
D13 External PWM Decay Mode. This bit determines the
current-decay mode when using ENABLE chopping for
external PWM current control.
D13
Mode
0
Fast
1
Slow
continued next page ...
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