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ADP5024CP-EVALZ
ADP5024
Data Sheet
Rev. E | Page 24 of 28
POWER DISSIPATION AND THERMAL CONSIDERATIONS
The
unit (microPMU), and, in most cases, the power dissipated in
the device is not a concern. However, if the device operates at
high ambient temperatures and maximum loading condition,
the junction temperature can reach the maximum allowable
operating limit (125°C).
When the temperature exceeds 150°C, the
When the temperature exceeds 150°C, the
all of the regulators allowing the device to cool down. When the
die temperature falls below 130°C, the
operation.
This section provides guidelines to calculate the power dissi-
This section provides guidelines to calculate the power dissi-
pated in the device and ensure that the
below the maximum allowable junction temperature.
The efficiency for each regulator on the
The efficiency for each regulator on the
100%
×
=
IN
OUT
P
P
η
(1)
where:
η is the efficiency.
P
P
IN
is the input power.
P
OUT
is the output power.
Power loss is given by
P
LOSS
= P
IN
− P
OUT
(2a)
or
P
LOSS
= P
OUT
(1− η)/η
(2b)
Power dissipation can be calculated in several ways. The most
intuitive and practical is to measure the power dissipated at the
intuitive and practical is to measure the power dissipated at the
input and at all of the outputs. Perform the measurements at the
worst-case conditions (voltages, currents, and temperature). The
difference between input and output power is dissipated in the
device and the inductor. Use Equation 4 to derive the power lost
in the inductor, and from this result use Equation 3 to calculate
in the inductor, and from this result use Equation 3 to calculate
the power dissipation in the
A second method to estimate the power dissipation uses the effi-
ciency curves provided for the buck regulator, and the power
lost on the LDO can be calculated using Equation 12. When
the buck efficiency is known, use Equation 2b to derive the
total power lost in the buck regulator and inductor, use Equa-
total power lost in the buck regulator and inductor, use Equa-
tion 4 to derive the power lost in the inductor, and then calculate
the power dissipation in the buck converter using Equation 3.
Add the power dissipated in the buck and in the LDO to find the
total dissipated power.
Note that the buck efficiency curves are typical values and may
not be provided for all possible combinations of V
Note that the buck efficiency curves are typical values and may
not be provided for all possible combinations of V
IN
, V
OUT
, and
I
OUT
. To account for these variations, it is necessary to include a
safety margin when calculating the power dissipated in the buck.
A third way to estimate the power dissipation is analytical and
A third way to estimate the power dissipation is analytical and
involves modeling the losses in the buck circuit provided by
Equation 8 to Equation 11 and calculating the losses in the LDO
provided by Equation 12.
BUCK REGULATOR POWER DISSIPATION
The power loss of the buck regulator is approximated by
P
LOSS
= P
DBUCK
+ P
L
(3)
where:
P
DBUCK
is the power dissipation on one of the
regulators.
P
L
is the inductor power loss.
The inductor losses are external to the device and they do not
have any effect on the die temperature.
The inductor losses are estimated (without core losses) by
The inductor losses are estimated (without core losses) by
P
L
≈ I
OUT1(RMS)
2
× DCR
L
(4)
where:
DCR
L
is the inductor series resistance.
I
OUT1(RMS)
is the rms load current of the buck regulator.
12
+
1
)
(
r
I
I
OUT1
RMS
OUT1
×
=
(5)
where r is the normalized inductor ripple current.
r = V
OUT1
× (1 − D)/(I
OUT1
× L × f
SW
)
(6)
where:
L is the inductance.
L is the inductance.
f
SW
is the switching frequency.
D is the duty cycle.
D = V
OUT1
/V
IN1
(7)
The buck regulator power dissipation, P
DBUCK
, of the
includes the power switch conductive losses, the switch losses, and
the transition losses of each channel. There are other sources of
loss, but these are generally less significant at high output load
loss, but these are generally less significant at high output load
currents, where the thermal limit of the application is located.
Equation 8 captures the calculation that must be made to
estimate the power dissipation in the buck regulator.
P
DBUCK
= P
COND
+ P
SW
+ P
TRAN
(8)
The power switch conductive losses are due to the output current,
I
OUT1
, flowing through the P-MOSFET and the N-MOSFET
power switches that have internal resistance, RDS
ON-P
and
RDS
ON-N
. The amount of conductive power loss is found by
P
COND
= [RDS
ON-P
× D + RDS
ON-N
× (1 − D)] × I
OUT1(RMS)
2
(9)
where RDS
ON-P
is approximately 0.2 Ω, and RDS
ON-N
is approxi-
mately 0.16 Ω at a junction temperature of 25°C and V
IN1
= V
IN2
=
3.6 V. At V
IN1
= V
IN2
= 2.3 V, these values change to 0.31 Ω and
0.21 Ω, respectively, and at V
IN1
= V
IN2
= 5.5 V, the values are
0.16 Ω and 0.14 Ω, respectively.