High Current embedded drivers

L6738A

20/32

Doc ID 18134 Rev 2

8

High Current embedded drivers

The L6738A 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 pin for supply and the GND pin for return.

The embedded driver embodies an anti-shoot-through and adaptive dead-time 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 diode. 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.

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

diode pin.

Figure 8.

Bootstrap capacitor design

8.2

Power dissipation

It is important to consider the power that the device is going to dissipate in driving the exter-

nal MOSFETs in order to avoid surpassing the maximum junction operative temperature.

0.0

0.5

1.0

1.5

2.0

2.5

010

20

3040506070

8090

100

High -Side MOSFET Gate Charge [nC]

Cboot = 47nF

Cboot = 100nF

Cboot = 220nF

Cboot = 330nF

Cboot = 470nF

0

500

1000

1500

2000

2500

0.0

0.2

0.4

0.6

0.8

1.0

Boot Cap Delta Voltage [V]

Qg = 10nC

Qg = 25nC

Qg = 50nC

Qg = 100nC