Device description and operation

L6747A

10/15

Doc ID 17126 Rev 1

The bootstrap capacitor value should be selected to obtain a negligible discharge due to the

turning on of the high-side MOSFET. It must provide a stable voltage supply to the high-side

driver during the MOSFET turn-on, and minimize the power dissipated by the embedded

boot diode. Figure 5 illustrates some guidelines on how to select the capacitance value for

the bootstrap according to the desired discharge, and the selected MOSFET.

To prevent the bootstrap capacitor from overcharging as a consequence of large negative

series with the BOOT pin.

Figure 5.

Bootstrap capacitance design

4.4

Power dissipation

The L6747A embeds high current 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 exceeding the maximum junction operating temperature.

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

●

supply pins and is easily quantifiable as follows:

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Driver power is the power needed by the driver to continuously switch the external

MOSFETs ON and OFF. 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

gate resistance (when present), intrinsic MOSFET resistance, and intrinsic driver

resistance. This last factor is the important one to be determined to calculate the device

power dissipation.

The total power dissipated to switch the MOSFETs is:

When designing an application based on the L6747A, it is recommended to take into

consideration the effect of external gate resistors on the power dissipated by the driver.

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

resulting in a general cooling of the device.

P

DC

V

CC

I

CC

V

PVCC

I

PVCC

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+

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P

SW

F

SW

Q

GHS

PVCC

⋅

Q

GLS

VCC

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+

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