MAX712/MAX713
NiCd/NiMH Battery
Fast-Charge Controllers
8 _______________________________________________________________________________________
_______________Detailed Description
The MAX712/MAX713 fast charge NiMH or NiCd batter-
ies by forcing a constant current into the battery. The
MAX712/MAX713 are always in one of two states: fast
charge or trickle charge. During fast charge, the
current level is high; once full charge is detected, the
current reduces to trickle charge. The device monitors
three variables to determine when the battery reaches
full charge: voltage slope, battery temperature, and
charge time.
Figure 1 shows the block diagram for the MAX712/
MAX713. The timer, voltage-slope detection, and temper-
ature comparators are used to determine full charge
state. The voltage and current regulator controls output
voltage and current, and senses battery presence.
Figure 2 shows a typical charging scenario with batteries
already inserted before power is applied. At time 1, the
MAX712/MAX713 draw negligible power from the bat-
tery. When power is applied to DC IN (time 2), the
power-on reset circuit (see the POWER
-
_ON
-
_RESET sig-
nal in Figure 1) holds the MAX712/MAX713 in trickle
charge. Once POWER
-
_ON
-
_RESET goes high, the device
enters the fast-charge state (time 3) as long as the cell
voltage is above the undervoltage lockout (UVLO) volt-
age (0.4V per cell). Fast charging cannot start until (bat-
tery voltage) / (number of cells) exceeds 0.4V.
When the cell voltage slope becomes negative, fast
charge is terminated and the MAX712/MAX713 revert
to trickle-charge state (time 4). When power is removed
(time 5), the device draws negligible current from the
battery.
Figure 3 shows a typical charging event using tempera-
ture full-charge detection. In the case shown, the bat-
tery pack is too cold for fast charging (for instance,
brought in from a cold outside environment). During
time 2, the MAX712/MAX713 remain in trickle-charge
state. Once a safe temperature is reached (time 3), fast
charge starts. When the battery temperature exceeds
the limit set by THI, the MAX712/MAX713 revert to trick-
le charge (time 4).
0
A
mA
µA
1
0.4
TIME
VOLTAGE
CELL VOLTAGE (V)CURRENT INTO CELL
CELL TEMPERATURE
1.4
1.5
1.3
2
4
53
1. NO POWER TO CHARGER
2. CELL VOLTAGE LESS THAN 0.4V
3. FAST CHARGE
4. TRICKLE CHARGE
5. CHARGER POWER REMOVED
TEMPERATURE
Figure 2. Typical Charging Using Voltage Slope
A
mA
µA
1
TIME
CELL TEMPERATURECURRENT INTO CELL
TLO
THI
243
1. NO POWER TO CHARGER
2. CELL TEMPERATURE TOO LOW
3. FAST CHARGE
4. TRICKLE CHARGE
Figure 3. Typical Charging Using Temperature
A
1.5
1.4
1.3
mA
µA
1
TIME
VREF = VLIMIT
CELL VOLTAGE (V)CURRENT INTO CELL
243
1. BATTERY NOT INSERTED
2. FAST CHARGE
3. TRICKLE CHARGE
4. BATTERY REMOVED
Figure 4. Typical Charging with Battery Insertion
