Leave regulated outputs of LTC3554 floating

We're evaluating the LTC3554 for our next design. We need a well-regulated 3.3V, running of a LiPo battery (one cell). The circuit must be able to switch seamlessly between battery supply and charging, as well as powering three high-intensity LEDS (total of ~50mA) during charging, while still being able to determine whether charging the battery or not.

Since the LTC3554 does not boost the battery voltage, we need a separate buck-boost, which we are fine with. However, we don't need the regulated 3.3V and 1.2V that the LTC3554 supplies. We will use Vout to supply a buck-boost (3.3V) and a boost (5V).

Our idea is to do the following.

  • Leave SW1, SW2, FB1 and FB2 floating
  • Tie ~ON, SUSP, PWR_ON1 and PWR_ON2 to ground
  • Tie STBY, and BVIN to VOUT

I'm curious if this will cause any issue during the start-up sequence or (if any adjustments are needed to control the start-up sequence) when "up and running"?

  • 0
    •  Analog Employees 
    on Jun 21, 2018 5:07 PM


    When the LTC3554 powers up by external power or by pushbutton it will try to power up the buck regulators for the first 5 seconds. FB1 and FB2 should be tied to VOUT to prevent the part from trying to switch. After the 5 seconds the bucks will turn off. This will not cause any issues and SW1 and SW2 can float.

    Was the LTC3554 chosen for the PB controller and the power path? We also have parts that have a power path and buck-boost like the LTC3566 or LTC3567 but these parts do not have the PB interface.



  • Thank you for your reply.

    I'll update the schematic so that FB1 and FB2 are connected to VOUT.

    The LTC3554 was chosen primarily because of size and power path. Its actually smaller to have the LTC3554 (3x3mm) and the buck-boost we found (1.41x1.41mm), than to have the LTC3566 or LTC3576 (4x4mm). PBSTAT and PGOOD are left floating.