UB2012XG-S08-R datasheet(14/17 Pages) UTC

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UB2012

Preliminary

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

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www.unisonic.com.tw

QW-R121-018 .b

APPLICATION INFORMATION(Cont.)

SELECTING AN EXTERNAL PASS-TRANSISTOR (Cont.)

P-CHANNEL MOSFET:

Selection steps for a P-channel MOSFET: Example: VI = 5.5 V, I(REG) = 500mA, 4.2-V single-cell Li-Ion (UTC

UB2012

C). VI is the input voltage to the charger and I (REG) is the desired charge current (see Figure 4).

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 VD is the forward voltage drop across the reverse-blocking diode (if one is used), and VCS is the voltage

drop across the current sense resistor.

PD = (VI-VD-V(CS)-V(BAT))×I(REG)

(10)

PD = (5.5-0.4-0.1-3)×0.5A

PD = 1W

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)

)

(11)

θJA =

)

150

(

θJA = 110°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 SOP-8 package, for instance, has typically a theta of 70°C/W.

3. Select a drain-source voltage, V(DS), rating greater than the maximum input voltage. A 12 V device will be

adequate in this example.

4. Select a device that has at least 50% higher drain current (ID) rating than the desired charge current I(REG).

5. Verify that the available drive is large enough to supply the desired charge current.

V(GS) = (VD+V(CS)+VOL(CC))-VI

(12)

V(GS) = ( 0.4+0.1+1.5)-5.5

V(GS) = -3.5

Where V(GS) is the gate-to-source voltage, VD is the forward voltage drop across the reverse-blocking diode (if

one is used), and VCS is the voltage drop across the current sense resistor, and VOL(CC) is the CC pin output low

voltage specification for the UTC UB2012.

Select a MOSFET with gate threshold voltage, V(GSTH), rating less than the calculated V(GS).

Now choose a P-channel MOSFET transistor that is rated for VDS≤-15 V, θJA ≤110°C /W, ID ≥1 A, V(GSTH)≥-3.5

V and in a SOP package.

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