DESCRIPTION
Demonstration circuit 783 is a complete Fast Battery
Charger capable of charging 1, 2, 3 or 4 series connected
NiMH or NiCd cells. Charge termination includes
-
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 conditions.
Jumpers are provided for programming
charge currents of 0.5A, 1A or 2A. Other charge currents
can be programmed by adding an external resistor
to the “PROG” terminal post. Charge current
greater than 2A can also be programmed by paralleling
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 included
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 battery
and a load (SYSTEM LOAD) when the input
power is removed. When the input voltage is present,
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 observed.