Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
Maxim Integrated 11
MAX1757
The MAX1757 includes a state machine that controls
the charging algorithm. Figure 4 shows the state dia-
gram. Table 1 is the charging state table. When power
is applied, or SHDN input is driven high, the part goes
into the reset state where the timers are reset to zero to
prepare for charging. From the reset state, it enters the
prequalification state. In this state, 1/10 of the fast-
charge current charges the battery, and the battery
temperature and voltage are measured. If the voltage is
above the undervoltage threshold and the temperature
is within the limits, then it will enter the fast-charge
state. If the battery voltage does not rise above the
undervoltage threshold before the prequalification timer
expires, the charging terminates and the FAULT output
goes low. The prequalification time is set by the
TIMER1 capacitor (C
TIMER1
). If the battery is outside
the temperature limits, charging and the timer are sus-
pended. Once the temperature is back within limits,
charging and the timer resume.
In the fast-charge state, the FASTCHG output goes low
and the batteries charge with a constant current (see
Charging Current Regulator
section). If the battery volt-
age reaches the voltage limit before the fast timer
expires, the part enters the full-charge state. If the fast-
charge timer expires before the voltage limit is
reached, charging terminates and the FAULT output
goes low. The fast-charge time limit is set by the
TIMER2 capacitor (C
TIMER2
). If the battery temperature
is outside the limits, charging pauses and the timers
are suspended until the temperature returns to within
the limits.
In the full-charge state, the FULLCHG output goes low
and the batteries charge at a constant voltage (see
Voltage
section). When the charging current drops
below 150mA (330mA peak inductor current), or if the
full-charge timer expires, the state machine enters the
top-off state. In the top-off state, the batteries continues
to charge at a constant voltage until the top-off timer
expires, at which time it enters the done state. In the
done state, charging stops until the battery voltage
drops below the recharge-voltage threshold, at which
time it enters the reset state to start the charging
process again. In the full-charge or the top-off state, if
the battery temperature is outside the limits, charging
pauses and the timers are suspended until the battery
temperature returns to within limits.
Voltage Regulator
Li+ batteries require a high-accuracy voltage limit while
charging. The MAX1757 uses a high-accuracy voltage
regulator (±0.8%) to limit the charging voltage. The bat-
tery regulation voltage is nominally set to 4.2V per cell
and can be adjusted ±5% by changing the voltage at
the V
ADJ
pin between reference voltage and ground.
By limiting the adjust range of the regulation voltage, an
overall voltage accuracy of better than 1% is main-
tained while using 1% resistors. CELL sets the cell
count from 1 to 3 series cells (see
Setting the Battery
Regulation Voltage
section).
An internal error amplifier (GMV) maintains voltage reg-
ulation (Figure 3). The GMV amplifier is compensated
at CCV. The component values shown in Figure 1 pro-
vide suitable performance for most applications.
Individual compensation of the voltage regulation and
current regulation loops allows for optimum stability.
Charging Current Regulator
The charging current-limit regulator limits the charging
current. Current is sensed by measuring the voltage
across the internal current-sense resistor R
CS
between
BATT and CS. The voltage at ISETOUT adjusts the
charging current. Full-scale charging current is
achieved when ISETOUT is connected to REF.
The charging current error amplifier (GMI) is compen-
sated at CCI. A 0.1µF capacitor at CCI provides suit-
able performance for most applications.
