MIC79050 datasheet(14/28 Pages) MICROCHIP

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MIC79050

DS20005771B-page 14



2017 - 2022 Microchip Technology Inc. and its subsidiaries.

FIGURE 5-6:

Pulse Charging for Top-Off

Voltage.

5.7

Charging Rate

Lithium-ion cells are typically charged at rates that are

fractional multiples of their rated capacity. The

maximum varies between 1C and 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 1200 mAh battery

charged with the MIC79050 can be charged at a

maximum of 0.5C. There are no adverse effects to

charging at lower charge rates; that charging will just

take longer. Charging at rates greater than 1C are not

recommended, nor do they decrease the charge time

linearly.

The MIC79050 is capable of providing 500 mA 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 600 mA to

700 mA 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 5-7 and Figure 5-8 show typical configurations

for PWM-based pulse-charging topologies. Figure 5-7

uses an external PWM signal to control the charger,

while Figure 5-8 uses the MIC4417 as a low duty cycle

oscillator to drive the base of Q1. Consult the battery

manufacturer for optimal pulse-charging techniques.

FIGURE 5-7:

External PWM Circuit

Design.

FIGURE 5-8:

PWM-Based Pulse

Charging Using an MIC4417.

Figure 5-9 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.

FIGURE 5-9:

High-Current Charging.

5.8

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 5-10 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 300 mA to 500 mA

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

Li-Ion

Cell

BAT

GND

MIC79050-4.2YMM

VDD OUT

GND

INP

100k

4.7μF

MIC834

VIN

GND

VREF=1.240V

VBAT(low) = VREF (1+

)

Li-Ion

Cell

4.7μF

BAT

GND

MIC79050-4.2YMM

VIN

External PWM

Li-Ion

Cell

4.7μF

200pF

40k

BAT

GND

MIC79050-4.2YMM

VIN=4.5V to 16V

MIC4417

BAT

4.7μF

GND

MIC79050-4.2YMM

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