BQ2057CTSTRG4 Datasheet, PDF (15/30 Pages) Texas Instruments – ADVANCED LINEAR CHARGE MANAGEMENT IC FOR SINGLE-AND TWO-CELL LITHIUM-ION AND LITHIUM-POLYMER

bq2057, bq2057C
bq2057T, bq2057W
SLUS025F − MAY 2001 − REVISED JULY 2002
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: V_I = 4.5 V, I_(REG) = 1 A, 4.2-V single-cell Li-Ion (bq2057C).
V_I is the input voltage to the charger and I_(REG) is the desired charge current (see Figure 1).
1. Determine the maximum power dissipation, P_D, 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 V_I is at its maximum.
Where V_CS is the voltage drop across the current sense resistor.
P_D = (V_I − V_CS – V _(BAT)) × IREG
(7)
P_D = (4.5 – 0.1 − 3) × 1 A
P_D= 1.4 W
2. 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.
ǒ Ǔ T(J)max * TA(max)
(8)
θ_JC +
PD
θ_JC
+
(150–40)
1.4
θ_JC + 78°CńW
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 I_C rating than the desired charge current I_(REG).
5. Using the following equation, calculate the minimum beta (β or h_FE) needed:
b_min
+
ICMAX
IB
(9)
b_min
+
1
0.035
b_min + 28
where I_max(C)) is the maximum collector current (in this case same as I_(REG)), and I_B 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, θ_JC ≤ 78°C/W, I_C ≥ 1.5 A, β_min ≥ 28 and that is in a
SOT223 package.
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