High impedance, strong pull-ups, weak current sources -- a little help?

Other Hardware General Electronics
1

Hi everyone. I'm trying to implement a battery charger IC into a project but I'm having a little trouble understanding how to read the charging state. The datasheet says:

When the battery is nearing full charge, the charger enters
the constant-voltage portion of the charge cycle and the
charge current begins to drop. When the charge current
drops below 1/10 of the programmed current, the charge
cycle ends, and the strong pull-down is replaced by the
20?A pull-down, indicating that the charge cycle has
ended. If the input voltage is removed or drops below the
undervoltage lockout threshold, the CHRG pin becomes
high impedance. Figure 3 shows that by using two different
value pull-up resistors, a microprocessor can detect
all three states from this pin.

To detect when the LTC4054L is in charge mode, force the
digital output pin (OUT) high and measure the voltage at
the CHRG pin. The N-channel MOSFET will pull the pin
voltage low even with the 2k pull-up resistor. Once the
charge cycle terminates, the N-channel MOSFET is turned
off and a 20?A current source is connected to the CHRG
pin. The IN pin will then be pulled high by the 2k pull-up
resistor. To determine if there is a weak pull-down current,
the OUT pin should be forced to a high impedance state.
The weak current source will pull the IN pin low through the
800k resistor; if CHRG is high impedance, the IN pin will
be pulled high, indicating that the part is in a UVLO state.

I'm not quite clear about the entire procedure. Part of the problem is that I still don't quite understand what a high impedance state is when it comes to microcontrollers, could someone explain that?

From what I get (given the proper wiring, the datasheet provided schematics so I should be set with that) -- I need to set an output pin high and read an input pin. If it reads low the IC is in a charging state, if it reads high the IC is done charging. If I then set the output pin low and read the input pin as high, the charger is in a UVLO state.

Is the above correct or have I made a mistake?

Thanks guys,
Alex V.

2

could you tell us which IC it is?

3

A high impedance (or high-Z) state is when a signal that is nominally an OUTPUT is disconnected so that it floats at the whim of an external pull-up or pull-down or whatever. CMOS outputs have normally either an n-chan FET pulling them low or a p-chan FET pulling them high. If neither output FET is activated the pin becomes high impedance (or high-Z or "tristated").

This is normally done to allow another chip to take over control of that signal (or to reduce power consumption).

4

Ok. that wasn't what I meant but I figured out which IC you are using.

The IC you are using is pretty uncommon, which is probably why you are having confusion. My best advice would be to use a Max1551 or a Max1555 instead. They are very small and simple.