LM2731

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SNVS217E – MAY 2004 – REVISED APRIL 2010

Figure 3. Recommended PCB Component Layout

Some additional guidelines to be observed:

1. Keep the path between L1, D1, and C2 extremely short. Parasitic trace inductance in series with D1 and C2

will increase noise and ringing.

2. The feedback components R1, R2 and CF must be kept close to the FB pin of U1 to prevent noise injection

on the FB pin trace.

3. If internal ground planes are available (recommended) use vias to connect directly to ground at pin 2 of U1,

as well as the negative sides of capacitors C1 and C2.

SETTING THE OUTPUT VOLTAGE

The output voltage is set using the external resistors R1 and R2 (see Basic Application Circuit). A value of

approximately 13.3 k

Ω is recommended for R2 to establish a divider current of approximately 92 µA. R1 is

calculated using the formula:

(3)

SWITCHING FREQUENCY

The LM2731 is provided with two switching frequencies: the “X” version is typically 1.6 MHz, while the “Y” version

is typically 600 kHz. The best frequency for a specific application must be determined based on the trade-offs

involved:

Higher switching frequency means the inductors and capacitors can be made smaller and cheaper for a given

output voltage and current. The down side is that efficiency is slightly lower because the fixed switching losses

occur more frequently and become a larger percentage of total power loss. EMI is typically worse at higher

switching frequencies because more EMI energy will be seen in the higher frequency spectrum where most

circuits are more sensitive to such interference.

Figure 4. Basic Application Circuit

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