[L * Fs]

[(1 - D) * Eff]

+

[2 * L * Fs]

|

|

2

LM3557

www.ti.com

SNVS338B – NOVEMBER 2004 – REVISED FEBRUARY 2013

(12)

where

•

•

•

Eff is the efficiency of the LM3557

•

Fs is the switching frequency

•

•

L is the inductance magnitude/inductor value

•

D is the duty cycle for CCM operation

•

•

(13)

The cycle-by-cycle peak inductor current for DCM operation can be computed with:

where

•

•

•

Eff is the efficiency of the LM3557

•

Fs is the switching frequency

•

•

L is the inductance magnitude/inductor value

•

D is the duty cycle for CCM operation

•

•

(14)

Some recommended inductor manufacturers are:

Coilcraft [www.coilcraft.com]

Coiltronics [www.cooperet.com]

TDK [www.tdk.com]

CAPACITOR SELECTION

Multilayer ceramic capacitors are the best choice for use with the LM3557. Multilayer ceramic capacitors have

the lowest equivalent series resistance (ESR). Applied voltage or DC bias, temperature, dielectric material type

(X7R, X5R, Y5V, etc), and manufacturer component tolerance have an affect on the true or effective capacitance

of a ceramic capacitor. Be aware of how your application will affect a particular ceramic capacitor by analyzing

the aforementioned factors of your application. Before selecting a capacitor always consult the capacitor

manufacturer’s data curves to verify the effective or true capacitance of the capacitor in your application.

INPUT CAPACITOR SELECTION

The input capacitor serves as an energy reservoir for the inductor. In addition to acting as an energy reservoir for

the inductor the input capacitor is necessary for the reduction in input voltage ripple and noise experienced by

the LM3557. The reduction in input voltage ripple and noise helps ensure the LM3557’s proper operation, and

reduces the effect of the LM3557 on other devices sharing the same supply voltage. To ensure low input voltage

ripple, the input capacitor must have an extremely low ESR. As a result of the low input voltage ripple

requirement multilayer ceramic capacitors are the best choice. A minimum capacitance of 2.0 µF is required for

normal operation, consult the capacitor manufacturer’s data curves to verify whether the minimum capacitance

requirement is going to be achieved for a particular application.

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