Datasheet

linear 4mA current source (PREQUAL1). When the cell
voltage exceeds 2V, the cell is charged with 10% of the
programmed fast-charge current (I
FASTCHG
) 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 t
PREQUAL1
and t
PREQUAL2
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 I
FASTCHG
. 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 t
PREQUAL1
(see the Timer
Capacitor and Fault Modes section) the charger enters
the Kelvin state (for 2 CT clock cycles, 60ms with C
CT
=
100nF) where charge current is interrupted so that the
battery voltage can be accurately measured.
The MAX1925/MAX1926 do not enter done mode until
t
FULLCHG
has been reached. If the battery is removed
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
______________________________________________________________________________________ 11
CVM
1V
2V
3V
4V
4mA C/10 CC/8
PREQUAL1
CHARGE CURRENT
CELL VOLTAGE
1V
2V
3V
4V
CHARGE CURRENT
2mA
C/10
C
CELL VOLTAGE
t
t
PREQUAL2
= t/16
4.2V
C/8
FAULT
NORMAL VOLTAGE
PROFILE
NORMAL
CURRENT
PROFILE
FASTCHG
FULL TOPOFF
CCM
CVM
CCM
FASTCHG
PREQUAL2
FAULT
t
PREQUAL1
= t/1024
CHARGE TIME (t
FULLCHG
= 3 HOURS WITH C
CT
= 100nF)
CHG LED OFF
CHG LED OFF
Figure 3. Charging Current and Voltage Timing Diagrams