10

LTC1734

known, Table 1 can be used as a guide in selecting some

to 0.8V all depending on the current. See the manufacturer’s

data sheet for details. All of the PNP transistors are rated

to carry at least 1A continuously as long as the power

dissipation is within limits. The Stability section addresses

caution in the use of high beta PNPs.

Should overheating of the PNP transistor be a concern,

protection can be achieved with a positive temperature

coefficient (PTC) thermistor, wired in series with the

current programming resistor and thermally coupled to

the transistor. The PTH9C chip series from Murata has a

steep resistance increase at temperature thresholds from

85

°C to 145°C making it behave somewhat like a thermo-

stat switch. For example, the model PTH9C16TBA471Q

thermistor is 470

Ω at 25°C, but abruptly increase its

resistance to 4.7k at 125

°C. Below 125°C, the device

exhibits a small negative TC. The 470

Ω thermistor can be

added in series with a 1.6k resistor to form the current

programming resistor for a 700mA charger. Should the

thermistor reach 125

°C, the charge current will drop to

238mA and inhibit any further increase in temperature.

Stability

The LTC1734 contains two control loops: constant voltage

and constant current. To maintain good AC stability in the

constant voltage mode, a capacitor of at least 4.7

µF is

usually required from BAT to ground. The battery and

interconnecting wires appear inductive at high frequen-

cies, and since these are in the feedback loop, this capaci-

tance may be necessary to compensate for the inductance.

This capacitor need not exceed 100

µF and its ESR can

range from near zero to several ohms depending on the

inductance to be compensated. In general, compensation

is optimal with a capacitance of 4.7

µF to 22µF and an ESR

of 0.5

Ω to 1.5Ω.

Using high beta PNP transistors (>300) and very low ESR

output capacitors (especially ceramic) reduces the phase

margin, possibly resulting in oscillation. Also, using high

value capacitors with very low ESRs will reduce the phase

margin. Adding a resistor of 0.5

Ω to 1.5Ω in series with

the capacitor will restore the phase margin.

In the constant current mode, the PROG pin is in the

feedback loop, not the battery. Because of this, capaci-

tance on this pin must be limited. Locating the program

resistor near the PROG pin and isolating the charge

current monitoring circuitry (if used) from the PROG pin

with a 1k to 10k resistor may be necessary if the capaci-

tance is greater than that given by the following equation:

C

k

R

MAX pF

PROG

400

Table 1. PNP Pass Transistor Selection Guide

Mounted on Board

Package Style

ZETEX Part Number

ROHM Part Number

Comments

0.5

SOT-23

FMMT549

0.625

SOT-23

FMMT720

1

SOT-89

FCX589 or BCX69

1.1

SOT-23-6

ZXT10P12DE6

1 to 2

SOT-89

FCX717

2

SOT-223

FZT589

2

SOT-223

BCP69 or FZT549

0.75

FTR

2SB822

1

ATV

2SB1443

2

SOT-89

2SA1797

TO-252

2SB1182