LTC3226 Output voltage regulation issue

Hello Nikhil,

NickAtlas said:

I have set CPO-FB (pin-10) in such a way that it will regulate 5V at output once input is disconnected or goes below PFI threshold (4.6V as per my circuit).

first you have to distinguish, what you call "the output"? The divider at CPO-FB does not regulate the output voltage Vout, it regulates the voltage at the supercap CPO. Of course Vout depends on CPO, but they are not identical. You have set the target for CPO to 5V, but you have set the target for Vout to 4,16V. Your LED is driven by Vout.

NickAtlas said:

Here, as voltages on CPO_FB pin goes below 1.11V, output should be disconnected;

No, Vout will not be switched off. The intetion of the LTC3226 is to deliver backup power as long as possible, not to switch off the output when the supercap is not completely charged.

As long as CPO is high enough, the LDO will regulate Vout to the intended voltage 4,16V. If CPO drops too low for output regulation, then Vout will follow CPO with a certain voltage drop. According to the graph on the first page of the datasheet, Vout will be ~500mV below CPO in that region. (In the graph this region is labeled as "Backup Mode (LDO in dropout)".

If you want to switch anything off when Vout drops too low, you may use RST_FB. Put a divider from Vout to this pin to define the minimum allowed Vout-value. If Vout drops below that voltage, the open drain output RST will swich and you may use this output signal to drive some external switch by yourself.

You may also use the open drain output CAPGOOD to control some external switch. But if you use CAPGOOD to switch your load, you are making poor use of the supercap's charge because CAPGOOD will switch already, when CO sinks just 7,5%.

best regards

Achim