SC440AEVB datasheet(14/21 Pages) SEMTECH

14 / 21 page

SC440A

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Applications Information (continued)

false protection, or, simply disable this feature by tying

Pin 15 to GND.

Parallel Operation

When two or more SC440As are operating in parallel for

a large-sized panel application, audible noise may be

observed due to a non-synchronous switching frequency.

The ripple voltage on the input voltage rail will be

modulated by the beat frequency resulting in audible

noise. This situation can be resolved by adding an input

inductor between the input voltage rail and the VIN pin.

This situation can also be improved by adding more input

decoupling capacitors.

Inductor Selection

The inductance value of the inductor affects the converter’s

steady state operation, transient response, and its loop

stability. Special attention needs to be paid to three

specifications of the inductor, its value, its DC resistance

and saturation current. The inductor’s inductance value

also determines the inductor ripple current. The converter

can operate in either CCM or DCM depending on its

working conditions. The inductor DC current or input

current can be calculated as,

OUT

- Input current;

OUT

– Output current;

OUT

– Boost output voltage;

– Input voltage;

η – Efficiency of the boost converter.

Then the duty ratio is,

OUT

– Forward conduction drop of the output rectifying

diode

When the boost converter runs in DCM ( L < L

boundary

), it

takes the advantages of small inductance and quick

transient response while avoiding the bandwidth-

limiting instability of the RHP zero found in CCM boost

converters.

The inductor peak current is,

The converter will work in CCM if L > L

boundary

. Generally

the converter has higher efficiency under CCM and the

inductor peak current is,

peak

For many applications, an inductor with value of 4.7

μH to

μH should be fine, such as for the typical case shown

on page 1. The inductor peak current must be less than its

saturation rating. When the inductor current is close to the

saturation level, its inductance can decrease 20% to 35%

from the 0A value depending on the vendor specifications.

Using a small value inductor forces the converter under

DCM in which case the inductor current ramps down to

zero before the end of each switching cycle. It reduces the

boost converter’s maximum output current, and produces

large input voltage ripple. An inductor with larger

inductance will reduce the bandwidth of the feedback

loop, possibly higher DC resistance (DCR). Inductor’s

DCR plays a significant role for the total efficiency since

the power transistor is integrated inside the SC440A. Of

course, there is a trade-off between the DCR and inductor

size.

Table 2 lists recommended inductors and their

vendors.

Table 2. Recommended Inductors

Inductor

Website

DR74, 4.7μH ~ 15μH

www.cooperet.com

IHLP-2525CZ-01, 4.7μ ~ 10μH

www.vishay.com

DS85LC, 6.8μH ~ 10μH

www.tokoam.com

peak

⋅

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