MAX1925 datasheet(11/16 Pages) MAXIM

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linear 4mA current source (PREQUAL1). When the cell

voltage exceeds 2V, the cell is charged with 10% of the

programmed fast-charge current (IFASTCHG) until it

reaches 3V. When the cell voltage is above 3V, fast

charging occurs at the full set current. If the cell fails to

reach the next prequalification threshold before a set

time (see tPREQUAL1 and tPREQUAL2 in the Timer

Capacitor and Fault Modes section), charging stops, a

fault alarm is set, and the CHG output blinks.

Figures 2 and 3 show charging behavior typical Li+ cell.

The MAX1925/MAX1926 remain in fast-charge mode

until the battery voltage reaches regulation and the

charge current drops below 1/8th of IFASTCHG. The

charger then enters full topoff mode and the CHG LED

is turned off. In full topoff mode, the controller continues

to operate as in fast-charge mode, except that it

remains in constant-voltage mode (CVM) unless the bat-

tery voltage falls. After every tPREQUAL1 (see the Timer

Capacitor and Fault Modes section) the charger enters

the Kelvin state (for 2 CT clock cycles, 60ms with CCT =

100nF) where charge current is interrupted so that the

battery voltage can be accurately measured.

The MAX1925/MAX1926 do not enter done mode until

tFULLCHG has been reached. If the battery is removed

Switch-Mode 1-Cell Li+ Chargers

______________________________________________________________________________________

CVM

4mA

C/10

C/8

PREQUAL1

CHARGE CURRENT

2mA

C/10

tPREQUAL2 = t/16

4.2V

C/8

FAULT

NORMAL VOLTAGE

PROFILE

NORMAL

CURRENT

PROFILE

FASTCHG

FULL TOPOFF

CCM

CVM

CCM

FASTCHG

PREQUAL2

FAULT

tPREQUAL1 = t/1024

CHARGE TIME (tFULLCHG = 3 HOURS WITH CCT = 100nF)

CHG LED OFF

Figure 3. Charging Current and Voltage Timing Diagrams

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