Datasheet

ADP5023 Data Sheet
Rev. D | Page 22 of 28
POWER DISSIPATION AND THERMAL CONSIDERATIONS
The ADP5023 is a highly efficient µPMU, and, in most cases,
the power dissipated in the device is not a concern. However,
if the device operates at high ambient temperatures and maxi-
mum loading condition, the junction temperature can reach
the maximum allowable operating limit (125°C).
When the temperature exceeds 150°C, the ADP5023 turns off
all the regulators, allowing the device to cool down. When the
die temperature falls below 130°C, the ADP5023 resumes
normal operation.
This section provides guidelines to calculate the power dissi-
pated in the device and ensure that the ADP5023 operates
below the maximum allowable junction temperature.
The efficiency for each regulator on the ADP5023 is given by
100%×=
IN
OUT
P
P
η
(1)
where:
η is the efficiency.
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
input and all 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 use Equation 3 to calculate
the power dissipation in the ADP5023 buck converter.
A second method to estimate the power dissipation uses the
efficiency 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 Equation 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
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
involves modeling the losses in the buck circuit provided by
Equation 8 to Equation 11 and 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 ADP5023 buck
regulators.
P
L
is the inductor power losses.
The inductor losses are external to the device and do not have
any effect on the die temperature.
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
)
(
1
r
II
OUT1
RMS
OUT
×=
(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.
f
SW
is the switching frequency.
D is the duty cycle.
D = V
OUT1
/V
IN1
(7)
ADP5023 buck regulator power dissipation, P
DBUCK
, includes the
power switch conductive losses, the switch losses, and the transi-
tion losses of each channel. There are other sources of loss, but
these are generally less significant at high output load currents,
where the thermal limit of the application is. 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 25°C junction temperature and VIN1 = VIN2 =
3.6 V. At VIN1 = VIN2 = 2.3 V, these values change to 0.31 Ω and
0.21 Ω, respectively, and at VIN1 = VIN2 = 5.5 V, the values are
0.16 Ω and 0.14 Ω, respectively.