BQ2057DGKR Datasheet, PDF (16/24 Pages) Texas Instruments – ADVANCED LINEAR CHARGE MANAGEMENT IC FOR SINGLE AND TWO CELL LITHIUM-POLYMER

bq2057, bq2057C
bq2057T, bq2057W
SLUS025F − MAY 2001 − REVISED JULY 2002
APPLICATION INFORMATION
selecting an external pass-transistor (continued)
P-channel MOSFET:
Selection steps for a P-channel MOSFET: Example: V_I = 5.5 V, I_(REG) = 500 mA, 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 4.)
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_D is the forward voltage drop across the reverse-blocking diode (if one is used), and V_CS is the
voltage drop across the current sense resistor.
P_D = (V_I – V_D − V_(CS) – V_(BAT)) × I_(REG)
(10)
P_D = (5.5 – 0.4 – 0.1 −3) × 0.5 A
P_D = 1 W
2. Determine the package size needed in order to keep the junction temperature below the manufacturer’s
recommended value, T_JMAX. Calculate the total theta, θ(°C/W), needed.
ǒ Ǔ Tmax(J)–TA(max)
(11)
θ_JC +
PD
θ_JC
+
(150–40)
1
θ_JC + 110°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 TSSOP-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 (I_D) 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) = (V_D +V_(CS) + V_OL(CC)) − V_I
(12)
V_(GS) = (0.4 + 0.1 + 1.5) – 5.5
V_(GS) = −3.5
Where V_(GS) is the gate-to-source voltage, V_D is the forward voltage drop across the reverse-blocking diode
(if one is used), and V_CS is the voltage drop across the current sense resistor, and V_OL(CC) is the CC pin
output low voltage specification for the bq2057.
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, θ_JC ≤ 110°C/W, I_D ≥ 1 A,
V_(GSth) ≥ −3.5 V and in a TSSOP package.
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