Stability considerations

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Stability considerations

The STBC08 contains two control loops: constant voltage and constant current. The

constant-voltage loop is stable without any compensation when a battery is connected with

low impedance leads. Excessive lead length, however, may add enough series inductance

to require a bypass capacitor of at least 1µF from BAT to GND. Furthermore, a 4.7µF

capacitor with a 0.2

Ω to 1Ω series resistor from BAT to GND is required to keep ripple

voltage low when the battery is removed.

High value capacitors with very low ESR (especially ceramic) reduce the constant-voltage

loop phase margin.

Ceramic capacitors up to 22µF may be used in parallel with a battery, but larger ceramics

should be decoupled with 0.2

Ω to 1Ω of series resistance.

In constant-current mode, the PROG pin is in the feedback loop, not the battery. Because of

the additional pole created by PROG pin capacitance, capacitance on this pin must be kept

to a minimum. With no additional capacitance on the PROG pin, the charger is stable with

program resistor values as high as 12k. However, additional capacitance on this node

reduces the maximum allowed program resistor.Therefore, if the PROG pin is loaded with a

Equation 1

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 2.

This design includes a 20k

Ω resistor between the PROG pin and the filter capacitor to

Figure 2.

Isolating capacitive load on PROG pin and filtering

R

PROG

1

2

π 510

•

5

C

PROG

•

•

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