MIC79050 Datasheet, PDF (12/20 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger Preliminary Information

MIC79050
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 operating
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
VIN=4.5V to 16V
External PWM
Figure 5A.
MIC79050-4.2BMM
IN BAT
EN FB
GND
4.7µF
1kΩ
Li-Ion
Cell
MIC4417
40kΩ
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
4.7µF
GND
Figure 6. High Current Charging
Micrel
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
monitors 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
Simple Charging
The MIC79050 is available in a three-terminal package,
allowing for extremely simple battery charging. When used
with a current-limited, low-power input supply, the MIC79050-
4.2BS completes a very simple, low-charge-rate, battery-
charger circuit. It provides the accuracy required for termina-
tion, while a current-limited input supply offers the constant-
current portion of the algorithm.
Thermal Considerations
The MIC79050 is offered in three packages for the various
applications. The SOT-223 is most thermally efficient of the
three packages, with the power SOP-8 and the power MSOP-8
following suit.
Power SOP-8 Thermal Characteristics
One of the secrets of the MIC79050’s performance is its
power SO-8 package featuring half the thermal resistance of
a standard SO-8 package. Lower thermal resistance means
more output current or higher input voltage for a given
package size.
Lower thermal resistance is achieved by joining the four
ground leads with the die attach paddle to create a single-
piece electrical and thermal conductor. This concept has
MIC79050
12
June 2000