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參數(shù)資料
| 型號： | LTC4090EDJC#TRPBR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 電源管理 |
| 英文描述： | 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO22 |
| 封裝： | 6 X 3 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, MO-229WXXX, DFN-22 |
| 文件頁數(shù)： | 17/28頁 |
| 文件大?。?/td> | 392K |
| 代理商： | LTC4090EDJC#TRPBR |
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LTC4090
24
4090f
From the Vishay Curve 2 R-T characteristics, rHOTis 0.5758
at 40°C. Using the above equation, RNOM should be set
to 14.0k. With this value of RNOM, the cold trip point is
about -7°C. Notice that the span is now 47°C rather than
the previous 50°C. This is due to the increase in “tem-
perature gain” of the thermistor as absolute temperature
decreases.
The upper and lower temperature trip points can be inde-
pendently programmed by using an additional bias resistor
as shown in Figure 9. The following formulas can be used
to compute the values of RNOM and R1:
R
rr
R
RR
r
NOM
COLD
HOT
C
NOM
HOT
=
–
.
.
–
2 815
1 0 409
25
R C
25
For example, to set the trip points to -5°C and 55°C with
a Vishay Curve 2 thermistor choose
Rk
k
NOM ==
3 532 0 3467
2 815 0 409
10
13 2
.– .
.
the nearest 1% value is 13.3k.
R1 = 0.409 13.3k – 0.3467 10k = 1.97k
the nearest 1% value is 1.96k. The nal solution is shown
in Figure 9 and results in an upper trip point of 55°C and
a lower trip point of -5°C.
Power Dissipation and High Temperature
Considerations
The die temperature of the LTC4090 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 LTC4090 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 LTC4090 is being powered from IN the
power dissipation can be calculated as follows:
PD = (VIN – VBAT) IBAT + (VIN – VOUT) IOUT
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:
PD = (5V – 3.7V) 0.4A + (5V – 4.75V) 0.1A
= 545mW
This examples 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 LTC4090 is being powered from HVIN, the power
dissipation can be estimated by calculating the regulator
power loss from an efciency measurement, and subtract-
ing the catch diode loss.
PV
I
V
D
HVOUT
BAT
OUT
D
HVOUT
=
+
()
(
)
1
η
V
II
V I
HVIN
BAT
OUT
BAT
++
()
)
.
03
where η is the efciency of the high voltage regulator and
VD is the forward voltage of the catch diode at I = IBAT
+ IOUT. The rst 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
V
D =
+
[]
(.
)
(
.
)
.
10 87
4
1
0 6
04
1
4
12
+
()+=
.
.
10 6
0 3
1
0 7
AA
V
A
W
This example assumes 87% efciency, VHVIN = 12V, VBAT
= 3.7V (VHVOUT is about 4V), IBAT = 1A, IOUT = 600mA
resulting in about 0.7W total dissipation.
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 LTC4090. Additional
vias should be placed near the catch diode. Adding more
copper to the top and bottom layers and tying this cop-
per 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 θJA
= 40°C/W.
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