Datasheet

23Maxim Integrated
Dual-Input Linear Chargers, Smart Power Selector
with Advanced Battery Temperature Monitoring
MAX8934A–MAX8934E
age where charge current starts tapering down. When
charge current decreases to 10% of the fast-charge
current, the charger enters a brief 15s top-off, (60min
for the MAX8934B and MAX8934D) and then charging
stops. If the battery voltage subsequently drops below
the recharge threshold, charging restarts and the timers
reset.
Charge Enable (CEN)
When CEN is low, the charger is on. When CEN is high,
the charger turns off. CEN does not affect the SYS out-
put. In many systems, there is no need for the system
controller (typically a microprocessor) to disable the
charger, because the MAX8934_ Smart Power Selector
circuitry independently manages charging and adapter/
battery power hand-off. In these situations, CEN can be
connected to ground.
Setting the Charge Current
ISET adjusts charge current to match the capacity of the
battery. A resistor from ISET to ground sets the maximum
fast-charge current:
I
CHGMAX
= 2000 x 1.5V/R
ISET
= 3000V/R
ISET
Determine the I
CHGMAX
value by considering the char-
acteristics of the battery. It is not necessary to limit the
charge current based on the capabilities of the expected
AC adapter/USB charging input, the system load, or
thermal limitations of the PCB. The MAX8934_ automati-
cally adjusts the charging algorithm to accommodate
these factors.
Monitoring the Charge Current
In addition to setting the charge current, ISET can also
be used to monitor the actual current charging the bat-
tery. See Figure 4. The ISET output voltage is:
V
ISET
= I
CHG
x 1.5V/I
CHGMAX
= I
CHG
x R
ISET
/2000
where I
CHGMAX
is the set fast-charge current and I
CHG
is the actual battery charge current. A 1.5V output indi-
cates the battery is being charged at the maximum set
fast charge current; 0V indicates no charging. This volt-
age is also used by the charger control circuitry to set
and monitor the battery current. Avoid adding more than
10pF capacitance directly to the ISET pin. If filtering of
the charge-current monitor is necessary, add a resistor
of 100kI or more between ISET and the filter capacitor
to preserve charger stability.
Note that the actual charge current can be less than the
set fast-charge current when the charger enters voltage
mode or when the input current limiter or thermal limiter
reduces charge current. This prevents the charger from
overloading the input source or overheating the system.
Charge Termination
When the charge current falls to the termination thresh-
old and the charger is in voltage mode, charging is
complete. Charging continues for a brief 15s top-off
period (60min for the MAX8934B and MAX8934D) and
then enters the DONE state where charging stops. The
DONE current threshold (I
DONE
) is set to 10% of the fast-
charge current setting. Note that if charge current falls
to I
DONE
as a result of the input or thermal limiter, the
charger does not enter the DONE state. For the charger
to enter the DONE state, the charge current must be
less than I
TERM
, the charger must be in voltage mode,
and the input or thermal limiter must not be reducing the
charge current. The charger exits the DONE state, and
fast-charge resumes if the battery voltage subsequently
drops 104mV or if CEN is cycled.
Charge Status Outputs
Charge Output (CHG)
CHG is an open-drain, active-low output that is low dur-
ing charging. CHG is low when the battery charger is in
its prequalification and fast-charge states. When charge
current falls to the charge termination threshold and the
charger is in voltage mode, CHG goes high impedance.
CHG goes high impedance if the thermistor causes the
charger to enter temperature suspend mode.
Figure 4. Monitoring the Battery Charge Current with V
ISET
1.5
V
ISET
0
0
2000 (1.5V/R
ISET
)
BATTERY CHARGING CURRENT (A)
MONITORING THE BATTERY
CHARGE CURRENT WITH V
ISET
DISCHARGING
V
ISET
(V)