Datasheet

MAX1925/MAX1926
Detailed Description
The MAX1925/MAX1926 switch-mode battery chargers
form a complete solution for a single-cell Li+ battery. The
devices include battery undervoltage/overvoltage fault
protection. The MAX1925/MAX1926 use EN and THRM
for shutdown, battery detection, and temperature monitor-
ing. The devices provide outputs to indicate charge sta-
tus (CHG) and presence of input power (ACON).
The MAX1925/MAX1926 include two prequalification
modes that must be passed before the charger enters
the fast-charge state. During fast charge, the charger
operates initially in constant-current mode until the bat-
tery voltage reaches 4.2V. When the battery voltage
has reached 4.2V, the charger operates in constant-
voltage mode. In constant-current mode, the charger
acts as a hysteretic current source, controlling the
inductor’s peak and valley currents. In constant-voltage
mode, the charger regulates the peak and valley of the
output ripple.
Charge Cycle
The MAX1925/MAX1926 initiate PREQUAL when one of
the following occurs:
• When an external power source is connected
• The cell voltage falls to 4V after charging is finished
• EB is toggled
• Input power is cycled
Some Li+ cells can be damaged when fast-charged
from a completely dead state. Moreover, an over-dis-
charged cell may indicate a dangerous abnormal cell
condition. As a built-in safety feature, the MAX1925/
MAX1926 use a two-level prequalification charge to
determine if it is safe to charge. When the cell voltage is
less than 2V, the cell is charged from an internal
Switch-Mode 1-Cell Li+ Chargers
10 ______________________________________________________________________________________
RESET
(ACON IS LOW)
OFF
(ACON HIGH Z)
SHDN
(ACON HIGH Z)
ANY STATE
(INCLUDING FAULT)
EN HIGH AND
IN WITHIN ACON
WINDOW*
EN LOW OR
IN OUTSIDE ACON
WINDOW*
V
BATT
> 2V
(DEBOUNCED)
V
BATT
> 3V
(DEBOUNCED)
FULL TOPOFF
(CHG LED OFF)
DONE
(CHG LED OFF)
KELVIN
(CHG LED OFF)
F_PQ1
F_VB
t
PREQUAL1
TIMES OUT
BEFORE V
BATT
> 2V
F_PQ2
F_FULL
t
PREQUAL2
TIMES OUT
BEFORE V
BATT
> 3V
t
FULLCHG
TIMES OUT
BEFORE ENTERING
FULL TOP-OFF
STATE
10-SECOND
TIMER
FULL-CHARGE
TIMER TIMES OUT
V
BATT
< 4.2V
*SEE TABLE 2 FOR ACON WINDOW
FOR MAX1925/MAX1926
V
BATT
> 4.35V
AND IN WITHIN ACON
WINDOW*
AND EN HIGH
V
IN
WITHIN ACON
WINDOW*
V
IN
< V
BATT
V
IN
> V
BATT
V
IN
< V
BATT
V
IN
> V
BATT
AND V
IN
OUTSIDE ACON WINDOW*
AUTOMATIC RESTART
BATT DROPS TO 4V
V
BATT
> 4.2V
F-TEMP
TEMP
IN RANGE
TEMP OUT
OF RANGE
V
IN
OUTSIDE ACON WINDOW*
PREQUAL1
(I
CHG
= 4mA)
(CHG LED ON)
PREQUAL2
(I
CHG
= C/10)
(CHG LED ON)
FASTCHG
(I
CHG
= C)
(CHG LED ON)
I
CHG
< C/8
(DEBOUNCED)
FAULT
(CHG LED BLINK AT 0.5Hz)
(EXT HIGH FET OFF)
Figure 2. MAX1925/MAX1926 State Diagram