LTC4057 Datasheet, PDF(9/12 Page) Linear Technology – Linear Li-Ion Battery Charger with Thermal Regulation in ThinSOT

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LTC4057 Datasheet, PDF (9/12 Pages) Linear Technology – Linear Li-Ion Battery Charger with Thermal Regulation in ThinSOT

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LTC4057- 4.2

APPLICATIO S I FOR ATIO

RCC

VCC

BAT

1ÂµF

LTC4057-4.2

PROG

GND

+ Li-Ion

CELL

RPROG

405742 F02

Figure 2. A Circuit to Maximize Thermal Mode Charge Current

Solving for IBAT using the quadratic formula1.

IBAT =

(VS â VBAT ) â

ï£«

ï£ï£¬

(VS

VBAT )2

4RCC (120Â°C

Î¸JA

)ï£¶

ï£¸ï£·

2RCC

Using RCC = 0.25â¦, VS = 5V, VBAT = 3.75V, TA = 25Â°C and

Î¸JA = 125Â°C/W, we can calculate the thermally

regulated charge current to be:

IBAT = 708.4mA

While this application delivers more energy to the battery

and reduces charge time in thermal mode, it may actually

lengthen charge time in voltage mode if VCC becomes low

enough to put the LTC4057 into dropout. Figure 3 shows

how this circuit can result in dropout as RCC becomes

large.

This technique works best when RCC values are minimized

to keep component size small and avoid dropout. Remem-

ber to choose a resistor with adequate power handling

capability.

1000

VS = 5V

800

CONSTANT

CURRENT

600

VS = 5.5V

VS = 5.25V

400 THERMAL

MODE

DROPOUT

200

VBAT = 3.75V

TA = 25Â°C

Î¸JA = 125Â°C/W

RPROG = 1.25kâ¦

0.25 0.5 0.75 1.0 1.25 1.5 1.75

RCC (â¦)

405442 F03

Figure 3. Charge Current vs RCC

Note 1: Large values of RCC will result in no solution for IBAT. This indicates that the LTC4057 will

not generate enough heat to require thermal regulation.

4057f

▷