ADP5024 Analog Devices, ADP5024 Datasheet - Page 24

• Manufacturer Part Number: ADP5024
• Description: Dual 3 MHz, 1200 mA Buck Regulators with One 300 mA LDO
• Manufacturer: Analog Devices

ADP5024
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
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-
pated in the device and ensure that the
below the maximum allowable junction temperature.
The efficiency for each regulator on the
where:
η is the efficiency.
P
P
Power loss is given by
or
Power dissipation can be calculated in several ways. The most
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
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-
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
I
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 calculating the losses in the LDO
provided by Equation 12.
OUT
IN
OUT
is the input power.
. To account for these variations, it is necessary to include a
ADP5024
P
P
η
is the output power.
LOSS
LOSS
=
P
= P
= P
P
OUT
IN
IN
OUT
×
is a highly efficient micropower management
− P
100%
(1− η)/η
OUT
ADP5024
ADP5024
buck converter.
ADP5024
ADP5024
ADP5024
resumes normal
operates
IN
is given by
, V
turns off
OUT
, and
(2b)
(2a)
Rev. A | Page 24 of 28
(1)
BUCK REGULATOR POWER DISSIPATION
The power loss of the buck regulator is approximated by
where:
P
regulators.
P
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
where:
DCR
I
where r is the normalized inductor ripple current.
where:
L is the inductance.
f
D is the duty cycle.
The buck regulator power dissipation, P
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
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.
The power switch conductive losses are due to the output current,
I
power switches that have internal resistance, RDS
RDS
where RDS
mately 0.16 Ω at a junction temperature of 125°C and V
3.6 V. At V
0.21 Ω, respectively, and at V
0.16 Ω and 0.14 Ω, respectively.
SW
OUT1(RMS)
OUT1
DBUCK
L
is the inductor power loss.
is the switching frequency.
, flowing through the P-MOSFET and the N-MOSFET
ON-N
P
P
r = V
D = V
P
P
L
I
LOSS
L
DBUCK
COND
is the inductor series resistance.
OUT1
is the power dissipation on one of the
≈ I
. The amount of conductive power loss is found by
is the rms load current of the buck regulator.
= P
OUT1
(
OUT1(RMS)
IN1
RMS
OUT1
= [RDS
ON-P
= P
= V
DBUCK
)
× (1 − D)/(I
/V
COND
is approximately 0.2 Ω, and RDS
=
IN2
IN1
I
2
ON-P
OUT1
+ P
× DCR
+ P
= 2.3 V, these values change to 0.31 Ω and
× D + RDS
L
SW
×
+ P
L
OUT1
1
+
IN1
TRAN
12
× L × f
r
= V
ON-N
IN2
SW
× (1 − D)] × I
= 5.5 V, the values are
)
DBUCK
ADP5024
, of the
Data Sheet
ON-N
ON-P
OUT1
is approxi-
ADP5024
IN1
and
2
buck
= V
IN2
(3)
(4)
(5)
(6)
(7)
(8)
(9)
=