Datasheet

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SLUS025F − MAY 2001 − REVISED JULY 2002
16
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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)
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.
θ
JC
+
ǒ
T
max(J)
–T
A(max)
Ǔ
P
D
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
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.
(10)
(11)
(12)