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LTC4095

4095fa

APPLICATIONS INFORMATION

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

back loop rather than the battery voltage. Because of the

additional pole created by any PROG pin capacitance,

capacitance on this pin must be kept to a minimum. With

no additional capacitance on the PROG pin, the battery

charger is stable with program resistor values as high

as 25k. However, additional capacitance on this node

reduces the maximum allowed program resistor. The pole

frequency at the PROG pin should be kept above 100kHz.

Therefore, if the PROG pin has a parasitic capacitance,

CPROG, the following equation should be used to calculate

the maximum resistance value for RPROG:

PROG

≤

210

π ••

The stability of the constant-current loop also needs to

be considered when average, rather than instantaneous,

battery current is of interest to the user. For example, if a

switching power supply operating in low current mode is

connected in parallel with the battery, the average current

being pulled out of the BAT pin is typically of more interest

than the instantaneous current pulses. In such a case, a

simple RC ﬁlter an be used on the PROG pin to measure

the average battery current as shown in Figure 7. A 10k

resistor has been added between the PROG pin and the

ﬁlter capacitor to ensure stability.

Board Layout Considerations

In order to deliver maximum charge current under all

conditions, it is critical that the exposed metal pad on

the backside of the LTC4095 package is soldered to the

PC board ground. Correctly soldered to a 2500mm2

double-sided 1oz. copper board the LTC4095 has a ther-

mal resistance of approximately 60°C/W. Failure to make

thermal contact between the Exposed Pad on the backside

of the package and the copper board will result in thermal

resistances far greater than 60°C/W. As an example, a

correctly soldered LTC4095 can deliver over 950mA to a

battery from a 5V supply at room temperature. Without

a backside thermal connection, this number could drop

to less than 500mA.

IN Bypass Capacitor

Many types of capacitors can be used for input bypassing;

however, caution must be exercised when using multi-layer

ceramic capacitors. Because of the self-resonant and high

Q characteristics of some types of ceramic capacitors, high

voltage transients can be generated under some start-up

conditions, such as connecting the charger input to a live

power source. For more information, refer to Application

Note 88.

4095 F04

CFILTER

CHARGE

CURRENT

MONITOR

CIRCUITRY

RPROG

LTC4095

PROG

GND

10k

Figure 7. Isolating Capacitive Load on PROG Pin and Filtering

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