MIC79050 datasheet(8/20 Pages) MICREL

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MIC79050

Micrel

MIC79050

June 2000

Functional Description

The MIC79050 is a high-accuracy, linear battery charging

circuit designed for the simplest implementation of a single

lithium-ion (Li-ion) battery charger. The part can operate from

a regulated or unregulated power source, making it ideal for

various applications. The MIC79050 can take an unregulated

voltage source and provide an extremely accurate termina-

tion voltage. The output voltage varies only 0.75% from

nominal over the standard temperature range for Li-ion

battery charging (–5

°C to 60°C). With a minimum of external

components, an accurate constant current charger can be

designed to provide constant current, constant voltage charg-

ing for Li-ion cells.

Input Voltage

The MIC79050 can operate with an input voltage up to 16V

(20V absolute maximum), ideal for applications where the

input voltage can float high, such as an unregulated wall

adapter that obeys a load-line. Higher voltages can be

sustained without any performance degradation to the output

voltage. The line regulation of the device is typically 0.009%/V;

that is, a 10V change on the input voltage corresponds to a

0.09% change in output voltage.

Enable

The MIC79050 has an enable pin that allows the charger to

be disabled when the battery is fully charged and the current

drawn by the battery has approached a minimum and/or the

maximum charging time has timed out. When disabled, the

regulator output sinks a minimum of current with the battery

voltage applied directly onto the output. This current is

typically 12

µA or less.

Feedback

The feedback pin allows for external manipulation of the

control loop. This node is connected to an external resistive

divider network, which is connected to the internal error

amplifier. This amplifier compares the voltage at the feed-

back pin to an internal voltage reference. The loop then

corrects for changes in load current or input voltage by

monitoring the output voltage and linearly controlling the

drive to the large, PNP pass element. By externally control-

ling the voltage at the feedback pin the output can be disabled

or forced to the input voltage. Pulling and holding the feed-

back pin low forces the output low. Holding the feedback pin

high forces the pass element into saturation, where the output

will be the input minus the saturation (dropout) voltage.

Battery Output

The BAT pin is the output of the MIC79050 and connects

directly to the cell to provide charging current and voltage.

When the input is left floating or grounded, the BAT pin limits

reverse current to <12

µA to minimize battery drain.

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