Standalone Linear Lithium Battery Charger

Page 9 of 11

3H20N Rev. P001

EC49017

Stability Considerations

The constant-voltage mode feedback loop is stable without an output capacitor provided a battery is connected to the

charger output. With no battery present, an output capacitor is recommended to reduce ripple voltage. When using

high value, low ESR ceramic capacitors, it is recom

mended to add a 1Ω resistor in series with the capacitor. No series

resistor is needed if tantalum capacitors are used.

In constant-current mode, the PROG pin is in the feedback loop, 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, the charger is stable

with program resistor values as high as 20k. However, additional capacitance on this node reduces the maximum

allowed program resistor. The pole frequency at the PROG pin should be kept above 100kHz.

V

Many types of capacitors can be used for 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 generated under some start-up conditions, such as connecting the charger input to a live

power source. Adding a 1.5Ω resistor in series with a ceramic capacitor will minimize start-up voltage transients.

Power Dissipation

The conditions that cause the EC49017 to reduce charge current through thermal feedback can be approximated by

considering the power dissipated in the IC. Nearly all of this power dissipation is generated by the internal

MOSFET

—this is calculated to be approximately:

P

D

= (V

CC

– V

BAT

) • I

BAT

The approximate ambient temperature at which the thermal feedback begins to protect the IC is:

T

A

= 120°C

– P

D

θ

JA

T

A

= 120°C

– (V

CC

– V

BAT

) • I

BAT

• θ

JA

Thermal Considerations

Because of the small size of the thin SOT23 package, it is very important to use a good thermal PC board layout to

maximize the available charge current. The thermal path for the heat generated by the IC is from the die to the copper

lead frame, through the package lead, (especially the ground lead) to the PC board copper. The PC board copper is

the heat sink. The footprint copper pads should be as wide as possible and expand out to larger copper areas to

spread and dissipate the heat to the surrounding ambient. Other heat sources on the board, not related to the charger,

must also be considered when designing a PC board layout because they will affect overall temperature rise and the

maximum charge current.

Application Hints