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參數(shù)資料
| 型號： | UB2012AG-S08-R |
| 廠商： | UNISONIC TECHNOLOGIES CO LTD |
| 元件分類： | 電源管理 |
| 英文描述： | POWER SUPPLY SUPPORT CKT, PDSO8 |
| 封裝： | HALOGEN FREE, SOP-8 |
| 文件頁數(shù)： | 5/17頁 |
| 文件大小： | 315K |
| 代理商： | UB2012AG-S08-R |
UB2012
Preliminary
LINEAR INTEGRATED CIRCUIT
UNISONICTECHNOLOGIESCO.,LTD
13 of 17
www.unisonic.com.tw
QW-R121-018 .b
APPLICATION INFORMATION(Cont.)
SELECTING AN EXTERNAL PASS-TRANSISTOR
The UTC UB2012 is designed to work with both PNP transistor and P-channel MOSFET. The device should be
chosen to handle the required power dissipation, given the circuit parameters, PCB layout and heat sink
configuration. The following examples illustrate the design process for either device:
PNP TRANSISTOR:
Selection steps for a PNP bipolar transistor: Example: VI = 4.5 V, I(REG) = 1 A, 4.2-V single-cell Li-Ion (UTC
UB2012C
). VI is the input voltage to the charger and I (REG) is the desired charge current (see Figure 1).
1. Determine the maximum power dissipation, PD, in the transistor.
The worst case power dissipation happens when the cell voltage, V(BAT), is at its lowest (typically 3 V at the
beginning of current regulation phase) and VI is at its maximum.
Where VCS is the voltage drop across the current sense resistor.
PD = (VI-V(CS)-V(BAT))×I(REG)
(7)
PD = (4.5-0.1-3)×1A
PD = 1.4W
2.
Determine the package size needed in order to keep the junction temperature below the manufacturer’s
recommended value, TJMAX. Calculate the total theta, θ (°C/W), needed.
θJA =
D
A(MAX)
MAX(J)
P
)
T
-
(T
(8)
θJA =
4
.
1
)
40
-
150
(
θJA = 78°C/W
Now choose a device package with a theta at least 10% below this value to account for additional thetas other
than the device. A SOT223 package, for instance, has typically a theta of 60°C/W.
3. Select a collector-emitter voltage, V(CE), rating greater than the maximum input voltage. A 15-V device will be
adequate in this example.
4. Select a device that has at least 50% higher drain current IC rating than the desired charge current I(REG).
5. Using the following equation calculate the minimum beta (β or hFE) needed:
βMIN =ICMAX / IB
(9)
βMIN =1 / 0.035
βMIN =28
Where IMAX(C)) is the maximum collector current (in this case same as I (REG)), and IB is the base current (chosen
to be 35 mA in this example).
Now choose a PNP transistor that is rated for V(CE) ≥15 V, θJA ≤ 78°C /W, IC ≥ 1.5 A, βMIN ≥ 28 and that is in a
SOT223 package.
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