FTD6821 datasheet(13/17 Pages) FS

13 / 17 page

Function Description(Cont.)

Inductor Core Selection

Different core materials and shapes will change the

size/current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy

materials are small and don’t radiate much energy, but

generally cost more than powdered iron core inductors

with similar electrical characteristics. The choice of

which style inductor to use often depends more on the

price vs size requirements and any radiated field/EMI

requirements than on what the FTD6821 requires to

operate. Table 1 shows some typical surface mount

inductors that work well in FTD6821 applications.

Table 1. Representative Surface Mount Inductors

CIN and COUT Selection

In continuous mode, the source current of the top

MOSFET is a square wave of duty cycle VOUT/VIN. To

prevent large voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS capacitor current is given by:

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 the

capacitor manufacturer’s ripple current ratings are often

based on 2000 hours of life. This makes it advisable to

further derate the capacitor, or choose a capacitor rated

at a higher temperature than required. Always consult

the manufacturer if there is any question.

The selection of COUT is driven by the required

effective series resistance (ESR). Typically, once the

ESR requirement for COUT has been met, the RMS

current rating generally far exceeds the IRIPPLE(P-P)

requirement. The output ripple DVOUT is determined

by:

where f = operating frequency, COUT = output

capacitanceand DIL = ripple current in the inductor. For

a fixed output voltage, the output ripple is highest at

maximum input voltage since DIL increases with input

voltage.

Aluminum electrolytic and dry tantalum capacitors are

both available in surface mount configurations. In the

case of tantalum, it is critical that the capacitors are

surge tested for use in switching power supplies. An

excellent choice is the AVX TPS series of surface mount

tantalum. These are specially constructed and tested for

low ESR so they give the lowest ESR for a given

volume. Other capacitor types include Sanyo POSCAP,

Kemet T510 and T495 series, and Sprague 593D and

595D series. Consult the manufacturer for other specific

recommendations.

2013.01. 28

13/17

SEMICONDUCTOR

TECHNICAL D ATA

FTD6821

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