Datasheet

24 Maxim Integrated
Dual-Input Linear Chargers, Smart Power Selector
with Advanced Battery Temperature Monitoring
MAX8934A–MAX8934E
When the MAX8934_ is used with a microprocessor (FP),
connect a pullup resistor between CHG and the logic I/O
voltage to indicate charge status to the FP. Alternatively,
CHG can sink up to 20mA for an LED indicator.
Charge DONE Output (DONE)
DONE is an open-drain, active-low output that goes low
when charging is complete. The charger enters its DONE
state 15s (60min for the MAX8934B and MAX8934D)
after the charge current falls to the charge-termination
threshold and the charger is in voltage mode. The char-
ger exits the DONE state, and fast-charge resumes, if the
battery voltage subsequently drops 104mV, or if input
power or CEN is cycled. When the MAX8934_ is used in
conjunction with a FP, connect a pullup resistor between
DONE and the logic I/O voltage to indicate charge status
to the FP. Alternatively, DONE can sink up to 20mA for
an LED indicator.
Fault Output (FLT) and Charge Timer
FLT is an open-drain, active-low output that goes low
during a battery fault. The fault state occurs when either
the prequal or fast-charge timer expires. The prequal
and fast-charge fault timers are set by C
CT
:
CT
PQ
C
PREQUAL: t 30min
0.068 F
= ×
F
CT
FC
C
FAST CHARGE: t 300min
0.068 F
= ×
F
TO
TOP OFF: t 15s (60 minutes for the
MAX8934B and MAX8934D)
− =
While in fast-charge mode, a large system load or device
self-heating can cause the MAX8934_ to reduce charge
current. Under these circumstances, the fast-charge
timer adjusts to ensure that adequate charge time is still
allowed. Consequently, the fast-charge timer is slowed
by 2x if charge current is reduced below 50% of the pro-
grammed fast-charge level. If charge current is reduced
to below 20% of the programmed level, the fast-charge
timer is paused. The fast-charge timer is not adjusted
if the charger is in voltage mode where charge current
reduces due to current tapering under normal charging.
To exit a fault state, toggle CEN or remove and reconnect
the input source(s). Note also that thermistor out of range
or on-chip thermal-limit conditions are not considered
faults. When the MAX8934_ is used in conjunction with a
FP, connect a pullup resistor between FLT and the logic
I/O voltage to indicate fault status to the FP. Alternatively,
FLT can sink up to 20mA for an LED indicator.
Thermistor Monitor
The MAX8934_ thermistor monitor is configured to
execute JEITA recommendations regarding Li+/Li-Poly
battery charging by adjusting the fast charge current
and/or the charge termination voltage accordingly (see
Figures 6 and 7). Connect the THM input to an external
negative temperature coefficient (NTC) thermistor to
monitor battery or system temperature. Since the therm-
istor monitoring circuit employs an external bias resistor
from THM to THMSW, the thermistor is not limited only
to 10kI (at +25NC). Any thermistor resistance can be
used as long as the value of R
THMSW
is equivalent to
the thermistor’s +25NC resistance. The MAX8934_ THM
thresholds are optimized for a thermistor Beta of 3964
or 3477 (see the Selector Guide). The general relation
of thermistor resistance to temperature is defined by the
following equation:
11
-
T 273 C 298 C
T 25
RR e




+° °


β
= ×
where:
R
T
= The resistance in ohms of the thermistor at
temperature T in Celsius
R
25
= The resistance in ohms of the thermistor at
+25NC
A = The material constant of the thermistor
T = The temperature of the thermistor in NC
Charging is suspended when the thermistor tempera-
ture is out of range (V
THM_T1
> V
THM
> V
THM_T4
). The
charge timers are also suspended and hold their state
but no fault is indicated. When the thermistor comes
back into range, charging resumes and the charge timer
continues from where it left off.
The THMEN input controls THMSW and the thermistor
monitor circuitry when the battery charger is disabled,
providing the user with the means to minimize the bat-
tery current drain caused by the thermistor monitor. The
THMEN input is ignored while the battery is charging,
since the thermistor must be monitored at all times.
While charging, the thermistor monitor is used to auto-
matically adjust the charge termination voltage and/or
the fast-charge current, depending on the sensed bat-
tery temperature and the safety region set at the factory.
While discharging, if the battery temperature exceeds
the THM hot overtemperature threshold and THMEN is
high, the OT flag pulls low. Typical systems connect OT
to a FP input so that the system can safely shut down.