Dual power rails to power the Notecard Question

I have a board that I am working on that works like this. I have a 48v battery that is going though a buck converter to drop the voltage to 5v. This 5v (when active) is then going to power the V_MODEM pins and the LDO that outputs 3v3 for the VIO power.

When the 5v is not present a 3.7v lipo will be enabled and now providing direct battery to V_MODEM pins as well as powering the 3v3 vregulator ldo for the VIO power. The schematic looks like this.

My question is is this ok? The MOSFET’s are going to switch power rails based on whether the 5v input is present or not. However, as you can see the 5V_VDD and the VBAT_OUT share the same pin. But only one is active at a time. Wanted to know if you thought this was ok or would go a different route to achieve this dual power switching mode type of circuit.

Hey @ril3y,

I got two notes from asking internally:

  • Why not always have the 5V exclusively running the Charger, and always have the LiPo powering VMODEM and the VIO?
  • Why not use an efficient 3v3 buck-boost regulator rather than an LDO, such as ISL9120 or TPS63001? That would allow the Notecard to work below 3.3v, which is important IF you want to get the most out of a lipo.


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fyi I just fixed a typo in my earlier reply (in case you had already seen this).

It looks like the TPS63001 only supports up to 1.6A and from what I read you need up to 2-3 A for the vmodem lines. This is why I was supplying the 5v to the vmodem when the battery was not connected.

Are you saying that this buck converter can power both vio and vm at the same time?

Never, ever put a regulator between the battery and VMODEM. We are only talking about VIO for the regulator.

VMODEM should be connected to the LiPo positive terminal directly.

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