34923

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

12

PWM Blank Timer

When a source driver turns on, a current spike occurs owing

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 comparator is blanked. The

blank timer runs after the off-time counter (see bits D2–D6) to

provide the programmable blanking function. The blank timer is

reset when PWM/ENABLE is chopped or DIR is changed. For

external PWM control, a DIR change or PWM/ENABLE on will

trigger the blanking function.

Synchronous Rectification

When a PWM off cycle is triggered, either by an PWM/

ENABLE chop command or internal fixed off-time cycle, load

current will recirculate according to the decay mode selected by

the control logic. The 34923 synchronous rectification feature

will turn on the opposite pair of outputs during the current decay

driver. This will reduce power dissipation significantly and can

eliminate the need for external Schottky diodes.

Synchronous rectification can be configured in active mode,

passive mode, or disabled via the serial port (bits D11 and

D12).

The active or passive mode selection has no impact in slow-

decay mode. With synchronous rectification enabled, the slow-

decay mode serves as an effective brake mode.

Current Regulation

Load current is regulated by an internal fixed off-time PWM

control circuit. When the outputs of the H-bridge are turned on,

the current increases in the motor winding until it reaches a trip

and serial data bit D16:

When SPAN

At the trip point, the sense comparator resets the source-

enable latch, turning off the source driver. The load inductance

then causes the current to recirculate for the serial-port-

programmed fixed off-time period. The current path during

recirculation is determined by the configuration of slow/mixed

current-decay mode (D17) and the synchronous rectification

control bits (D11 and D12).

Internal PWM (Current Mode) PWM Frequency

The internal PWM opeating frequency is set by the sum of

“Off Time”, as determined by bits D2 through D6, “Blank Time”,

as determined by bits D0 and D1, and the time constant of the

motor.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com