When Vin is greater than Vout on a boost converter

[lindworm]

As the SX1308 is a really cheap, I'd really like to get an update if you ended up testing it and if it worked as intended or if you had to use another solution.

[NiHaoMike]

Can you use a 3S pack? They're 12.6V fully charged or 12.3V with a conservative 4.1V/cell charge voltage.

[lindworm]

Thanks for your reply, but my voltage requirements are different than the original OPs, so specific chemistries based on the original post aren't exactly what I'm looking for.
I was mainly interested in experiences with using a cheap boost converter when V_in > V_out.

My specific application is powering a circuit which can only operate between 3.3 and 3.6V from 5V USB and a LiIon Battery (~3V cutoff - 4.2V when charging).

[NiHaoMike]

My specific application is powering a circuit which can only operate between 3.3 and 3.6V from 5V USB and a LiIon Battery (~3V cutoff - 4.2V when charging).

Just use a 100% duty cycle capable buck converter and either a linear or switching charger depending on efficiency requirements. You won't gain much between 3.3V and 3V.

[bson]

You can also get a boost-LDO for low power applications.  Such as some variants of the TI TPS61098x.  These have the benefit of always offering an LDO output, even during boost operation.

[lindworm]

Thanks for the tip, at first glance the TPS61098X chips look really nice. But after reading the datasheet I realized that the LDO output is only supposed to be used for low-power sensors and can only supply <200mA and not the full 500mA of the boost circuit. There is also a significant voltage drop/difference. E.g. the TPS610981DSE supplies 3.3V on the main output (passtrough when V_in>V_out) but only 3.0V on the LDO output (p. 7).

But if you know of any similar devices with higher output  currents I'd be stoked.
edit: I found the TPS61025 from a very similar series which seems really nice. I has only one output and can apparently also linearly regulate the full 1A it can step-up.

I've also thought about highly integrated solutions of battery charger + boost converter. I've found plenty with a fixed boost voltage of 5V@500mA for USB-OTG, the cheapest (~0,7€) ones beeing the FAN54015 and FAN54005. 500mA are great, but I'd need another buck or LDO to get my 3.3V from the 5V supply. This again leads to increased size and BOM cost while not beeing ideal efficiency wise.

Then there are some with an additional 1.8V output, but I've only come across one (well two extremely similar ones) with 3.3V output (with an internal buck converter + LDO). That would be the TI BQ25120A BQ25122, but they can only supply 300mA via the buck converter. In theory they can spit out another 100mA via the LDO, but until someone corrects me I'd be pessimistic about simply connecting the outputs of both to get up to 400mA.
Anyway, the BGA packages of all mentioned chips are a real "treat" to solder manually.

So my next idea was maybe a SEPIC design behind the charger with something like the TPS61170 (DFN), the LM3481 (more power, sweet VSSOP package) ot the LM2735 in an awesome SOT23 variant. They are a bit more expensive tough. They also need at least 3V, which is fine for my application but would make them unusable to use with NiMH and other chemistries.

If someone knows of the proverbial jack of all trades device or other nifty solutions, let me know!

[NiHaoMike]

Wouldn't using a LDO defeat the point of being able to use the extra last 0.3V of the discharge curve? As in at the 3.7V or so over the majority of the discharge curve, a 3.3V LDO would waste about 11% of the power.

[lindworm]

Yeah, for now I decided to use a TPS63031 and to stick with the MCP73831 charging IC.
It's the cheapest combination I could find of chips from known manufacturers which are actually available in relevant numbers.
Even though the BOM cost increases because of the second inductor, X7R caps and so on it's still a lot cheaper compared to an all-in-one solution, which are pretty close to unobtainium or can't deliver enough power.

OT: By the way, before I used the MCP1603 and Microchip demands in the datasheet (p.17, lower right corner) that the output capacitor should not exceed 22µF. I guess that's because more capacitance could destabilize the resonant circuit. However I haven't come across this requirement in most other datasheets and the following circuitry explicitly requires several 22µF caps not even counting the C_out of the MCP.
Before deciding to use the TPS I added a lowpass filter with a 4,7µF coil + 47µF inductor to effectively isolate the 2MHz oscillation from whatever sits behind it. Do you think this would have actually been necessary or would it have sufficed to have a few cm of trace behind the output cap, leading to enough phase distortion to not influence the resonant circuit too badly?