MIC79050_05 Datasheet, PDF (6/14 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger

MIC79050
Block Diagrams
V_IN
IN
V_{B A T}
Bandgap
Ref.
Current Limit
Thermal Shutdown
MIC79050-x.xBS
GND
3-Pin Version
Micrel, Inc.
V_IN
IN
FB
V_{B A T}
Bandgap
V^R_R^e_E^f._F
EN
Current Limit
Thermal Shutdown
MIC79050-x.xBMM/M
GND
5-Pin Version
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 ac-
curate termination 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 volt-
age charging 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 typi-
cally 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 am-
plifier. This amplifier compares the voltage at the feedback
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 controlling the voltage at
the feedback pin the output can be disabled or forced to the
input voltage. Pulling and holding the feedback 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
6
August 2005