3948

DMOS FULL-BRIDGE

PWM MOTOR DRIVER

www.allegromicro.com

5

FUNCTIONAL DESCRIPTION

Serial Interface. The A3948 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

D3

Off Time Bit 1

D4

Off Time Bit 2

D5

Off Time Bit 3

D6

Off Time MSB

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

D15

Phase

D16

Reference Range Select

D17

Internal PWM Mode

D18

Test Use Only

D19

Sleep Mode

D0 – D1 Blank Time. The current-sense comparator is

blanked when any output driver is switched on, according

D1

D0

Blank Time

0

0

0

1

1

0

1

1

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

where N = 0 … 31

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.

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

operate in the fast-decay mode. The fast decay portion is

defined by

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 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

D11

Mode

0

Active

1

Passive

D12 Synchronous Rectification Enable.

D12

Synchronous Rect.

0

Disabled

1

Enabled

D13 External PWM Decay Mode. Bit D13 determines

the current-decay mode when using ENABLE chopping

for external PWM current control.

D13

Mode

0

Fast

1

Slow

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.

ENABLE

D14

Mode

0

0

Chopped

10

On

01

On

1

1

Chopped