Analog Devices ADP2291 Evaluation Board ADP2291RM-EVALZ ADP2291RM-EVALZ 数据表
产品代码
ADP2291RM-EVALZ
ADP2291
Rev. A | Page 13 of 20
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.
must be kept low, a P MOSFET is recommended. Connect the
MOSFET as shown in Figure 21.
ADP2291
RS
INPUT
4.6V–12V
IN
CHG
CIN
CS
DRV
04873-021
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.
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 V
voltage that exceeds the maximum adapter voltage. A V
CEO
rating of at least 15 V is recommended.
Providing a charge current of I
MAX
with a minimum base drive
of 40 mA requires a PNP beta of at least
β
MIN
=
mA
40
MAX
MAX
I
I
I
=
Β
(5)
Note that the beta of a transistor drops off with collector
current. Therefore, make sure the beta at I
current. Therefore, make sure the beta at I
MAX
meets the
minimum requirement.
For cases where the adapter voltage is low (less than 5.5 V),
calculate the saturation voltage by
calculate the saturation voltage by
V
CE(SAT)
=
V
ADAPTER(MIN)
−
V
PROTECT
−
V
RS
−
V
BAT
(6)
where V
PROTECT
is the forward drop of the reverse input
protection.
The power handling capability of the PNP pass transistor is
another important parameter. The maximum power dissipation
of the pass transistor is estimated using
another important parameter. The maximum power dissipation
of the pass transistor is estimated using
P
DISS
(W) = I
MAX
× (V
ADAPTER(MAX)
− V
PROTECT
− V
RS
− V
BAT
) (7)
where
V
RS
= 50 mV to 150 mV at V
ADJ
= 1.5 V to 3.0 V,
V
BAT
= 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.
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:
V
ADAPTER(MIN)
= 5.0 V
V
ADAPTER(MAX)
= 6.0 V
I
MAX
= 500 mA
V
PROTECT
= 0.2 V at 500 mA
V
ADJ
= 3 V
V
RS
= 150 mV
β
MIN
=
5
.
12
mA
40
mA
500
=
=
Β
I
I
MAX
V
CE(SAT)
=
V
ADAPTER(MIN)
−
V
PROTECT
−
V
RS
−
V
BAT
= 5.0 V − 0.2 V − 0.15 V − 4.2 V
= 0.45 V
= 0.45 V
P
DISS
(W) = I
MAX
× (
V
ADAPTER(MAX)
−
V
PROTECT
−
V
RS
−
V
BAT
)
= 0.50 A × (6.0 V − 0.2 V − 0.15 V − 2.8 V)
= 1.4 W
= 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
Package
Max PD @ 25°C
Beta @ 1 A
VCE (SAT)
Fairchild
FSB6726
NZT45H8
NZT45H8
SuperSOT
SOT223
SOT223
0.5 W
1.5 W
1.5 W
150
110
110
0.5 V
0.1 V
0.1 V
ON Semiconductor®
MTB35200
BCP53T1
MMJT9435
BCP53T1
MMJT9435
TSOP-6
SOT223
SOT223
SOT223
SOT223
0.625 W
1.5 W
1.6 W
1.5 W
1.6 W
200
35
200
35
200
0.175 V
0.3 V
0.18 V
0.3 V
0.18 V
Philips
BCP51
SOT223
1.3 W
50
0.5 V
ZETEX
ZXT10P20DE6
ZXT2M322
FZT549
FMMT549
ZXT2M322
FZT549
FMMT549
SOT23-6
2 mm × 2 mm MLP
SOT223
SOT23
2 mm × 2 mm MLP
SOT223
SOT23
1.1 W
1.5 W
2 W
0.5 W
1.5 W
2 W
0.5 W
270
270
130
130
270
130
130
0.17 V
0.17 V
0.25 V
0.25 V
0.17 V
0.25 V
0.25 V