L6228Q_10 Datasheet, PDF (16/32 Pages) STMicroelectronics – DMOS driver for bipolar stepper motor

Circuit description
Figure 15. Area where t_ON can vary maintaining the PWM regulation
100
L6228Q
10
2.5μs (typ. value)
1
0.1
1
10
100
Coff [nF]
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 16 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 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
monostable times out, the power MOS are turned on again after some delay set by the dead
time to prevent cross conduction.
Figure 17 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 synchronous 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.
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Doc ID 14321 Rev 4