SPMD150STP

Function description and application information

Doc ID 17160 Rev 1

15/23

Figure 14.

VREFx from internal source

4.4

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 15 shows the operation of the bridge in the fast decay mode. At the start of the off

time, both of the power MOSFETs 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

MOSFET in parallel with the conducting diode is turned on in synchronous rectification

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 rectification mode. When the

mono-stable circuit times out, the power FETs are turned on again after some delay set by

the dead time to prevent cross conduction.