LP4054 datasheet(12/14 Pages) ETC

12 / 14 page

Preliminary

LP4054

LP4054-datasheet V1.0

Sep.-2006

12-12

possible and expand out to larger copper areas to

spread and dissipate the heat to the surrounding

ambient. Feedthrough vias to inner or backside copper

layers are also useful in improving the overall thermal

performance of the charger .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.

Increasing Thermal Regulation Current

Reducing the voltage drop across the internal MOSFET

can significantly decrease the power dissipation in the

IC. This has the effect of increasing the current

delivered to

the battery during thermal regulation.

One method is by dissipating some of the power

through an external component, such as a

resistor or diode.

Example: An LP4054 operating from a 5V wall adapter

is programmed to supply 800mA full-scale current to a

discharged Li-Ion battery with a voltage of 3.75V.

Assuming θJA is 125℃/W, the approximate charge

current at an ambient temperature of 25℃ is:

By dropping voltage across a resistor in series with a 5V

wall adapter (shown in Figure 9), the on-chip power

dissipation can be decreased, thus increasing the

thermally regulated charge current.:

Solving for IBAT using the quadratic formaula

Using RCC = 0.25Ω, VS = 5V, VBAT = 3.75V, TA = 25℃

and θJA = 125℃/W we can calculate the thermally

regulated charge current to be:

While this application delivers more energy to the

battery and reduces charge time in thermal mode, it

may actually lengthen charge time in voltage mode if

VCC becomes low enough to put the LP4054 into

dropout.

This technique works best when RCC values are

minimized to keep component size small and avoid

dropout. Remember to choose a resistor with adequate

power handling capability.

VCC Bypass Capacitor

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 an X5R

ceramic capacitor will minimize start-up voltage

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