Which DC-DC is better to use for 2.7-4.2 => 4v conversion?

Question

Do it yourself

Good day!
Tell me how best to power the GSM module from li-ion, provided that the module is outside in winter and summer, i.e. temperatures down to -20, while the lithium voltage drops and the GSM module does not start if directly powered.
The allowable range is 3.4-4.5v, current up to 1.5A at peak (but here you can smooth it out with conders)
so far I see 2 solutions:
1. sepic
2. step-up with switching to direct battery power Please
advise a decent step-up, so that it can work in bypass mode if the input voltage is greater than it should be at the output? Well, or sepic?

Answer 1

In particular, I am strongly advised to use the LM2621 in SEPIC enable mode for a similar task. Unless the task is somewhat different - the voltage at the output is 3.0 ~ 3.3V, so an automatic Step-Up / Step-Dn is required. Unless, it seems, another chip will have to be installed, probably LTC3113 / TPS63020 and others. In general, it is a difficult task.
Maybe use two cells and put Step-Down?
Naturally, with cell balancing.