QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 783

NIMH/NICD CHARGER

1

LTC4060

DESCRIPTION

Demonstration circuit 783 is a complete Fast Battery

Charger capable of charging 1, 2, 3 or 4 series con-

nected NiMH or NiCd cells. Charge termination in-

cludes - V, peak voltage and a safety timer. Included

on the board are jumpers to select NTC thermistor

functions, battery chemistry, number of series cells,

safety timer length, recharge voltage and two LEDs

to indicate charger status. A power path FET is also

included that provides a low loss path between the

battery and an external system load when the input

power is removed. Terminals are included for PAUSE

and SHUTDOWN.

The power portion of the circuit consists of a PNP

power transistor constant current source which can

provide up to 2A of charge current under some con-

ditions. Jumpers are provided for programming

charge currents of 0.5A, 1A or 2A. Other charge cur-

rents can be programmed by adding an external re-

sistor to the “PROG” terminal post. Charge current

greater than 2A can also be programmed by parallel-

ing an external current sense resistor (R5) with the

internal LTC4060 sense resistor). See “Extending

Charge Current” section of the data sheet for more

information. For more power dissipation capability

than the pc board can provide, terminals are in-

cluded for adding an external PNP pass transistor

mounted on a suitable heat sink.

Also included on this board is a power path MOSFET

that provides a low resistance path between the bat-

tery and a load (SYSTEM LOAD) when the input

power is removed. When the input voltage is pre-

sent, the load is powered from the input voltage.

The pc board used for this demo circuit is a 4 layer

board with generous amounts of copper surrounding

the heat producing components. It also includes vias

to internal copper layers to maximize the power

dissipation capabilities. Since this is a linear charger,

under some conditions, the power dissipated by the

charger can be high. To minimize power dissipation,

close attention to the input voltage must be ob-

served.

Charging NiMH or NiCd batteries.

Nickel based batteries are typically charged with a

constant current and allowing the battery voltage to

rise to the level it requires (within limits) to force this

current. When batteries are fast charged (1 to 2

hours) the charge cycle must be terminated when

the battery reaches full charge. This demo circuit

determines when the battery is fully charged using

the - V method with peak voltage and timer as a

backup termination.

The – V method measures the battery voltage as it

is being charged with a constant current. The voltage

will rise as the battery accepts charge. When it ap-

proaches full charge, the battery voltage will begin

rising faster, reach a peak, then begin to drop. When

the battery voltage drops a fixed number of mV (de-

pending on battery chemistry), the battery is fully

charged and the charge cycle terminates.

For the - V charge termination method to work cor-

rectly, the charge current must be programmed to a

sufficiently high current level (C/2 to 2C). “C” is a

measure of cell capacity in mAHr. For example, to

charge an 1800 mAHr battery at 1C would require a

charge current of 1.8A.

Design files for this circuit board are available.

Call the LTC Factory.