TPS54232 in "pass through" mode

Even at 12.6 V input, you may not regulate well to 12 V output as that is over 95 % duty cycle.  There are some countermeasures you can take to prevent the BOOT UVLO from activating.  A better choice for your application may be TPS54335 (synchronous converter) or TPS54340 (integrated BOOT charge pull down).

Thank you for your answer. I was not concerned about UVLO but about minimum clamp. So TPS54335 would pass input voltage directly throu when it becomes lower than set voltage (12V)?

Chris Glaser said:

Another option is the TPS62110 with its true 100% duty cycle mode.  When Vin drops too low, the high side switch turns on to keep Vout as high as possible.

After inspecting datasheed of TPS62110 I realized maximum input voltage is only 17V, but our apilcations may see input voltageas as high as 26V (6S LiPo batteries). Is there another DC/DC switcher that has 100% duty cycle mode and can handle such high input voltages? 

The trick here is keeping the high side FET on.  For n channel high side, a BOOT voltage above VIN is required.  When that BOOT charge runs out, the the high side is turned off so the BOOT cap can be recharged.  For TPS54232, if the circuit is in eco-mode, the switching ode may not be pulled down to ground so the BOOT cap cannot recharge until Vout falls to Vin - 2.1 V.  TPS54340 has  pull down FET.  TPS62110 uses a p channel high side switch which does not require a BOOT voltage.  I have not tested TPS54335 in your exact condition yet.

No, there is no higher voltage version of the TPS62110.

But how do you have 12.6V at the input with a 6-cell battery stack?

The product will operate with 3S up to 6S batteries. Is there maby some other IC with 100% duty cicle capability?

With an input voltage range that wide, you will need to make some tradeoffs in the power supply design.

As well, your 3 cell pack might have a minimum voltage down to 9V or so.  So, your 12V output would see less than that due to the IR drops.  If the load is ok with seeing maybe 8.5V on its input, then why not just set the output voltage to 8.5V or lower to begin with?  This would greatly reduce high duty cycle issues.

LiPo batteries for use in radio controlled equipment should never go below 3.4V per cell (10.4V on 3S pack). Problem is that load will be LED strip designed for 12V and driving it with lower voltage will reduce LED brightness...

Should we be looking at a 4 switch buck boost instead?