Datasheet

MAX712/MAX713
NiCd/NiMH Battery
Fast-Charge Controllers
______________________________________________________________________________________ 15
Linear-Mode, High Series Cell Count
The absolute maximum voltage rating for the BATT+ pin
is higher when the MAX712/MAX713 are powered on. If
more than 11 cells are used in the battery, the BATT+
input voltage must be limited by external circuitry when
DC IN is not applied (Figure 15).
Efficiency During Discharge
The current-sense resistor, R
SENSE
, causes a small
efficiency loss during battery use. The efficiency loss is
significant only if R
SENSE
is much greater than the
battery stack’s internal resistance. The circuit in Figure
16 can be used to shunt the sense resistor whenever
power is removed from the charger.
Status Outputs
Figure 17 shows a circuit that can be used to indicate
charger status with logic levels. Figure 18 shows a
circuit that can be used to drive LEDs for power and
charger status.
Q1
D1
R2
150Ω
DC IN
33k
Q2
500Ω
DRV
BATT+
TO 
BATTERY 
POSITIVE 
TERMINAL
MAX712
MAX713
Figure 15. Cascoding to Accommodate High Cell Counts for
Linear-Mode Circuits
100k
100k
MAX712
MAX713
D1
V+
GND
R
SENSE
>4 CELLS
LOW R
ON

LOGIC LEVEL
N-CHANNEL
POWER
MOSFET
*
*
Figure 16. Shunting R
SENSE
for Efficiency Improvement
10k
V
CC
OV = NO POWER
5V = POWER
OV = FAST
V
CC
= TRICKLE OR 
NO POWER
MAX712
MAX713
V+
FASTCHG
MAX712
MAX713


Figure 17. Logic-Level Status Outputs
V+
MAX712
MAX713
R1
470ΩMIN
FASTCHG
DC IN
CHARGE POWER
FAST CHARGE
Figure 18. LED Connection for Status Outputs