# AP1509 DC-DC Power Supply calculating Inductance?

I'm working on an embedded system which I want to enable USB Host on for a few devices. As I'm supplying USB Devices with current and am not sure of my own on board current requirements I decided I'd aim high and go for the Diodes INC AP1509 which can supply up to 2Amps. Along with the data sheet there is an Application note ANP013 which shows how to calculate the required Inductor and Capacitor.

My question is that the formula used to calculate the Inductance required uses Time on (Ton) in the formula. But there is no indication as to what Ton is. I know that this DC to DC converter will turn on and off at high frequency to give the required output Voltage and current but I've no idea what Ton or Toff will be. Am I missing something? How do you calculate Inductance and Capacitance? I'm a Software Engineer really so I'm sure there might be something very basic here I've missed.

## 1 Answer

For a buck operating in CCM (continuous conduction mode), which is what ANP013 is addressing:

$V_{\text{out}}$ = $D V_{\text{in}}$

and $T_{\text{on}}$ = $\frac{D}{F}$ while $T_{\text{off}}$ = $\frac{1-D}{F}$

The easiest way to calculate the inductance is:

L = $\frac{T_{\text{off}} V_{\text{out}}}{2 I_{\text{Load} (\min) }}$

There is more about choosing buck inductors here. The equation in the referenced question will give an over estimate of inductance, which will be somewhat higher than you get from the equation shown above.