Datasheet

ADP1828
Rev. C | Page 28 of 36
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
)(
DLDH
SW
IND
QQfVP += (52)
where:
V
IN
is the voltage applied to IN.
f
SW
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:
JA
D
A
J
θPTT
+
=
(53)
The thermal resistance (θ
JA
) of the package is 83°C/W depending
on board layout, and the maximum specified junction temperature
ans 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.
is 125°C, which me