BQ2057CSN BENCHMARQ, BQ2057CSN Datasheet - Page 15

• Manufacturer Part Number: BQ2057CSN
• Description: PROG BATTERY CHARGER, 2057, SOIC8
• Manufacturer: BENCHMARQ

Specifications of BQ2057CSN

• Battery Type: Li-Ion / Li-Polymer
• Input Voltage: 15V
• Battery Charge Voltage: 4.2V
• Charge Current Max: 2A
• Battery Ic Case Style: SOIC
• No. Of Pins: 8
• No. Of Series Cells: 1
• Operating Temperature: RoHS Compliant

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
V
1. Determine the maximum power dissipation, P
2. Determine the package size needed in order to keep the junction temperature below the manufacturer’s
3. Select a collector-emitter voltage, V
4. Select a device that has at least 50% higher drain current I
5. Using the following equation, calculate the minimum beta (β or h
I
is the input voltage to the charger and I
The worst case power dissipation happens when the cell voltage, V
beginning of current regulation phase) and V
Where V
recommended value, T
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.
be adequate in this example.
where I
(chosen to be 35 mA in this example).
Now choose a PNP transistor that is rated for V
SOT223 package.
θ
θ
θ
b
b
b
P
P
P
JC
JC
JC
min
min
min
D
D
D
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
the device. This beta should be larger than the minimum required beta.
max(C))
= 1.4 W
= (V
= (4.5 – 0.1 − 3) × 1 A
+
+
+ 78°C W
CS
+
+
+ 28
( 150–40 )
is the voltage drop across the current sense resistor.
I
I
T
0.035
CMAX
− V
(J)max
1.4
I
1
is the maximum collector current (in this case same as I
B
CS
– V
P
* T
D
(J)max
(BAT)
A(max)
. Calculate the total theta, θ(°C/W), needed.
) × IREG
APPLICATION INFORMATION
(CE)
(REG)
, rating greater than the maximum input voltage. A 15-V device will
(REG)
www.ti.com
is the desired charge current (see Figure 1).
NOTE:
I
and the minimum ambient temperature when choosing
(CE)
D
is at its maximum.
, in the transistor.
I
≥15 V, θ
= 4.5 V, I
C
JC
(REG)
rating than the desired charge current I
≤ 78°C/W, I
FE
= 1 A, 4.2-V single-cell Li-Ion (bq2057C).
(BAT)
) needed:
SLUS025F − MAY 2001 − REVISED JULY 2002
, is at its lowest (typically 3 V at the
(REG)
C
≥ 1.5 A, β
), and I
min
B
is the base current
≥ 28 and that is in a
(REG)
(7)
(8)
(9)
15
.