Datasheet

Data Sheet ADP5065
Rev. D | Page 35 of 40
POWER DISSIPATION AND THERMAL CONSIDERATIONS
The ADP5065 is a highly efficient USB compliant charger.
However, if the device operates at high ambient temperatures
and maximum current charging and loading conditions, the
junction temperature can reach the maximum allowable
operating limit (125°C).
When the temperature exceeds 140°C, the ADP5065 turns off
allowing the device to cool down. When the die temperature
falls below 110°C and the TSD 140°C fault bit in Register 0x0D
is cleared by an I
2
C write, the ADP5065 resumes normal
operation.
This section provides guidelines to calculate the power dissi-
pated in the device and ensure that the ADP5065 operates
below the maximum allowable junction temperature.
The output power of the ADP5065 charger is gived by
P
OUT
= V
ISO_S
× I
LOAD
+ V
ISO_B
× I
CHG
(1)
where:
P
OUT
is the total output power to the system and battery.
V
ISO_S
is the ISO_Sx pin voltage.
I
LOAD
is the load current from ISO_Sx node.
V
ISO_B
is the battery voltage.
I
CHG
is the charge current.
The efficiency of the ADP5065 is given by
100%×=
IN
OUT
P
P
η
(2)
where:
η is the efficiency.
P
IN
is the input power.
Power loss is given by
P
LOSS
= P
IN
− P
OUT
(3a)
or
P
LOSS
= P
OUT
(1− η)/η (3b)
Power dissipation can be calculated in several ways. The most
intuitive and practical is to measure the power dissipated at the
input and both outputs (ISO_Sx and ISO_Bx). Perform the mea-
surements 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 5
to derive the power lost in the inductor and, from this, use
Equation 4 to calculate the power dissipation in the ADP5065
charger.
A second method to estimate the power dissipation uses the
system voltage and charging efficiency curves provided for the
ADP5065. When the efficiency is known, use Equation 3b to
derive the total power lost in the dc-to-dc converter, isolation
FET and inductor; use Equation 5 to derive the power lost in
the inductor, and then calculate the power dissipation in the
buck converter using Equation 4.
Note that the ADP5065 efficiency curves are typical values and
may not be provided for all possible combinations of V
IN
, V
OUT
,
and I
OU T.
To account for these variations, it is necessary to
include a safety margin when calculating the power dissipated in
the charger.
CHARGER POWER DISSIPATION
The power loss of the step-down charger is approximated by
P
LOSS
= P
DCHG
+ P
L
(4)
where:
P
DCHG
is the power dissipation of the ADP5065 charger.
P
L
is the inductor power losses.
The inductor losses are external to the device, and they do not
have any effect on the die temperature. Equation 5 estimates the
inductor losses without core losses. Some inductor manufacturers
provide web tools to estimate power inductor core losses based
on inductor type, switching frequency, and ripple current. At a
switching frequency of 3 MHz, the core losses can add inductor
losses significantly.
P
L
≈ I
OUT(RMS)
2
× DCR
L
(5)
where:
DCR
L
is the inductor series resistance.
I
OUT(RMS)
is the summary of rms load current and charging
current (I
LOAD(RMS)
+ I
CHG
).
12
+1
)(
r
II
OUT
RMSOUT
×=
(6)
where r is the normalized inductor ripple current.
r = V
OUT
× (1 − D)/(I
OUT
× L × f
SW
) (7)
where:
L is the inductance.
f
SW
is the switching frequency.
D is the duty cycle.
D = V
OUT
/V
IN
(8)