ADP5041 Analog Devices, ADP5041 Datasheet - Page 34

• Manufacturer Part Number: ADP5041
• Description: Micro PMU with 1.2 A Buck, Two 300 mA LDOs, Supervisory, Watchdog and Manual Reset
• Manufacturer: Analog Devices

ADP5041
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).
POWER DISSIPATION/THERMAL CONSIDERATIONS
The
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
turns off all the regulators, allowing the device to cool down.
Once the die temperature falls below 135°C, the
resumes normal operation.
This section provides guidelines to calculate the power dissi-
pated in the device and to make sure the
below the maximum allowable junction temperature.
ADP5041
SUBROUTINE
PROGRAM
SET WDI
RETURN
SET WDI
is a highly efficient micropower management
START
CODE
HIGH
LOW
Figure 112. Watchdog Flow Diagram
INFINITE LOOP:
WATCHDOG
ADP5041
TIMES OUT
RESET
ADP5041
operates
ADP5041
Rev. 0 | Page 34 of 40
The efficiency for each regulator on the
where:
η is efficiency.
P
P
Power loss is given by
or
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
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 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 the losses in the LDOs provided
by Equation 12.
Buck Regulator Power Dissipation
The power loss of the buck regulator is approximated by
where:
P
P
The inductor losses are external to the device and they do not
have any effect on the die temperature.
OUT
IN
OUT
DBUCK
L
is the inductor power losses.
is the input power.
. To account for these variations, it is necessary to include a
P
P
P
is the output power.
η
LOSS
LOSS
LOSS
is the power dissipation on the
=
P
= P
= P
buck regulator.
= P
P
OUT
IN
IN
OUT
DBUCK
×
− P
100%
(1-η)/η
+ P
OUT
L
ADP5041
ADP5041
Data Sheet
buck regulator.
is given by
IN
, V
OUT
, and
(2b)
(2a)
(1)
(3)