where

(

)

2

u

K

LM3555

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SNVS594E – JANUARY 2009 – REVISED NOVEMBER 2011

INDUCTOR SELECTION

of the biggest sources of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible

series resistance is important. Additionally, the saturation rating of the inductor should be greater than the

maximum operating peak current of the LM3555. This prevents excess efficiency loss that can occur with

inductors that operate in saturation and prevents over heating of the inductor and possible damage. For proper

inductor operation and circuit performance ensure that the inductor saturation and the peak current limit setting of

Table 1. Recommended Inductors

Manufacturer

Part#

Toko

FDSE312-2R2M

2.2 µH / 2.3A

Coilcraft

LPS4012-222ML

2.2 µH / 2.3A

TDK

VLF4014ST-2R2M1R9

2.2 µH / 2.0A

CAPACITOR SELECTION

mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very

low equivalent series resistance (ESR <20 m

Ω typ.). Tantalum capacitors, OS-CON capacitors, and aluminum

electrolytic capacitors are not recommended for use with the LM3555 due to their high ESR, as compared to

ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with

the LM3555. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over

temperature (X7R: ±15% over

−55°C to 125°C; X5R: ±15% over −55°C to 85°C).

Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM3555.

Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and

vary significantly over temperature (Y5V: +22%, -82% over

−30°C to +85°C range; Z5U: +22%, -56% over +10°C

to +85°C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only

0.1 µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum

capacitance requirements of the LM3555.

The recommended voltage rating for the input capacitor is 10V (min = 6.3V). The recommended output

capacitor voltage rating is 16V (min = 10V). The recommended value takes into account the DC bias

capacitance losses, while the minimum rating takes into account the OVP trip levels.

LAYOUT CONSIDERATIONS

The micro SMD is a chip-scale package with good thermal properties. For more detailed instructions on handling

and mounting micro SMD packages, please refer to Texas Instruments Application Note AN-1112.

The high switching frequencies and large peak currents make the PCB layout a critical part of the design. The

proceeding steps must be followed to ensure stable operation and proper current source regulation.

1. Connect the inductor as close as possible to the SW pin. This reduces the inductance and resistance of the

switching node which minimizes ringing and excess voltage drops.

2. Connect the return terminals of the input capacitor and the output capacitor as close as possible to the two

ground pins (PGND and SGND) and through low impedance traces.

Copyright © 2009–2011, Texas Instruments Incorporated

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