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LTC4089 Datasheet(PDF) 20 Page - Linear Technology

Part # LTC4089
Description  USB Power Manager with High Voltage Switching Charger
PDF  24 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC4089 Datasheet(HTML) 20 Page - Linear Technology

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LTC4089/LTC4089-5
20
40895fb
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 fol-
lows:
PV
V
I
V
V
I
D
IN
BAT
BAT
IN
OUT
OUT
=−
+
−
() •
(
) •
where PD is the power dissipated, IBAT is the battery
charge current, and IOUT is the application load current.
For a typical application, an example of this calculation
would be:
PV
V
A
V
V
A
mW
D =−
+
−
=
(.
) • .
(
.
) • .
53 7
0 4
5
4 75
0 1
545
This example assumes VIN = 5V, VOUT = 4.75V, VBAT =
3.7V, IBAT = 400mA, and IOUT = 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 efficiency measurement, and subtract-
ing the catch diode loss.
PV
I
I
V
V
V
D
HVOUT
BAT
OUT
D
HVOUT
H
=−
+
−
−
(
) •(
•(
))
•
1
1
η
V
VIN
BAT
OUT
BAT
II
V I
⎛
⎝⎜
⎞
⎠⎟
++
•(
)
.
•
03
where is the efficiency of the high voltage regulator and
VD is the forward voltage of the catch diode at I = IBAT
+ IOUT. The first term corresponds to the power lost in
converting VHVIN to VHVOUT, the second term subtracts
the catch diode loss, and the third term is the power dis-
sipated in the battery charger. For a typical application,
an example of this calculation would be:
PV
A
A
V
V
V
D =−
+
[]−
−
⎛
⎝
(
.
)•
•( .
.
)
.
•
10 87
4
0 7
0 3
0 4
1
4
12
⎜⎜
⎞
⎠⎟
++
=
•( .
.
)
.
• .
07
03
03
07
463
AA
V
A
mW
This example assumes 87% efficiency, VHVIN = 12V, VBAT
= 3.7V (VHVOUT is about 4V), IBAT = 700mA, IOUT = 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 efficiency measurement and subtract-
ing the catch diode loss.
PV I
I
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:
PV
A
A
V
V
V
D =−
+
−−
(.
) • (
• ( .
.
))
.• (
)
1 0 87
5
0 7
0 3
04
1
5
12
••( .
.
)
(.
) • .
,
07
03
53 7
0 71 327
AA
VV
A
mW
+
+−
=
Like the LTC4089 example, this example assumes 87%
efficiency, VHVIN = 12V, VBAT = 3.7V, IBAT = 700mA, IOUT
= 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.
APPLICATIO S I FOR ATIO



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