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DECAY MODES

The CONTROL input is used to select the behavior of the bridge during the off time. When the CONTROL pin

is low, the Fast Decay mode is selected and both transistors in the bridge are switched off during the off time.

When the CONTROL pin is high, the Slow Decay mode is selected and only the low side transistor of the bridge

is switched off during the off time.

Figure 14 shows the operation of the bridge in the Fast Decay mode. At the start of the off time, both of the

power MOS are switched off and the current recirculates through the two opposite free wheeling diodes. The

current decays with a high di/dt since the voltage across the coil is essentially the power supply voltage. After

the dead time, the lower power MOS in parallel with the conducting diode is turned on in synchronous rectifica-

tion mode. In applications where the motor current is low it is possible that the current can decay completely to

zero during the off time. At this point if both of the power MOS were operating in the synchronous rectification

mode it would then be possible for the current to build in the opposite direction. To prevent this only the lower

power MOS is operated in synchronous rectification mode. This operation is called Quasi-Synchronous Recti-

fication Mode. When the monostable times out, the power MOS are turned on again after some delay set by the

dead time to prevent cross conduction.

Figure 15 shows the operation of the bridge in the Slow Decay mode. At the start of the off time, the lower power

MOS is switched off and the current recirculates around the upper half of the bridge. Since the voltage across

the coil is low, the current decays slowly. After the dead time the upper power MOS is operated in the synchro-

nous rectification mode. When the monostable times out, the lower power MOS is turned on again after some

delay set by the dead time to prevent cross conduction.

Figure 14. Fast Decay Mode Output Stage Configurations

Figure 15. Slow Decay Mode Output Stage Configurations

STEPPING SEQUENCE GENERATION

The phase sequence generator is a state machine that provides the phase and enable inputs for the two bridges

to drive a stepper motor in either full step or half step. Two full step modes are possible, the Normal Drive Mode

where both phases are energized each step and the Wave Drive Mode where only one phase is energized at a

A) ON TIME

B) 1

µs DEAD TIME

C) QUASI-SYNCHRONOUS

RECTIFICATION

D) 1

µs SLOW DECAY

D01IN1335

A) ON TIME

B) 1

µs DEAD TIME

C) SYNCHRONOUS

RECTIFICATION

D) 1

µs DEAD TIME

D01IN1336