I am attempting to use the LTC4002 Li-Ion switch-mode battery charger in my project. I'm fairly green with electronics, so some of the conventions used in data sheets probably evade me.
On page 8 of the Linear Technology-supplied data sheet (link below), I see the following description for pin 1:

COMP (Pin 1/Pin 1): Compensation, Soft-Start and Shut- down Control Pin. The COMP pin is the control signal of the inner loop of the current mode PWM. Charging begins when the COMP pin reaches 800mV. The recommended compensation components are a 0.47μF (or larger) capacitor and a 2.2k series resistor. A 100μA current into the compensation capacitor also sets the soft-start slew rate. Pulling the COMP pin below 360mV will shut down the charger.

Ok. Nice description.

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But when I look at their suggested schematic (shown on both page 1 and page 14), it shows Pin 1 (the Comp pin) connected to a 0.47μF cap and then to a 2.2K ohm resistor, then on to ground.

From the description of pin 1, it seems they are implying that I need to be supplying the 800mV, like I need to supply the 100μA of current. But how would I do that when they show the pin tied to ground through a cap and a resistor?

Right now, my board is otherwise complete and ready for fab, but I am loathe to write that check until I'm sure the device will work. I have the parts, but they are all extremely tiny fine-pitch surface-mount parts that are difficult to bread-board without breakout boards...

So: What will the behavior of this part be if laid out like their schematic? Can I leave it as it is? Does the description simply provide some details in case I want more granular control over the charging process?

Any assistance will be hugely appreciated! Thanks!

The data sheet can be found here


1 Answer 1


The 100µA current source on the COMP pin is internal to the part -- see the block diagram on page 9 of the datasheet. This means that the soft-start time is controlled by the value of the capacitor in the RC network connected to the pin -- there's no need to inject an external current into COMP for such a basic function.

  • \$\begingroup\$ Wow! Thanks for the snappy response time! So, when it talks about the charge cycle starting when the voltage is at 800mV and pulling it below 360mV to disable charging, the are implying that I could, if I desire, tap into that signal between the pin and the cap, and either monitor the voltage for obtaining status, or pull the signal to ground, or both, using a gpio pin on my mcu, is that correct? \$\endgroup\$ Dec 28, 2016 at 5:37
  • \$\begingroup\$ @MikeMetcalf -- you can pull the signal to ground with a transistor switched from a GPIO, yes. \$\endgroup\$ Dec 28, 2016 at 12:42

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