Datasheet

Data Sheet ADP5062
Rev. B | Page 39 of 44
POWER DISSIPATION AND THERMAL CONSIDERATIONS
CHARGER POWER DISSIPATION
When the ADP5062 charger operates at high ambient tempera-
tures and at maximum current charging and loading conditions,
the junction temperature can reach the maximum allowable
operating limit of 125°C.
When the junction temperature exceeds 140°C, the ADP5062
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 ADP5062 resumes
normal operation.
This section provides guidelines to calculate the power dissi-
pated in the device to ensure that the ADP5062 operates below
the maximum allowable junction temperature.
To determine the available output current in different operating
modes under various operating conditions, use the following
equations:
P
D
= P
LDOFET
+ P
ISOFET
(1)
where:
P
LDOFET
is the power dissipated in the input LDO FET.
P
ISOFET
is the power dissipated in the battery isolation FET.
Calculate the power dissipation in the LDO FET and the battery
isolation FET using Equation 2 and Equation 3.
P
LDOFET
= (V
IN
– V
ISO_Sx
) × (I
CHG
+ I
LOAD
) (2)
P
ISOFET
= (V
ISO_Sx
– V
ISO_Bx
) × I
CHG
(3)
where:
V
IN
is the input voltage at the VINx pins.
V
ISO_Sx
is the system voltage at the ISO_Sx pins.
V
ISO_Bx
is the battery voltage at the ISO_Bx pins.
I
CHG
is the battery charge current.
I
LOAD
is the system load current from the ISO_Sx pins.
LDO Mode
The system regulation voltage is user-programmable from 4.3 V
to 5.0 V. In LDO mode (charging disabled, EN_CHG = low),
calculation of the total power dissipation is simplified, assuming
that all current is drawn from the VINx pins and the battery is
not shared with ISO_Sx.
P
D
= (V
IN
– V
ISO_Sx
) × I
LOAD
Charging Mode
In charging mode, the voltage at the ISO_Sx pins depends on
the battery level. When the battery voltage is lower than V
ISO_SFC
(typically 3.8 V), the voltage drop over the battery isolation FET
is higher and the power dissipation must be calculated using
Equation 3. When the battery voltage level reaches V
ISO_SFC
, the
power dissipation can be calculated using Equation 4.
P
ISOFET
= R
DSON
_
ISO
× I
CHG
(4)
where:
R
DSON_ISO
is the on resistance of the battery isolation FET
(typically 110 mΩ during charging).
I
CHG
is the battery charge current.
The thermal control loop of the ADP5062 automatically limits
the charge current to maintain a die temperature below T
LIM
(typically 115°C).
The most intuitive and practical way to calculate the power
dissipation in the ADP5062 device is to measure the power
dissipated at the input and all of the outputs. Perform the
measurements at the worst case conditions (voltages, currents,
and temperature). The difference between input and output
power is the power that is dissipated in the device.
JUNCTION TEMPERATURE
In cases where the board temperature, T
A
, is known, the
thermal resistance parameter, θ
JA
, can be used to estimate the
junction temperature rise. T
J
is calculated from T
A
and P
D
using
the formula
T
J
= T
A
+ (P
D
× θ
JA
) (5)
The typical θ
JA
value for the 20-lead LFCSP is 35.6°C/W (see
Table 5). A very important factor to consider is that θ
JA
is based
on a 4-layer, 4 in × 3 in, 2.5 oz. copper board as per JEDEC
standard, and real-world applications may use different sizes
and layers. It is important to maximize the copper to remove the
heat from the device. Copper exposed to air dissipates heat
better than copper used in the inner layers.
If the case temperature can be measured, the junction
temperature is calculated by
T
J
= T
C
+ (P
D
× θ
JC
) (6)
where T
C
is the case temperature and θ
JC
is the junction-to-case
thermal resistance provided in Table 5.
The reliable operation of the charger can be achieved only if the
estimated die junction temperature of the ADP5062 (Equation 5)
is less than 125°C. Reliability and mean time between failures
(MTBF) are greatly affected by increasing the junction temperature.
Additional information about product reliability can be found in
the ADI Reliability Handbook located at the following URL:
www.analog.com/reliability_handbook.