Data Sheet

ADP5135

Rev. 0 | Page 21 of 24

POWER DISSIPATION AND THERMAL CONSIDERATIONS

The ADP5135 is a highly efficient micro PMU 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 conditions, the junction temperature

can reach the maximum allowable operating limit (125°C).

When the temperature exceeds 150°C, the ADP5135 turns off

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

die temperature falls below 130°C, the ADP5135 resumes

normal operation.

This section provides guidelines to calculate the power dissi-

pated in the device and to ensure that the ADP5135 operates

below the maximum allowable operating junction temperature.

The efficiency for each regulator on the ADP5135 is given by

100%





IN

OUT

P

P



(1)

where:

η is the efficiency.

Power loss is given by

(2a)

or

(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 3 to derive the power

lost in the inductor and, from this, use Equation 7 to calculate

the power dissipation in the ADP5135 buck converter.

A second method to estimate the power dissipation uses the

efficiency curves provided for the buck regulator. When the

buck efficiency is known, use Equation 2b to derive the total

power lost in the buck regulator and inductor, use Equation 3 to

derive the power lost in the inductor, and then calculate the

power dissipation in the buck converter using Equation 7. Add

the power dissipated in the three bucks to find the total

dissipated power.

Note that the buck efficiency curves are typical values and may

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.

BUCK REGULATOR POWER DISSIPATION

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

(3)

where:

12

+

1

)

(

r

I

I

OUT1

RMS

OUT1





(4)

where r is the normalized inductor ripple current.

(5)

where:

L is the inductance.

D is the duty cycle.

(6)

The power loss of the buck regulator is approximated by

(7)

where:

regulators.

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. Equation 8

captures the calculation that must be made to estimate the

power dissipation in the buck regulator.

(8)

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

loss is found by

(9)

respectively.