L6726ATR datasheet(11/35 Pages) STMICROELECTRONICS

11 / 35 page

L6726A

Driver section

Doc ID 12754 Rev 4

11/35

5.1

Power dissipation

L6726A 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, the voltage supply of the

driver 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:

where VBOOT - VPHASE is the voltage across the bootstrap capacitor.

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.

Figure 4.

Soft start (left) and disable (right)

BOOT

()

⋅

gHS

BOOT

PHASE

–

() Q

gLS

⋅

[]

⋅

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