BQ2057TSTR datasheet(15/24 Pages) TI

15 / 24 page

bq2057, bq2057C

bq2057T, bq2057W

SLUS025F − MAY 2001 − REVISED JULY 2002

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APPLICATION INFORMATION

selecting an external pass-transistor

The bq2057 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 (bq2057C).

VI is the input voltage to the charger and I(REG) is the desired charge current (see Figure 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 − VCS – V (BAT)) × IREG

PD = (4.5 – 0.1 − 3) × 1 A

PD= 1.4 W

Determine the package size needed in order to keep the junction temperature below the manufacturer’s

recommended value, T(J)max. Calculate the total theta, θ(°C/W), needed.

JC +

(J)max *

A(max)

JC +

(150–40)

1.4

JC +

°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.

Select a collector-emitter voltage, V(CE), rating greater than the maximum input voltage. A 15-V device will

be adequate in this example.

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

Using the following equation, calculate the minimum beta (

β or hFE) needed:

min +

CMAX

min +

0.035

min +

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).

NOTE:

The beta of a transistor drops off by a factor of 3 over temperature and also drops off with load.

Therefore, note the beta of device at I(REG) and the minimum ambient temperature when choosing

the device. This beta should be larger than the minimum required beta.

Now choose a PNP transistor that is rated for V(CE) ≥15 V, θJC ≤ 78°C/W, IC ≥ 1.5 A, βmin ≥ 28 and that is in a

SOT223 package.

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BQ2057TSTR Datasheet(PDF) 15 Page - Texas Instruments

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