L6208Q View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6208Q' PDF : 34 Pages View PDF
Circuit description
L6208Q
5.4
Decay mode
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 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 freewheeling diodes. The current decays with a high di/dt since the voltage
across the coil is essentially the power supply voltage. After the deadtime, 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 may
decay completely to zero during the OFF time. At this point, if both of the power MOSFETs
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 MOSFET is
operated in synchronous rectification mode. This operation is called quasi-synchronous
rectification mode. When the monostable times out, the power MOSFETs are turned on
again after some delay set by the deadtime to prevent cross conduction.
Figure 17 shows the operation of the bridge in slow decay mode. At the start of the OFF
time, the lower power MOSFET 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 deadtime the upper power MOSFET is operated in the synchronous rectification mode.
When the monostable times out, the lower power MOSFET is turned on again after some
delay set by the deadtime to prevent cross conduction.
Figure 16. Fast decay mode output stage configurations
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Figure 17. Slow decay mode output stage configurations
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