where V
SOURCE
= (V
OV
x 0.04) + V
2
in volts, and C
rss
is the MOSFET’s reverse transfer capacitance in pF.
Any external capacitance between GATE and PGND
adds up to C
rss
.
Power Dissipation/Junction Temperature
During normal operation, the MAX15009/MAX15011
have two main sources of internal power dissipation:
the LDO and the switched output.
The internal power dissipation due to the LDO can be
calculated as:
where V
IN
is the LDO input supply voltage in volts,
V
OUT_LDO
is the output voltage of the LDO in volts,
I
OUT_LDO
is the LDO total load current in mA, and
I
OUT_SW
is the switch load current in mA.
Calculate the power dissipation due to the switch as:
where ΔV
SW
is the switch dropout voltage in volts for
the given I
OUT_SW
current in mA.
The total power dissipation P
DISS
in mW as:
P
DISS
= P
LDO
+ P
SW
For prolonged exposure to overvoltage events, use the
V
IN
voltage expected during overvoltage conditions.
Under these circumstances the corresponding internal
power dissipation contribution, P
OVP
, calculated in the
previous section should also be included in the total
power dissipation, P
DISS
.
For a given ambient temperature, T
A
, calculate the
junction temperature, T
J
, as follows:
T
J
= T
A
+ P
DISS
x θ
JA
where T
J
and T
A
are in °C and θ
JA
is the junction-to-
ambient thermal resistance in °C/W as listed in the
Absolute Maximum Ratings
section.
The junction temperature should never exceed +150°C
during normal operation.
Thermal Protection
When the junction temperature exceeds T
J
= +160°C,
the MAX15009/MAX15011 shut down to allow the
device to cool. When the junction temperature drops to
+140°C, the thermal sensor turns all enabled blocks on
again, resulting in a cycled output during continuous
thermal-overload conditions. Thermal protection pro-
tects the MAX15009/MAX15011 from excessive power
dissipation. For continuous operation, do not exceed
the absolute maximum junction temperature rating of
+150°C.
