Rainbow Electronics MAX8903A - Manual (Page 17)

MAX8903A
2A 1-Cell Li+ DC-DC Charger for USB*
and Adapter Power
______________________________________________________________________________________ 17
When V BATT is below 3V, the charger enters prequal
mode and the battery charges at 10% of the maximum
fast-charge rate until the voltage of the deeply dis-
charged battery recovers. When the battery voltage
reaches 4.2V and the charge current drops to 10% of
the maximum fast-charge current, the charger enters
the DONE state. The charger restarts a fast-charge
cycle if the battery voltage drops by 100mV.
Charge Termination
When the charge current falls to the termination thresh-
old (I
TERM
) and the charger is in voltage mode, charg-
ing is complete. Charging continues for a brief 15s
top-off period and then enters the DONE state where
charging stops.
Note that if charge current falls to I
TERM
as a result of
the input or thermal limiter, the charger does not enter
DONE. For the charger to enter DONE, charge current
must be less than I
TERM , the charger must be in volt-
age mode, and the input or thermal limiter must not be
reducing charge current.
Charge Status Outputs
Charge Output ( CHG)
CHG is an open-drain, active-low output that indicates
charger status. CHG is low when the battery charger is
in its prequalification and fast-charge states. CHG goes
high impedance if the thermistor causes the charger to
go into temperature suspend mode.
When used in conjunction with a microprocessor (µP),
connect a pullup resistor between CHG and the logic
I/O voltage to indicate charge status to the µP.
Alternatively, CHG can sink up to 20mA for an LED
charge indicator.
Fault Output (
FLT)
FLT is an open-drain, active-low output that indicates
charger status. FLT is low when the battery charger has
entered a fault state when the charge timer expires.
This can occur when the charger remains in its prequal
state for more than 33 minutes or if the charger remains
in fast-charge state for more than 660 minutes (see
Figure 6). To exit this fault state, toggle CEN or remove
and reconnect the input source.
When used in conjunction with a microprocessor (µP),
connect a pullup resistor between FLT and the logic I/O
voltage to indicate charge status to the µP.
Alternatively, FLT can sink up to 20mA for an LED fault
indicator. If the FLT output is not required, connect FLT
to ground or leave unconnected.
Charge Timer
A fault timer prevents the battery from charging indefi-
nitely. The fault prequal and fast-charge timers are con-
trolled by the capacitance at CT (C
CT ).
While in fast-charge mode, a large system load or device
self-heating may cause the MAX8903A to reduce charge
current. Under these circumstances, the fast-charge
timer is slowed by 2x if the charge current drops below
50% of the programmed fast-charge level, and suspend-
ed if the charge current drops below 20% of the pro-
grammed level. The fast-charge timer is not affected at
any current if the charger is regulating the BAT voltage
at 4.2V (i.e., the charger is in voltage mode).
Thermistor Input (THM)
The THM input connects to an external negative tem-
perature coefficient (NTC) thermistor to monitor battery
or system temperature. Charging is suspended when
the thermistor temperature is out of range. The charge
timers are suspended and hold their state but no fault is
indicated. When the thermistor comes back into range,
t
C
F
t
C
F
t s
PREQUAL
CT
FST CHG
CT
TOP OFF
= Ч
= Ч
=
−
−
33
0 15
660
0 15
15
min
.
min
.
μ
μ
1.5
0
MONITORING THE BATTERY
CHARGE CURRENT WITH V ISET
0 1200/R ISET
BATTERY CHARGING CURRENT (A)
DISCHARGING
V ISET (V)
Figure 5. Monitoring the Battery Charge Current with the
Voltage from ISET to GND