NiCd/NiMH Battery

Fast-Charge Controllers

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11

The MAX712/MAX713 internally set the trickle-charge

current by increasing the current amplifier gain (Figure

table).

Nonstandard Trickle-Charge

Current Example

Configuration:

Typical Operating Circuit

2 x Panasonic P-50AA 500mAh AA NiCd batteries

C/3 fast-charge rate

264-minute timeout

Negative voltage-slope cutoff enabled

Minimum DC IN voltage of 6V

Settings:

Use MAX713

PGM0 = V+, PGM1 = open, PGM2 = BATT-,

ed to 31.3mV during trickle charge, and the current is

20.7mA. Thus the trickle current is actually C/25, not

C/16.

Further Reduction of Trickle-Charge

Current for NiMH Batteries

The trickle-charge current can be reduced to less than

C/16 using the circuit in Figure 7. In trickle charge,

some of the current will be shunted around the battery,

since Q2 is turned on. Select the value of R7 as follows:

where

current setting

Regulation Loop

The regulation loop controls the output voltage between

the BATT+ and BATT- terminals and the current

through the battery via the voltage between BATT- and

GND. The sink current from DRV is reduced when the

output voltage exceeds the number of cells times

grammed charging current.

For a linear-mode circuit, this loop provides the following

functions:

1) When the charger is powered, the battery can be

removed without interrupting power to the load.

2) If the load is connected as shown in the

Typical

Operating Circuit, the battery current is regulated

regardless of the load current (provided the input

power source can supply both).

Voltage Loop

The voltage loop sets the maximum output voltage

2.5V, then:

(number of cells as determined by PGM0, PGM1)

VLIMIT should be set between 1.9V and 2.5V. If VLIMIT

is set below the maximum cell voltage, proper

termination of the fast-charge cycle might not occur.

Cell voltage can approach 1.9V/cell, under fast charge,

operation .

With the battery removed, the MAX712/MAX713 do not

provide constant current; they regulate BATT+ to the

maximum voltage as determined above.

OPEN

2C

Fast-Charge Rate

Trickle-Charge

V+

4C

BATT-

C/2

PGM3

REF

C

Table 5. Trickle-Charge Current

Determination from PGM3

MAX712

BATTERY

R7

Q2

10k

V+

10k

DRV

D1

Q1

DC IN

GND

Figure 7. Reduction of Trickle Current for NiMH Batteries

(linear mode)