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ADP5041

Data Sheet

Rev. B | Page 34 of 40

Watchdog Software Considerations

In implementing the watchdog strobe code of the micro-

processor, quickly switching WDI low to high and then high to

low (minimizing WDI high time) is desirable for current

consumption reasons. However, a more effective way of using

the watchdog function can be considered.

A low-to-high-to-low WDI pulse within a given subroutine

prevents the watchdog from timing out. However, if the sub-

routine is held in an infinite loop, the watchdog cannot detect

this because the subroutine continues to toggle WDI.

A more effective coding scheme for detecting this error involves

using a slightly longer watchdog timeout. In the program that

calls the subroutine, WDI is set high. The subroutine sets WDI

low when it is called. If the program executes without error, WDI

is toggled high and low with every loop of the program. If the

subroutine enters an infinite loop, WDI is kept low, the watchdog

times out, and the microprocessor is reset (see Figure 112).

START

SET WDI

HIGH

PROGRAM

CODE

SUBROUTINE

SET WDI

LOW

RETURN

INFINITE LOOP:

WATCHDOG

TIMES OUT

RESET

Figure 112. Watchdog Flow Diagram

POWER DISSIPATION/THERMAL CONSIDERATIONS

The ADP5041 is a highly efficient micropower management

unit (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 with maximum loading

conditions, the junction temperature can reach the maximum

allowable operating limit (125°C).

When the junction temperature exceeds 150°C, the ADP5041

turns off all the regulators, allowing the device to cool down.

Once the die temperature falls below 135°C, the ADP5041

resumes normal operation.

This section provides guidelines to calculate the power dissi-

pated in the device and to make sure the ADP5041 operates

below the maximum allowable junction temperature.

The efficiency for each regulator on the ADP5041 is given by

100%

OUT

 

(1)

where:

η is 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)

The power dissipation of the supervisory function is small and

negligible.

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. The measurements should be

performed 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

ADP5041

buck regulator.

A second method to estimate the power dissipation uses the

efficiency curves provided for the buck regulator, wheras the

power lost on a LDO is 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 LDOs 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 the losses in the LDOs 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 the ADP5041 buck regulator.

PL is the inductor power losses.

The inductor losses are external to the device and they do not

have any effect on the die temperature.

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