ADP1823 Datasheet, PDF (24/32 Pages) Analog Devices – Dual, Interleaved, Step-Down DC-to-DC Controller with Tracking

ADP1823
THERMAL CONSIDERATIONS
The current required to drive the external MOSFETs comprises
the vast majority of the power dissipation of the ADP1823. The
on-chip LDO regulates down to 5 V, and this 5 V supplies the
drivers. Because the full gate drive current passes through the
LDO and then is dissipated in the gate drive, effectively the full
gate charge comes from the input voltage and dissipated on the
ADP1823 is
P_D = V_IN f _SW (Q_{DH 1} + Q_DL1 + Q_{DH 2} + Q_DL2 )
(68)
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 the ADP1823. As the switching
frequency, the input voltage, and the MOSFET size increase, the
power dissipation on the ADP1823 increases. Care must be taken
not to exceed the maximum junction temperature. To calculate
the junction temperature from the ambient temperature and
power dissipation:
T_J = T_A + P_D θ_JA
(69)
The thermal resistance, θJA, of the package is typically 40°C/W
depending on board layout, and the maximum specified
junction temperature is 125°C, which means that at maximum
ambient of 85°C without airflow, the maximum dissipation
allowed is about 1 W.
A thermal shutdown protection circuit on the ADP1823 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 relied upon for system reliability.
Rev. A | Page 24 of 32