ADP1828YRQZ-R7 datasheet(27/32 Pages) AD

27 / 32 page

ADP1828

Rev. 0 | Page 27 of 32

THERMAL CONSIDERATIONS

The current required to drive the external MOSFETs comprises

the vast majority of the power dissipation of the ADP1828. The

on-chip LDO regulates down to 5 V, and this 5 V supplies the

drivers. The full gate drive current passes through the LDO and

is then dissipated in the gate drivers. The power dissipated in

the gate drivers on the ADP1828 is

)

(

(52)

where:

VIN is the voltage applied to IN.

fSW is the switching frequency.

Q numbers are the total gate charge specifications from the

selected MOSFET data sheets.

The power dissipation heats up the ADP1828. As the switching

frequency, the input voltage, and the MOSFET size increase, the

power dissipation on the ADP1828 increases. Care must be taken

not to exceed the maximum junction temperature. To calculate

the junction temperature from the ambient temperature and

power dissipation, use the following formula:

(53)

The thermal resistance (θJA) of the package is 83°C/W depending

on board layout, and the maximum specified junction temperature

is 125°C, which means that at maximum ambient temperature

of 85°C without airflow, the maximum dissipation allowed is

about 1 W.

A thermal shutdown protection circuit on the ADP1828 shuts

off the LDO and the controllers if the die temperature exceeds

approximately 145°C, but this is a gross fault protection only

and should not be depended on for system reliability.

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