DocID15086 Rev 4

13/21

PM8834

Design guidelines

21

DocID15086 Rev 4

13/21

PM8834

Design guidelines

21

5.2

Power dissipation

The PM8834 embeds two high-current low-side drivers that can be used to drive high

capacitive MOSFETs. This section estimates the power dissipated inside the device in

normal applications.

Two main terms contribute to the device’s power dissipation: bias power and the power of

the driver.



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

pins and it is simply obtained as follows:

Equation 1



The power of the driver is defined as 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 is dissipated by three main

factors: external gate resistance, intrinsic MOSFET resistance and intrinsic driver

resistance. This last term has to be determined to calculate the device power

dissipation. The total power dissipated by each section to switch an external MOSFETs

with gate charge QG is:

Equation 2

When designing an application based on the PM8834 it is recommended to take into

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

External gate resistors help 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.

Referring to Figure 6, a typical MOSFET driver can be represented by a push-pull output

stage with two different MOSFETs: P-DMOS to drive the external gate high and N-DMOS to

drive the external gate low (with their own RdsON: Rhi, Rlo). The external power MOSFET

can be represented in this case as a capacitance (CG) that stores the gate-charge (QG)

required by the external power MOSFET to reach the driving voltage (VCC). This

capacitance is charged and discharged at the driver switching frequency FSW.The total

power PSW is dissipated among the resistive components distributed along the driving path.

According to the external gate resistance and the power MOSFET intrinsic gate resistance,

the driver dissipates only a portion of PSW (per section) as follows:

Equation 3

PDC VCC ICC



=

PSW FSW QG VCC







=

PSW

1

2

--- CG VCC



2

FSW

Rhi

Rhi RGate Ri

++

----------------------------------------------

RIo

RIo RGate Ri

++

-----------------------------------------------

+













=