Data Sheet

LT7200S

analog.com

Rev 0

18 of 35

Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard”, which means that

inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in

inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate.

To avoid overheating and poor efficiency, choose an inductor with an RMS current rating that is greater than the

maximum expected output load of the application. In addition, the saturation current (typically labeled ISAT) rating

of the inductor must be higher than the load current plus 1/2 of inductor ripple current:

IL(PEAK) = IOUT(MAX) +

∆IL

2

Different core materials and shapes change the size/current and price/current relationship of an inductor. Toroid or

shielded pot cores in ferrite or permalloy materials are small and do not radiate much energy, but generally cost

more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly

depends on the price versus size requirements and any radiated field/EMI requirements. New designs for surface

mount inductors are available from Coilcraft, Toko, Vishay, NEC/Tokin, Cooper, TDK and Wurth Electronik. Refer to

Table 5 for more details.

Table 5. Inductor Selection Table

INDUCTANCE (μH)

DCR (mΩ)

MAX CURRENT (A)

DIMENSIONS (mm)

HEIGHT (mm)

Coilcraft XEL5030 Series

0.42

3

23.5

5.28 x 5.48

3.2

0.6

4.44

22

5.28 x 5.48

3.2

1

7

16.9

5.28 x 5.48

3.1

1.2

8.8

15.3

5.28 x 5.48

3.1

1.5

9.9

15

5.28 x 5.48

3.1

Selecting the Input Capacitor (CIN)

The input capacitance, CIN, is needed to filter the square wave current at the drain of the top power MOSFET. To

prevent large voltage transients from occurring, use a low ESR input capacitor sized for the maximum RMS current.

The maximum RMS current is given by:

IRMS ≅ IOUT(MAX)

VOUT

VIN

√

VIN

VOUT

− 1

This formula has a maximum at VIN = 2VOUT, where:

IRMS ≅

IOUT

2

This simple worst-case condition is commonly used for design because even significant deviations do not offer much

relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life, which

makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required.

Several capacitors may also be paralleled to meet size or height requirements in the design. For low input voltage

applications, sufficient bulk input capacitance is needed to minimize transient effects during output load changes.