DN1 • 7
Since we want the voltage appearing at the IC to be equivalent to one cell,
we first must “divide” the cell voltage by the number of cells in the pack.
The ladder resistors R2 -R24 form an effective voltage divider circuit so that
the BAT (pin 7) voltage will be about 1.25 V per cell. The switch can increase
or decrease the BAT voltage by adding or subtracting “rungs” from the
voltage divider ladder. Another divider network consists of resistors R14 and
R16. This voltage sets up the MCV voltage for the BQ2003 IC. This should
measure 1.8 V when in operation.
Seeing how you’ll want to charge your batteries quickly, you need a high
charging current power supply to back you up. Transistors Q2, Q1, and
components D1 and L1 form the “high current” portion of our “switched-
mode-regulator” circuit. When the MOD output goes “high”, transistor Q2 is
turned on, like a switch. This current then flows into the battery. Resistor R29
(and/or R27) is in series with the current flow and the voltage drop across it
is sensed by IC pin 9, the sense pin. When the sense pin reaches its trigger
point, the transistor is abruptly turned off. When this occurs, the magnetic
field around the coil quickly collapses and causes a reverse voltage “spike”
which is routed through the “catch” diode D1. This energy is recovered and
delivered to the battery cells being charged. This is what provides us with the
high current to quickly charge the cell, but does not dissipate power in the
FET or NPN transistor, making the switched power much more efficient than
a conventional pass transistor type of supply. Another contributing factor to
the charging circuit is the charge rate setup, which is configured using
resistors R26 and 27, as well as test points A - F.
Transistor Q3 is the integral part of our constant current discharging circuit.
When the chip sees a positive going pulse at the DCMD pin, it initiates the
DIS discharge output. With switch S1:10 closed diodes D2 and D4 are
forward biased, causing 1.4VDC to be present at the base of Q3. With 1.4 V
at the base, there is .7 VDC at the emitter, a diode drop in potential lost
through the transistor. With the emitter at .7 VDC, the current through
resistors R10 and R22 is about 140 mA, regardless of the cell voltages. If
switch S1:10 is opened the potential increases to 1.4 VDC. increasing the
current to 280 mA. This will continue to discharge the batteries until they
reach a potential of about .9 volts per cell. The Benchmarq chip then initiates
its own charging sequence.
A few final points concerning the TM1 and TM2 time-out, which are
configured using points G - J. They are dependant on the charge capacity, or
“C” of the pack. We’ll discuss this in more detail when it comes time to
configure these jumpers.