Applications Information

The MAX1679 was designed to offer the maximum inte-

gration and functionality in the smallest, most basic

application circuit possible. The only necessary exter-

nal components are a current-limited wall cube, a

PMOS FET, two small capacitors, and a 10k

Ω thermis-

tor/resistor. This simple application circuit appears in

Figure 3. Optionally (as shown in Figure 4), an LED can

can be used to trim down the maximum charge voltage

from 4.2V, and/or a reverse-current-protection diode

can be added in line at the source.

If the input is shorted, the MAX1679 will not allow current

to flow from BATT back through IN to the source.

However, the body diode inherent in the enhancement-

mode FET would still allow the cell to discharge rapidly.

To prevent this, add a power Schottky diode between

the source and IN as in Figure 4.

Adjusting the Battery Regulation Voltage

A typical Li+ cell should be charged at a constant cur-

rent until it reaches a voltage of about 4.2V, then

charged at this voltage until the current decays below a

predetermined level. The MAX1679 provides a simple

way to reduce this maximum target voltage with a sin-

gle resistor between ADJ and GND. Internally, ADJ

connects to a precision 1.4V reference through a 10k

Ω

resistor. Leave ADJ open for a battery regulation volt-

GND to form a voltage divider for lower battery regula-

A 1% tolerance resistor at ADJ degrades system accu-

racy by only a fraction of a percent. For example, an

multiplied by the percent change in the battery regula-

tion voltage, or (1%)(2.4%) = 0.024%.

R

10k

V

V'

1

ADJ

BR

BR

=

−

Ω

¢

Single-Cell Li+ Battery Charger

for Current-Limited Supply

_______________________________________________________________________________________

9

CHG

High impedance

No battery or no charger, or

cell voltage < 2.2V

CONDITION

Low (LED on)

Fast-charge or pulsed top-

off charge in progress

LED blinking 0.5sec on (low),

3.5sec off (high impedance)

Charge cycle complete

2Hz, 50% duty factor (LED

flashing)

Fast-charge timer expiration

or initial prequalification state

perature fault)

Table 2. CHG Output States

THERM

ADJ

0.001

µF

IN

CHG

GATE

PMOS FET

FAIRCHILD FDC638P

-4.5A, -20V

0.07

CURRENT-LIMITED

WALL CUBE

(800mA, 6V max)

BATT

TSEL*

10k

SINGLE

Li+

CELL

2.2

µF

*SEE TABLE 3 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.

MAX1679

NTC THERMISTOR

FENWAL

140-103LAG-RBI

(10k

Ω AT 25°C)

THERM

IN

CHG

GATE

PMOSFET

FAIRCHILD FDC638P

-4.5A, -20V

0.07

SCHOTTKY DIODE

30V, 1A

ZETEX ZHCS1000

CURRENT LIMITED

WALL CUBE

(800mA, 6V max)

BATT

TSEL*

410k

1%**

SINGLE

Li+

CELL

2.2

µF

0.001

µF

5mA

LED

*SEE TABLE 3 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.

FOR 4.2V.

MAX1679

ADJ

Figure 3. Simple Application Circuit

Figure 4. Application Circuit Including LED, Thermistor, and

Reverse-Current Protection Diode