ADP2291_15 Datasheet, PDF (13/20 Pages) Analog Devices – Compact, 1.5 A Linear Charger for Single-Cell Li Battery

ADP2291
In cases where the voltage drop across the protection device
must be kept low, a P MOSFET is recommended. Connect the
MOSFET as shown in Figure 21.
INPUT
4.6V–12V
CIN
RS
CS
DRV
IN
ADP2291
CHG
Figure 21. Reverse Input Protection
EXTERNAL PASS TRANSISTOR
Choose the external PNP pass transistor based on the given
operating conditions and power handling capabilities. The pass
device is determined by the base drive available, the input and
output voltage, and the maximum charge current.
Select the pass transistor with a collector-emitter breakdown
voltage that exceeds the maximum adapter voltage. A VCEO
rating of at least 15 V is recommended.
Providing a charge current of IMAX with a minimum base drive
of 40 mA requires a PNP beta of at least
βMIN = ^IMAX ₌ ^IMAX
(5)
IΒ 40 mA
Note that the beta of a transistor drops off with collector
current. Therefore, make sure the beta at IMAX meets the
minimum requirement.
For cases where the adapter voltage is low (less than 5.5 V),
calculate the saturation voltage by
The power handling capability of the PNP pass transistor is
another important parameter. The maximum power dissipation
of the pass transistor is estimated using
PDISS (W) = IMAX × (VADAPTER(MAX) − VPROTECT − VRS − VBAT) (7)
where VRS = 50 mV to 150 mV at VADJ = 1.5 V to 3.0 V,
VBAT = 2.8 V, the lowest cell voltage where fast charge can occur.
Note that the adapter voltage can be either preregulated or
unregulated. In the preregulated case, the difference between
the maximum and minimum adapter voltage is small. In this
case, use the maximum regulated adapter voltage to determine
the maximum power dissipation. In the unregulated case, the
adapter voltage can have a wide range specified. However, the
maximum voltage specified is usually with no load applied.
Therefore, the worst-case power dissipation calculation often
leads to an over-specified pass device. In either case, it is best to
determine the load characteristics of the adapter to optimize the
charger design.
For example:
VADAPTER(MIN) = 5.0 V
VADAPTER(MAX) = 6.0 V
IMAX = 500 mA
VPROTECT = 0.2 V at 500 mA
VADJ = 3 V
VRS = 150 mV
βMIN = ^IMAX ₌ ^{500 mA} _{= 12.5}
IΒ 40 mA
VCE(SAT) = VADAPTER(MIN) − VPROTECT − VRS − VBAT
= 5.0 V − 0.2 V − 0.15 V − 4.2 V
= 0.45 V
VCE(SAT) = VADAPTER(MIN) − VPROTECT − VRS − VBAT
(6)
where VPROTECT is the forward drop of the reverse input
protection.
PDISS (W) = IMAX × (VADAPTER(MAX) − VPROTECT − VRS − VBAT)
= 0.50 A × (6.0 V − 0.2 V − 0.15 V − 2.8 V)
= 1.4 W
A guide for selecting the PNP pass transistor is shown in Table 5.
Table 5. PNP Pass Transistor Selection Guide
Vendor
Part Number
Fairchild
FSB6726
NZT45H8
ON Semiconductor®
MTB35200
BCP53T1
MMJT9435
Philips
BCP51
ZETEX
ZXT10P20DE6
ZXT2M322
FZT549
FMMT549
Package
SuperSOT
SOT223
TSOP-6
SOT223
SOT223
SOT223
SOT23-6
2 mm × 2 mm MLP
SOT223
SOT23
Max PD @ 25°C
0.5 W
1.5 W
0.625 W
1.5 W
1.6 W
1.3 W
1.1 W
1.5 W
2W
0.5 W
Beta @ 1 A
150
110
200
35
200
50
270
270
130
130
VCE (SAT)
0.5 V
0.1 V
0.175 V
0.3 V
0.18 V
0.5 V
0.17 V
0.17 V
0.25 V
0.25 V
Rev. A | Page 13 of 20