Function description and application information

SPMD150STP

16/23

Doc ID 17160 Rev 1

Figure 15.

Fast decay mode output stage configurations

Figure 16 shows the operation of the bridge in the slow decay mode. At the start of the Off-

time, the lower power FET 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 FET is operated in the synchronous rectification mode. When

the mono-stable circuit times out, the lower power FET is turned on again after some delay

set by the dead time to prevent cross conduction.

Figure 16.

Slow decay mode output stage configurations

4.5

Stepping sequence generation

The phase sequence generator is a state machine that provides the phase and the 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 time. The drive mode is

selected by the HALF/FULL input and the current state of the sequence generator as

described below. A rising edge of the CLOCK input advances the state machine to the next

state. The direction of rotation is set by the CW/CCW input. The RESET input resets the

state machine to Home State.

4.5.1

Half step mode

A logic level high on the HALF/FULL input selects half step mode. Figure 17 shows the

motor current waveforms and the state diagram for the phase sequencer generator. At start-

up or after a RESET the phase sequencer is at state 1, home state. After each clock pulse

the state changes following the sequence 1,2,3,4,5,6,7,8,… if CW/ CCW is high (Clockwise

movement) or 1,8,7,6,5,4,3,2,… if CW/CCW is low (Counterclockwise movement).