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L6739

High current embedded drivers

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8

High current embedded drivers

The L6739 device provides high current driving control. The driver for the high-side

MOSFET uses the BOOT pin for supply and the PHASE pin for return. The driver for the

low-side MOSFET uses the VCCDR voltage for supply and the GND pin for return. An

embedded linear regulator is provided to guarantee proper supply voltage to the high-side

and low-side drivers which are optimized for such regulated voltage to guarantee minimum

deadtimes and higher efficiency. The LDO can be bypassed, in case of low 5 V VCC

applications, by properly shorting VCCDR to VCC. In this case the LDO automatically

disables reducing consumptions.

The embedded driver embodies an anti shoot-through and adaptive deadtime control to

minimize the low-side body diode conduction time maintaining good efficiency and saving

the use of Schottky diodes: when the high-side MOSFET turns off, the voltage on its source

begins to fall; when the voltage reaches about 2 V, the low-side MOSFET gate drive voltage

is suddenly applied. When the low-side MOSFET turns off, the voltage at the LGATE pin is

sensed. When it drops below about 1 V, the high-side MOSFET gate drive voltage is

suddenly applied. If the current flowing in the inductor is negative, the source of the high-

side MOSFET never drops. To allow the low-side MOSFET to turn-on even in this case,

a watchdog controller is enabled: if the source of the high-side MOSFET doesn't drop, the

low-side MOSFET is switched on, so allowing the negative current of the inductor to

recirculate. This mechanism allows the system to regulate even if the current is negative.

8.1

Boot capacitor design

The bootstrap capacitor needs to be designed in order to show a negligible discharge due to

the high-side MOSFET turn-on. In fact, it must give a stable voltage supply to the high-side

driver during the MOSFET turn-on, also minimizing the power dissipated by the embedded

boot switch. Figure 8 gives some guidelines on how to select the capacitance value for the

bootstrap according to the desired discharge and depending on the selected MOSFET.

Figure 8. Bootstrap capacitor design