Driver section

L6728D

10/33

Doc ID 16498 Rev 1

6

Driver section

The integrated high-current drivers permit the use of different types of power MOSFETs

transition.

The driver for the high-side MOSFET uses the BOOT pin for supply and the PHASE pin for

return. The driver for low-side MOSFET uses the VCC pin for supply and the GND pin for

return.

The controller embodies an anti-shoot-through and adaptive dead-time control to minimize

low side body diode conduction time, maintaining good efficiency while eliminating the need

for a Schottky diode:

●

to check the high-side MOSFET turn-off, the PHASE pin is sensed. When the voltage

at the PHASE pin drops, the low-side MOSFET gate drive is suddenly applied

●

to check the low-side MOSFET turn-off, the LGATE pin is sensed. When the voltage at

LGATE has fallen, the high-side MOSFET gate drive is suddenly applied

If the current flowing in the inductor is negative, voltage on the PHASE pin 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 does not drop, the low side MOSFET is

switched on, thereby allowing the negative current of the inductor to recirculate. This

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

Power conversion input is flexible: 5 V, 12 V bus or any bus that allows the conversion (see

maximum duty cycle limitations) to be chosen freely.

6.1

Power dissipation

The L6728D embeds high current MOSFET drivers for both high side and low side

MOSFETs. It is therefore important to consider the power that the device is going to

dissipate in driving them, in order to avoid overcoming the maximum junction operating

temperature.

Two main factors contribute to device power dissipation: bias power and driver power.

●

supply pins, and is quantifiable as follows (assuming HS and LS drivers with the same

VCC of the device):

●

Driver power is the power needed by the driver to continuously switch on and off the

external MOSFETs. It is a function of the switching frequency and total gate charge of

dissipated to switch the MOSFETs (easily calculable) is dissipated by three main

factors: external gate resistance (when present), intrinsic MOSFET resistance and

intrinsic driver resistance. This last factor is the most important one to be determined to

calculate the device power dissipation. The total power dissipated to switch the

MOSFETs is:

P

DC

V

CC

I

CC

I

BOOT

+

()

⋅

=

P

SW

F

SW

Q

gHS

V

BOOT

Q

gLS

V

CC

⋅

+

⋅

()

⋅

=