MIC79050 datasheet(13/28 Pages) MICROCHIP

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DS20005771B-page 13

MIC79050

5.4

Zero-Output Impedance Source

Charging

Input voltage sources that have very low output

impedances can be a challenge due to the nature of the

source. Using the circuit in Figure 5-5 will provide a

constant-current and constant voltage charging

algorithm with the appropriate end-of-charge

termination. The main loop consists of an op-amp

controlling the feedback pin through the schottky diode,

D1. The charge current through RS is held constant by

the op-amp circuit until the output draws less than the

set charge-current. At this point, the output goes

constant-voltage. When the current through RS gets to

less than 50 mA, the difference amp output becomes

less than the reference voltage of the MIC834 and the

output pulls low. This sets the output of the MIC79050

less than nominal, stopping current flow and

terminating charge.

FIGURE 5-5:

Zero-Output Impedance Source Charging.

5.5

Lithium-Ion Battery Charging

Single lithium-ion cells are typically charged by

providing a constant current and terminating the charge

with constant voltage. The charge cycle must be

initiated by ensuring that the battery is not in deep

discharge. If the battery voltage is below 2.5V, it is

commonly recommended to trickle charge the battery

with 5 mA to 10 mA of current until the output is above

2.5V. At this point, the battery can be charged with

constant current until it reaches its top off voltage (4.2V

for a typical single lithium-ion cell) or a time-out occurs.

For the constant-voltage portion of the charging circuit,

an extremely accurate termination voltage is highly

recommended. The higher the accuracy of the

termination circuit, the more energy the battery will

store. Because lithium-ion cells do not exhibit a

memory effect, less accurate termination does not

harm the cell, but simply stores less usable energy in

the battery. The charge cycle is completed by disabling

the charge circuit after the termination current drops

below a minimum recommended level, typically 50 mA

less,

depending

the

manufacturer’s

recommendation, or if the circuit times out.

5.6

Time-Out

The time-out aspect of lithium-ion battery charging can

be added as a safety feature of the circuit. Often times

this function is incorporated in the software portion of

an application using a real-time clock to count out the

maximum amount of time allowed in the charging cycle.

When the maximum recommended charge time for the

specific cell has been exceeded, the enable pin of the

MIC79050 can be pulled low, and the output will float to

the battery voltage, no longer providing current to the

output.

As a second option, the feedback pin of the MIC79050

can be modulated as in Figure 5-6. It shows a simple

circuit where the MIC834, an integrated comparator

and reference, monitors the battery voltage and

disables the MIC79050 output after the voltage on the

battery exceeds a set value. When the voltage decays

below this set threshold, the MIC834 drives Q1 low

allowing the MIC79050 to turn on again and provide

current to the battery until it is fully charged. This form

of pulse charging is an acceptable way of maintaining

the full charge on a cell until it is ready to be used.

MIC79050-4.2YM

MIC834

SD101

1/2 MIC7122

BAT

GND

8.06M

4.7μF

R2=124k

R3=1k

R4=124k

0.01μF

VDD OUT

GND

INP

R1=1k

221k

16.2k

16k

10k

ICC=

80mV

IEOC=

1.24V × R1

R2 × RS

LM4041

CIM3-1.2

Li-Ion

Cell

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