11

LTC1734L

1734lf

Higher charge currents require lower program resistor

values which can tolerate more capacitive loading on the

PROG pin. Maximum capacitance can be as high as 50pF

Figure 4 is a simple test circuit for checking stability in both

the constant current and constant voltage modes. With

input power applied and a near fully charged battery

connected to the charger, driving the PROG pin with a

pulse generator will cycle the charger in and out of the

manual shutdown mode. Referring to Figure 5, after a

short delay, the charger will enter the constant current

mode first, then if the battery voltage is near the pro-

grammed voltage of 4.2V, the constant voltage mode will

begin. The resulting waveform on the PROG pin is an

indication of stability.

The double exposure photo in Figure 5 shows the effects

of capacitance on the program pin. The middle waveform

is typical while the lower waveform indicates excessive

program pin capacitance resulting in constant current

mode instability. Although not common, ringing on the

constant voltage portion of the waveform is an indication

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

of instability due to any combination of extremely low ESR

values, high capacitance values of the output capacitor or

very high PNP transistor beta. To minimize the effect of the

scope probe capacitance, a 10k resistor is used to isolate

the probe from the program pin. Also, an adjustable load

resistor or current sink can be used to quickly alter the

charge current when a fully charged battery is used.

Reverse Input Voltage Protection

In some applications, protection from reverse voltage on

series blocking diode can be used. In other cases, where

the voltage drop must be kept low, a P-channel FET as

shown in Figure 6 can be used.

Figure 5. Stability Waveforms

LTC1734L

PROG

Li-Ion*

20

Ω TO

200

Ω

*FULLY CHARGED CELL

10k

3k

TO SCOPE

1734 F04

BAT

2.5V

f = 1kHz

0V

+

Figure 4. Setup for AC Stability Testing

5V

0V

PROG PIN

(20pF ON PIN)

PROG PIN

(200pF ON PIN)

PULSE

GENERATOR

2V

1V

0V

2V

SHUT

DOWN

DELAY

CONSTANT

CURRENT

HORIZONTAL SCALE: 100

µs/DIV

1V

0V

CONSTANT

VOLTAGE

*

1734 F06

LTC1734L

*DRAIN-BULK DIODE OF FET

Figure 6. Low Loss Reverse Voltage Protection

Many types of capacitors with values ranging from 1

µF to

10

µF located close to the LTC1734L will provide adequate

input bypassing. However, caution must be exercised

when using multilayer ceramic capacitors. Because of the

self resonant and high Q characteristics of some types of

ceramic capacitors, high voltage transients can be gener-

ated under some start-up conditions, such as connecting

the charger input to a hot power source. To prevent these

transients from exceeding the absolute maximum voltage

rating, several ohms of resistance can be added in series

with the ceramic input capacitor.

Internal Protection

Internal protection is provided to prevent excessive DRIVE

LTC1734L during a fault condition. The faults can be

generated from a shorted DRIVE pin or from excessive

DRIVE pin current to the base of the external PNP

transistor when it’s in deep saturation from a very low

ing of the external pass transistor. Indirectly though, self-

heating of the PNP thermally conducting to the LTC1734L