LTC1734 データシートの表示（PDF） - Linear Technology

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LTC1734

Lithium-Ion Linear Battery Charger in ThinSOT

Linear Technology 

LTC1734

APPLICATIONS INFORMATION

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

for a charge current of 200mA (RPROG = 7.5k).

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 4.1V or 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

10k

TO SCOPE

RPROG

3k

2.5V

0V

f = 1kHz

PROG BAT

+

LTC1734

Li-Ion*

6Ω TO

20Ω

1734 F04

*FULLY CHARGED CELL

Figure 4. Setup for AC Stability Testing

PULSE 5V

GENERATOR 0V

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

VCC is desired. If the supply voltage is high enough, a

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.

*

VIN

VCC

LTC1734

1734 F06

*DRAIN-BULK DIODE OF FET

Figure 6. Low Loss Reverse Voltage Protection

VCC Bypass Capacitor

Many types of capacitors with values ranging from 1µF to

10µF located close to the LTC1734 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.

2V

PROG PIN

(20pF ON PIN)

1V

0V

2V

PROG PIN

(200pF ON PIN)

1V

0V

SHUT DELAY CONSTANT

DOWN

CURRENT

CONSTANT

VOLTAGE

HORIZONTAL SCALE: 100µs/DIV

Figure 5. Stability Waveforms

Internal Protection

Internal protection is provided to prevent excessive DRIVE

pin currents (IDSHRT) and excessive self-heating of the

LTC1734 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 too low a VCE.

This protection is not designed to prevent overheating of

the external pass transistor. Indirectly though, self-heating

of the PNP thermally conducting to the LTC1734 and

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.

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