3972

DUAL DMOS FULL-BRIDGE

MICROSTEPPING PWM MOTOR DRIVER

www.allegromicro.com

7

V

the sink-side DMOS outputs. V

in the case of a fault condition, the outputs of the device are

disabled. The V

capacitor to ground.

Current Regulation. The reference voltage can be set by

analog input to the REF terminal, or via the internal 2 V

precision reference. The choice of reference voltage and sense

resistor set the maximum trip current.

I

Microstepping current levels are set according to the following

equations:

I

V

where DAC input code equals 1 to 63 and Range is 4 or 8 as

selected by Word 0, D18. Programming the DAC input code to

zero disables the bridge, and results in minimum load current.

PWM Timer Function. The PWM timer is programmable via

the serial port to provide fixed off-time PWM signals to the

control block. In mixed-decay mode, the first portion of the off

time operates in fast decay, until the fast-decay time count is

reached, followed by slow decay for the rest of the fixed off-

time period. If the fast-decay time is set longer than the off-

time, the device effectively operates in fast-decay mode.

Oscillator. The PWM timer is based on an oscillator input,

typically 4 MHz. The A3972SB can be configured to select

either a 4 MHz internal oscillator or, if more precision is

required, an external clock can be connected to the OSC

terminal. If an external clock is used, three internal divider

choices are selectable via the serial port to allow flexibility in

choosing f

oscillator option is used, the absolute accuracy is dependent on

the process variation of resistance and capacitance. A precision

resistor can be connected from the OSC terminal to V

further improve the tolerance. The frequency will be:

f

9

/R

OSC

If the internal oscillator is used without the external resistor, the

OSC terminal should be connected to ground.

Sleep Mode. The input terminal SLEEP is dedicated to

putting the device into a minimum current draw mode. When

pulled low, the serial port will be reset to all zeros and all

circuits will be disabled.

Shutdown. In the event of a fault due to excessive junction

temperature, or low voltage on V

device are disabled until the fault condition is removed. At

power up, or in the event of low V

disables the drivers and resets the data in the serial port to zeros.

Synchronous Rectification. When a PWM off-cycle is

triggered, either by a bridge disable command or internal fixed

off-time cycle, the load current will recirculate according to the

decay mode selected by the control logic. The A3972SB

synchronous rectification feature will turn on the appropriate

MOSFET(s) during the current decay and effectively short out

the body diodes with the low r

power dissipation significantly and can eliminate the need for

external Schottky diodes for most applications.

Four distinct modes of operation can be configured with the two

serial port control bits:

1.

Active Mode. Prevents reversal of load current by

turning off synchronous rectification when a zero current

level is detected.

2.

Passive Mode. Allows reversal of current but will turn

off the synchronous rectifier circuit if the load current

inversion ramps up to the current limit.

3.

Disabled. MOSFET switching will not occur during load

recirculation. This setting would only be used with four

external clamp diodes per bridge.

4.

Low Side Only. The low-side MOSFETs will switch on

during the off time to short out the current path through the

MOSFET body diode. With this setting, the high-side

MOSFETs will not synchronously rectify so four external

diodes from output to supply are recommended. This mode

is intended for use with high-power applications where it is

desired to save the expense of two external diodes per

bridge. In this mode, the sink-side MOSFETs are chopped

during the PWM off time. In all other cases, the source-

side MOSFETs are chopped in response to a PWM off

command.

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FUNCTIONAL DESCRIPTION (continued)