L6208Q datasheet(18/34 Pages) STMICROELECTRONICS

18 / 34 page

Circuit description

L6208Q

18/34

DocID018710 Rev 3

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

Figure 17. Slow decay mode output stage configurations

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