L6717ATR datasheet(48/57 Pages) STMICROELECTRONICS

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High current embedded drivers

L6717A

48/57

DocID024465 Rev 1

High current embedded drivers

L6717A provides high-current driving control for CORE and NB sections. The driver for the

high-side MOSFET use BOOTx pin for supply and PHASEx pin for return. The driver for the

low-side MOSFET use the VCCDR pin for supply and GND pin for return.

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

minimize low-side body diode conduction time maintaining good efficiency 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 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 high-side

MOSFET will never drop. 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.

10.1

Boot capacitor design

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 diode.

Figure 16 gives some guidelines on how to select the capacitance value for the

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

To prevent bootstrap capacitor to extra-charge as a consequence of large negative spikes,

an external series resistance R

BOOT

(in the range of few ohms) may be required in series to

BOOT pin.

Figure 16. Bootstrap capacitor design

0.0

0.5

1.0

1.5

2.0

2.5

2030

506070

100

High-Side MOSFET Gate Charge [nC]

Cboot = 47nF

Cboot = 100nF

Cboot = 220nF

Cboot = 330nF

Cboot = 470nF

500

1000

1500

2000

2500

0.00.2

0.40.6

0.81.0

Boot Cap Delta Voltage [V]

Qg = 10nC

Qg = 25nC

Qg = 50nC

Qg = 100nC

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