UB2012BL-S08-R datasheet(13/17 Pages) UTC

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UB2012

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

13 of 17

www.unisonic.com.tw

QW-R121-018 .G



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 = 4.5V, I(REG) = 1A, 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 Fig. 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 3V 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

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 =

A(MAX)

MAX(J)

)

(8)

θJA =

)

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 SOT-223 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

SOT-223 package.

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