L6718 datasheet(65/71 Pages) STMICROELECTRONICS

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L6718

Power dissipation and application details

13.1

High-current embedded drivers

The L6718 integrates 3 high-current drivers in control which can work for multi-rail and

single-rail. By reducing the number of external components, this integration optimizes the

cost and space of the motherboard solution.

The driver for the high-side MOSFET uses the BOOTx pins for supply and the PHASEx pins

for return. The driver for the low-side MOSFET uses the VCC12 pin for supply and the GND

exposed pad for return.

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

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

use of 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 does not drop, the

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

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

13.2

Boot diode and capacitor design

The bootstrap capacitor must 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.

To prevent the bootstrap capacitor from extra-charging as a consequence of large negative

spikes, an external series resistance RBOOT (in the range of few Ohm) may be required in

series to the BOOTx pins.

One external Schottky boot diode must be added to each channel, between the high-side

driver power supply and the BOOTx pins.

13.3

Device power dissipation

As the L6718 embeds three high-current MOSFET drivers for both high-side and low-side

MOSFETs, it is important to consider the power the device is going to dissipate in driving

them, in order to avoid the maximum junction operative temperature being exceeded.

The exposed pad (PGND pin) must be soldered to the PCB power ground plane through

several VIAs in order to facilitate the heat dissipation.

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

•

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

supply pins and it is simply quantifiable as follows (assuming HS and LS drivers are

supplied with the same VCC of the device):

L6718 Datasheet(PDF) 65 Page - STMicroelectronics

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