5

Rev.1.3

11 -

Where Vfb is the feedback voltage and Vout is the output voltage. Thus the output voltage is:

Vout = 0.923 × (R1 + R2) / R2

R2 can be as high as 100kΩ, but a typical value is 10kΩ. Using the typical value for R2, R1 is determined by:

R1 = 10.83 × (Vout − 0.923) (kΩ)

3. Inductor

The inductor is required to supply constant current to the output load while being driven by the switched input

voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage.

However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower satura-

tion current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the

inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor

current is below the maximum switch current limit. The inductance value can be calculated by:

L = [ Vout / (fs × ΔIl) ] × (1 − Vout/Vin)

Where Vout is the output voltage, Vin is the input voltage, fs is the switching frequency, and ΔIl is the peak-to-

peak inductor ripple current.

Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current

can be calculated by:

Ilp = Iload + [ Vout / (2 × fs × L) ] × (1 − Vout/Vin)

Where Iload is the load current.

The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI re-

quirements.

4. Optional Schottky diode

During the transition between high-side switch and low-side switch, the body diode of the low-side power

MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky

diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 1 lists example

Schottky diodes and their Manufacturers.

Part number Voltage and current rating

Vendor

B130

30V, 1A

Diodes Inc.

SK13

30V, 1A

Diodes Inc.

MBRS130

30V, 1A

International Rectifier

Table 1: Diode selection guide.

5. Input capacitor

The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the

AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the

best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also

suffice. Choose X5R or X7R dielectrics when using ceramic capacitors. Since the input capacitor (Cin) absorbs

the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor

can be estimated by:

The worst-case condition occurs at Vin = 2Vout, where Icin = Iload/2. For simplification, choose the input ca-

pacitor whose RMS current rating greater than half of the maximum load current.