ADP2291

Rev. 0 | 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.

ADP2291

RS

INPUT

4.6V–12V

IN

CHG

CIN

CS

DRV

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

rating of at least 15 V is recommended.

of 40 mA requires a PNP beta of at least

mA

I

I

I

MAX

MAX

40

=

Β

(5)

Note that the beta of a transistor drops off with collector

minimum requirement.

For cases where the adapter voltage is low (less than 5.5 V)

calculate the saturation voltage using the following equation:

(6)

protection.

The power handling capabilities of the PNP pass transistor is

another important parameter. The maximum power dissipation

of the pass transistor is estimated using

2.8 V, the lowest cell voltage where fast charge can occur

It should be noted that the adapter voltage can be either pre-

regulated 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. How-

ever, 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:

= 5.0 V

= 6.0 V

= 500 mA

= 0.2 V at 500 mA

= 150 mV

5

.

12

40

500

=

=

Β

mA

mA

I

= 5.0 V − 0.2 V − 0.15 V − 4.2 V

= 0.45 V

= 0.50 A × (6.0 V − 0.2 V − 0.15 V − 2.8 V)

= 1.4 W

A guide for selecting the PNP transistor is given in Table 5.

Table 5. PNP Pass Transistor Selection Guide

Vendor

Part Number

Package

Max PD@ 25°C

Beta @1A

VCE (SAT)

Fairchild

FSB6726

NZT45H8

SuperSOT

SOT223

0.5 W

1.5 W

150

110

0.5 V

0.1 V

ON Semi

MTB35200

BCP53T1

MMJT9435

TSOP-6

SOT223

SOT223

0.625 W

1.5 W

1.6 W

200

35

200

0.175 V

0.3 V

0.18 V

Philips

BCP51

SOT223

1.3 W

50

0.5 V

ZETEX

ZXT10P20DE6

ZXT2M322

FZT549

FMMT549

SOT23-6

2 mm × 2 mm MLP

SOT223

SOT23

1.1 W

1.5 W

2 W

0.5 W

270

270

130

130

0.17 V

0.17 V

0.25 V

0.25 V