ADP5024ACPZ-R7 datasheet(24/28 Pages) AD

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ADP5024

Data Sheet

Rev. A | Page 24 of 28

POWER DISSIPATION AND THERMAL CONSIDERATIONS

The ADP5024 is a highly efficient micropower management

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 ADP5024 turns off

all of the regulators allowing the device to cool down. When the

die temperature falls below 130°C, the ADP5024 resumes normal

operation.

This section provides guidelines to calculate the power dissi-

pated in the device and ensure that the ADP5024 operates

below the maximum allowable junction temperature.

The efficiency for each regulator on the ADP5024 is given by

100%

OUT

(1)

where:

η is the efficiency.

PIN is the input power.

POUT is the output power.

Power loss is given by

PLOSS = PIN − POUT

(2a)

PLOSS = POUT (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 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 ADP5024 buck converter.

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 VIN, VOUT, and

IOUT. 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 calculating the losses in the LDO

provided by Equation 12.

BUCK REGULATOR POWER DISSIPATION

The power loss of the buck regulator is approximated by

PLOSS = PDBUCK + PL

(3)

where:

PDBUCK is the power dissipation on one of the ADP5024 buck

regulators.

PL 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

PL ≈ IOUT1(RMS)2 × DCRL

(4)

where:

DCRL is the inductor series resistance.

IOUT1(RMS) is the rms load current of the buck regulator.

)

(

OUT1

RMS

OUT1

(5)

where r is the normalized inductor ripple current.

r = VOUT1 × (1 − D)/(IOUT1 × L × fSW)

(6)

where:

L is the inductance.

fSW is the switching frequency.

D is the duty cycle.

D = VOUT1/VIN1

(7)

The buck regulator power dissipation, PDBUCK, of the ADP5024

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.

PDBUCK = PCOND + PSW + PTRAN

(8)

The power switch conductive losses are due to the output current,

IOUT1, flowing through the P-MOSFET and the N-MOSFET

power switches that have internal resistance, RDSON-P and

RDSON-N. The amount of conductive power loss is found by

PCOND = [RDSON-P × D + RDSON-N × (1 − D)] × IOUT12

(9)

where RDSON-P is approximately 0.2 Ω, and RDSON-N is approxi-

mately 0.16 Ω at a junction temperature of 125°C 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.

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