MIC79050_05 Datasheet, PDF (10/14 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger

MIC79050
MIC79050-4.2BMM
IN BAT
VIN _{EN FB}
GND
4.7µF
100k
GND
MIC834
VDD OUT
R1
INP GND
R2
Li-Ion
Cell
V_BAT(low) = V_REF (1+ ^RR12)
V_REF=1.240V
Figure 4. Pulse Charging For
Top-off Voltage
Charging Rate
Lithium-ion cells are typically charged at rates that are frac-
tional multiples of their rated capacity. The maximum varies
between 1C – 1.3C (1× to 1.3× the capacity of the cell). The
MIC79050 can be used for any cell size. The size of the cell
and the current capability of the input source will determine the
overall circuit charge rate. For example, a 1200mAh battery
charged with the MIC79050 can be charged at a maximum of
0.5C. There is no adverse effects to charging at lower charge
rates; that charging will just take longer. Charging at rates
greater than 1C are not recommended, or do they decrease
the charge time linearly.
The MIC79050 is capable of providing 500mA of current at its
nominal rated output voltage of 4.2V. If the input is brought
below the nominal output voltage, the output will follow the
input, less the saturation voltage drop of the pass element.
If the cell draws more than the maximum output current of
the device, the output will be pulled low, charging the cell at
600mA to 700mA current. If the input is a fixed source with a
low output impedance, this could lead to a large drop across
the MIC79050 and excess heating. By driving the feedback
pin with an external PWM-circuit, the MIC79050 can be used
to pulse charge the battery to reduce power dissipation and
bring the device and the entire unit down to a lower operat-
ing temperature. Figure 5 shows a typical configuration for a
PWM-based pulse-charging topology. Two circuits are shown
in Figure 5: circuit a uses an external PWM signal to control
the charger, while circuit b uses the MIC4417 as a low duty-
cycle oscillator to drive the base of Q1. (Consult the battery
manufacturer for optimal pulse-charging techniques).
MIC79050-4.2BMM
VIN
IN BAT
EN FB
GND
4.7µF Li-Ion
Cell
External PWM
Figure 5A.
VIN=4.5V to 16V
MIC79050-4.2BMM
IN BAT
EN FB
GND
Micrel, Inc.
4.7µF
1k
40k
Li-Ion
Cell
MIC4417
200pF
Figure 5B. PWM Based Pulse-charging
Applications
Figure 6 shows another application to increase the output
current capability of the MIC79050. By adding an external
PNP power transistor, higher output current can be obtained
while maintaining the same accuracy. The internal PNP now
becomes the driver of a darlington array of PNP transistors,
obtaining much higher output currents for applications where
the charge rate of the battery is much higher.
MIC79050-4.2BMM
IN BAT
EN FB
GND
4.7µF
Figure 6. High Current Charging
Regulated Input Source Charging
When providing a constant-current, constant-voltage, charger
solution from a well-regulated adapter circuit, the MIC79050
can be used with external components to provide a constant
voltage, constant-current charger solution. Figure 7 shows a
configuration for a high-side battery charger circuit that moni-
tors input current to the battery and allows a constant current
charge that is accurately terminated with the MIC79050. The
circuit works best with smaller batteries, charging at C rates in
the 300mA to 500mA range. The MIC7300 op-amp compares
the drop across a current sense resistor and compares that
to a high-side voltage reference, the LM4041, pulling the
feedback pin low when the circuit is in the constant-current
mode. When the current through the resistor drops and the
battery gets closer to full charge, the output of the op-amp
rises and allows the internal feedback network of the regulator
take over, regulating the output to 4.2V.
R_S
LM4041CIM3-1.2
16.2k
221k
MIC79050-4.2BMM
IN BAT
EN FB
GND
MIC7300
4.7µF
SD101
I_CC
=
80mV
R_S
10k
0.01µF
Figure 7. Constant Current,
Constant Voltage Charger
MIC79050
10
August 2005