LTC1733

13

sn1733 1733fs

Stability

The constant-voltage mode feedback loop is stable

without any compensation when a battery is connected.

However, a 1

µF capacitor with a 1Ω series resistor to GND

is recommended at the BAT pin to keep ripple voltage low

when the battery is disconnected.

In the constant-current mode it is the PROG pin that is in

the feedback loop and not the battery. The constant-

current mode stability is affected by the impedance at the

PROG pin. With no additional capacitance on the PROG

pin, stability is acceptable with program resistor values as

high as 50k. However, additional capacitance on this node

reduces the maximum allowed program resistor. The pole

frequency at the PROG pin should be kept above 500kHz.

Therefore, if the PROG pin is loaded with a capacitance, C,

the following equation should be used to calculate the

Average, rather than instantaneous, battery current may

be 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 can be used on the PROG pin to measure the average

battery current as shown in Figure 4. A 10k resistor is

added between the PROG pin and the filter capacitor and

monitoring circuit to ensure stability.

5

7

CHARGE

CURRENT

MONITOR

CIRCUITRY

10k

LTC1733

1733 F04

GND

PROG

Figure 4. Isolating Capacitive Load on PROG Pin and Filtering.

APPLICATIO S I FOR ATIO

I

CC

VV

C W

C

CA

A

°°

°

=

°

°

=

105

55

5

3 75

40

50

50

1

–

(– .

)•

/

/

Furthermore, the voltage at the PROG pin will change

proportionally with the charge current as discussed in the

Programming Charge Current section.

It is important to remember that LTC1733 applications do

not need to be designed for worst-case thermal conditions

since the IC will automatically reduce power dissipation

when the junction temperature reaches approximately

105

°C. See Design Note 283 for additional information.

Board Layout Considerations

In order to be able to deliver maximum charge current

under all conditions, it is critical that the exposed pad on

the backside of the LTC1733 package is soldered to the

1oz. copper board the LTC1733 has a thermal resistance

of approximately 40

°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 40

°C/W. As an example, a correctly soldered

LTC1733 can deliver over 1250mA to a battery from a 5V

supply at room temperature. Without a backside thermal

connection, this number could drop to less than 500mA.

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 capaci-

tors, high voltage transients can be generated under some

start-up conditions, such as connecting the charger input

to a hot power source. For more information refer to

Application Note 88.