Can anyone explain this LED backlight boost circuit to me?

[JGAN]

I'm confused as to why this LED backlight boost circuit has two inductors and two diodes but one output. Normally I would only expect one inductor and one diode. The closest datasheet I could find was for the LM3533, which only has one inductor and diode in the example circuit.


What is the reason behind the extra inductor and diode? PP_VCC_MAIN is nominally 4.2v if that helps at all.

[Siwastaja]

Maybe two-phase operation for reduced ripple and better capacitor utilization? This gets more important as the voltage ratio rises. 4.2V to 35V sounds somewhat challenging, so that could be the reason.

Edit: Taking a better look, this can't be the reason because the inductors are massively different. Another guess: two different converters for different power levels for maximizing low-load efficiency? The 15uH one would be used by switching at a lot lower frequency when drawing nearly no power, to minimize switching losses.

[MagicSmoker]

As mentioned above, this is probably for two different brightness (ie - backlight power) levels, the 15uH inductor reduces the rms current (though not the average) drawn from the battery at low brightness while the 1.5uH indcutor will, presumably, have lower I²R losses at high brightness/power level.

This still wouldn't make much sense (why not wind use a bigger core for the 15uH inductor and wind it with bigger wire) *except* that this schematic claims to be from the iPhone 6S and so one might surmise that the height of any component would be severely constrained, and in this case it probably works out that 2 inductors take up more board area than 1, but at a lot lower height, and the latter was deemed more important than anything else.

[JGAN]

Thanks for the replies,
I was also looking at this patent and it notes that operation of both switches can be simultaneous. It looks like the intention is to allow for a wider range of input voltages. Is the patent design different from my schematic?