MIC79050 Datasheet, PDF (8/20 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger Preliminary Information

MIC79050
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
Micrel
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 12µA 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 <12µA to minimize battery drain.
MIC79050
8
June 2000