LTC4089/LTC4089-5
20
40895fc
Power Dissipation and High Temperature
Considerations
The die temperature of the LTC4089/LTC4089-5 must
be lower than the maximum rating of 110°C. This is
generally not a concern unless the ambient temperature
is above 85°C. The total power dissipated inside the
LTC4089/LTC4089-5 depends on many factors, including
input voltage (IN or HVIN), battery voltage, programmed
charge current, programmed input current limit, and load
current.
In general, if the LTC4089/LTC4089-5 is being powered from
IN the power dissipation can be calculated as follows:
PVV I VV I
D IN BAT BAT IN OUT OUT
=− +−()•()•
where P
D
is the power dissipated, I
BAT
is the battery
charge current, and I
OUT
is the application load current.
For a typical application, an example of this calculation
would be:
PVV AV VA
mW
D
=− +− =(.)•.(.)•.53704 547501
545
This example assumes V
IN
= 5V, V
OUT
= 4.75V, V
BAT
= 3.7V,
I
BAT
= 400mA, and I
OUT
= 100mA resulting in slightly more
than 0.5W total dissipation.
If the LTC4089 is being powered from HVIN, the power
dissipation can be estimated by calculating the regulator
power loss from an effi ciency measurement, and subtract-
ing the catch diode loss.
PVIIV
V
V
D HVOUT BAT OUT D
HVOUT
H
=− + −
−
( )•( •( )) •1
1
η
VVIN
BAT OUT BAT
II VI
⎛
⎝
⎜
⎞
⎠
⎟
++•( ) . •03
where is the effi ciency of the high voltage regulator
and V
D
is the forward voltage of the catch diode at I =
I
BAT
+ I
OUT
. The fi rst term corresponds to the power lost
in converting V
HVIN
to V
HVOUT
, the second term subtracts
the catch diode loss, and the third term is the power
dissipated in the battery charger. For a typical application,
an example of this calculation would be:
PVAAV
V
V
D
=− +
[]
−
−
⎛
⎝
( . )• •( . . ) . •1087 4 07 03 04
1
4
12
⎜⎜
⎞
⎠
⎟
++ =•( . . ) . • .07 03 03 07 463AA VA mW
This example assumes 87% effi ciency, V
HVIN
= 12V, V
BAT
=
3.7V (V
HVOUT
is about 4V), I
BAT
= 700mA, I
OUT
= 300mA
resulting in less than 0.5W total dissipation.
If the LTC4089-5 is being powered from HVIN, the power
dissipation can be estimated by calculating the regulator
power loss from an effi ciency measurement and subtract-
ing the catch diode loss.
PVII
V
V
V
D BAT OUT
D
HVIN
=− +
−−
⎛
⎝
⎜
⎞
⎠
( )•( •( ))
•
15
1
5
η
⎟⎟
+
+−
•( )
()•
II
VV I
BAT OUT
BAT BAT
5
The difference between this equation and the LTC4089 is
the last term which represents the power dissipation in
the battery charger. For a typical application, an example
of this calculation would be:
PVAA
V
V
V
D
=− +
−−
(.)•(•(. .))
.•( )
1 0 87 5 0 7 0 3
04 1
5
12
••( . . )
(.)•.,
07 03
537071327
AA
VVA mW
+
+− =
Like the LTC4089 example, this example assumes 87%
effi ciency, V
HVIN
= 12V, V
BAT
= 3.7V, I
BAT
= 700mA, I
OUT
=
300mA resulting in 1.3W total dissipation.
To prevent power dissipation of this magnitude from
causing high die temperature, it is important to solder the
exposed backside of the package to a ground plane. This
ground should be tied to other copper layers below with
thermal vias; these layers will spread the heat dissipated
by the LTC4089. Additional vias should be placed near the
catch diodes. Adding more copper to the top and bottom
layers, and tying this copper to the internal planes with
vias, can reduce thermal resistance further. With these
steps, the thermal resistance from die (i.e., junction) to
ambient can be reduced to
JA
= 40°C/W.
APPLICATIONS INFORMATION
