# USB powered Switch Mode Li-ion Charger

I want to design a Switch Mode Li-ion charger which uses 5V as input and charges a single cell Li-ion cell.
I am referring this AppNote

On page 4 they have given the equation to find inductor value i.e. $$L = (V_i – V_{sat} – V_o) \times \frac{T_{on}}{2 \times I_{max}}$$ If I use 5V Vin and want to charge at 1A current, the inductor value comes out to be very low. I assume Vo=4.2V. Ton=1.32$\mu$s (50% duty cycle) for 376KHz PWM.

So I am confused as to how they use switch mode chargers in mobile phones.

This is the schematic of typical charger:

simulate this circuit – Schematic created using CircuitLab

so I was wondering with 5V input and voltage drops at MOSFET and schottky diode would I have enough voltage to charge the battery?

• What is the problem? Why would that not work for phones? One of the main points of modern switch mode converter design is to minimise inductor value, as otherwise they are bulky and heavy among other things. Don't forget that a 1H inductor is enormous and weighs a ton. – CL22 Apr 7 '15 at 11:27
• @Jodes I have added my doubt in the question. – Sajid Apr 8 '15 at 7:12