L6728 datasheet(10/31 Pages) STMICROELECTRONICS

10 / 31 page

Driver section

L6728

10/31

Driver section

The integrated high-current drivers allow using different types of power MOSFET (also

multiple MOSFETs to reduce the equivalent RDS(on)), maintaining fast switching transition.

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

The driver for low-side MOSFET uses the VCC pin for supply and 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 saving the use of

Schottky diode:

●

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

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

●

to check low-side MOSFET turn off, 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 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 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.

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

maximum duty cycle limitations) can be chosen freely.

6.1

Power dissipation

L6728 embeds high current MOSFET drivers for both high side and low side MOSFETs: it is

then important to consider the power that the device is going to dissipate in driving them in

order to avoid overcoming the maximum junction operative temperature.

Two main terms contribute in the device power dissipation: bias power and drivers' power.

●

Device bias power (PDC) depends on the static consumption of the device through the

supply pins and it is simply quantifiable as follow (assuming to supply HS and LS

drivers with the same VCC of the device):

●

Drivers 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

the selected MOSFETs. It can be quantified considering that the total power PSW

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

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

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

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

MOSFETs results:

External gate resistors helps the device to dissipate the switching power since the same

power PSW will be shared between the internal driver impedance and the external resistor

resulting in a general cooling of the device.

BOOT

()

⋅

gHS

BOOT

gLS

⋅

()

⋅

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