MAX1679 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – Single-Cell Li Battery Charger for Current-Limited Supply

Single-Cell Li+ Battery Charger
for Current-Limited Supply
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 10kΩ thermis-
tor/resistor. This simple application circuit appears in
Figure 3. Optionally (as shown in Figure 4), an LED can
be added as a charge-state indicator, a resistor (R_ADJ)
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-
Table 2. CHG Output States
CONDITION
No battery or no charger, or
cell voltage < 2.2V
CHG
High impedance
Fast-charge or pulsed top-
off charge in progress
Low (LED on)
Fast-charge timer expiration
or initial prequalification state
(V_BATT < 2.5V or initial tem-
perature fault)
2Hz, 50% duty factor (LED
flashing)
Charge cycle complete
LED blinking 0.5sec on (low),
3.5sec off (high impedance)
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 10kΩ
resistor. Leave ADJ open for a battery regulation volt-
age (V_BR) of 4.2V; connect a 1% resistor from ADJ to
GND to form a voltage divider for lower battery regula-
tion voltage (V_BR¢)_. Select the external value using:
R_ADJ =
10kΩ
V_{BR − 1}
V_BR '_¢
A 1% tolerance resistor at ADJ degrades system accu-
racy by only a fraction of a percent. For example, an
R_ADJ of 410kΩ ±1% reduces the battery regulation volt-
age by 2.4% (V_BR¢ = 4.1V from equation above, and
(V_BR¢ - V_BR) / V_BR = (4.1 - 4.2) / 4.2 = -2.4%). Therefore,
the additional system error is simply the R_ADJ tolerance
multiplied by the percent change in the battery regula-
tion voltage, or (1%)(2.4%) = 0.024%.
CURRENT-LIMITED
WALL CUBE
(800mA, 6V max)
PMOS FET
FAIRCHILD FDC638P
-4.5A, -20V
0.07Ω AT V_GS = -2.5V
CURRENT LIMITED _{SCHOTTKY DIODE}
WALL CUBE
30V, 1A
(800mA, 6V max) _{ZETEX ZHCS1000}
PMOSFET
FAIRCHILD FDC638P
-4.5A, -20V
0.07Ω AT V_GS = -2.5V
GATE
IN
BATT
MAX1679
CHG
0.001µF
ADJ THERM
10k
TSEL*
2.2µF
SINGLE
Li+
CELL
0.001µF
GATE
5mA
LED
R_ADJ
410k
1%**
IN
BATT
MAX1679
CHG
TSEL*
ADJ THERM
2.2µF
NTC THERMISTOR
FENWAL
140-103LAG-RBI
(10kΩ AT 25°C)
SINGLE
Li+
CELL
*SEE TABLE 3 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.
Figure 3. Simple Application Circuit
*SEE TABLE 3 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.
** R_ADJ SETS BATTERY REGULATION VOLTAGE TO 4.10V; LEAVE OPEN
FOR 4.2V.
Figure 4. Application Circuit Including LED, Thermistor, and
Reverse-Current Protection Diode
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