TI Confidential - NDA Restrictions

LM2731

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SNVS217F – MAY 2004 – REVISED NOVEMBER 2012

FEED-FORWARD COMPENSATION

Although internally compensated, the feed-forward capacitor Cf is required for stability (see Figure 29). Adding

this capacitor puts a zero in the loop response of the converter. The recommended frequency for the zero fz

should be approximately 6 kHz. Cf can be calculated using the formula:

Cf = 1 / (2 X

π X R1 X fz)

(1)

SELECTING DIODES

The external diode used in the typical application should be a Schottky diode. A 20V diode such as the

MBR0520 is recommended.

The MBR05XX series of diodes are designed to handle a maximum average current of 0.5A. For applications

exceeding 0.5A average but less than 1A, a Microsemi UPS5817 can be used.

LAYOUT HINTS

High frequency switching regulators require very careful layout of components in order to get stable operation

and low noise. All components must be as close as possible to the LM2731 device. It is recommended that a 4-

layer PCB be used so that internal ground planes are available.

As an example, a recommended layout of components is shown:

Figure 28. 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 Figure 29). A minimum value of 13.3 k

Ω is

recommended for R2 to establish a divider current of approximately 92 µA. R1 is calculated using the formula:

(2)

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