TPS53126 dual switcher suggested design

Hello Bruno,

First, you'll need to decide at which switching frequency you want to run the TPS53126?  The TPS53126 allows the user to select 350kHz or 700kHz.  350kHz will offer higher efficiency, but will require larger inductors and ouptut capacitors than the higher 700kHz switching frequency.

Once you decide on a switching frequency, you'll be able to size your inductors.  Inductors are typically selected to generate a peak to peak ripple current of 20-40% of the full load current.  Larger inductance will give you less ripple at the output while smaller inductors will be physically smaller and allow faster transient response.  Since you have commented about the size of the existing solution, consider 40% of your load currents.

To calculate the inductor you want, divide the Volt-Seconds you'll apply to the inductor by the peak to peak ripple current you'll allow in the inductor.  This looks like:

L = (Vin - Vout) / Ipkpk * Vin/Vout * 1/Fsw

For 700kHz, 12V to 1V and a 1.2A peak to peak ripple (about 40% of full load) that comes to  1.0uH.  The inductor needs to be designed for the full load current 3.0A (thermal current rating) and the peak current (Iout(max) + 1/2 Ipk-pk) or 4.1A (saturation rating).  Since the over current protection will be set above your maximum load current and the TPS53126 uses valley detection for current limit, you may want to consider a higher saturation current.

IHLP1212BZER1R0M11 meets these requirements in a 0.14" x 0.12" package and will offer you smaller size.

For MOSFETs, at these low current levels you can consider Dual MOSFETs (a single SO-8 package with both high-side and low-side MOSFET) but this will require a comprimise between the high-side and low-side MOSFET performance since most dual packaged FETs offered use the same MOSFET for both devices.

There are some smaller packages available for single FETs such as TI's 3mmx3mm QFN or Vishay's 1206-8 and PowerPAK SC-70-8 that will offer the power handling capabilities necessary for this application.  I would look for a high-side MOSFET with Rdson between 15 and 20mOhms when driven to 5V and a low-side MOSFET with 8-12mOhms Rdson, again when driven to 5V.

I hope this helps.