LM2736 datasheet(13/31 Pages) TI

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1 =

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VREF

x R2

LM2736

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SNVS316F – SEPTEMBER 2004 – REVISED APRIL 2013

CATCH DIODE

The catch diode (D1) conducts during the switch off-time. A Schottky diode is recommended for its fast switching

times and low forward voltage drop. The catch diode should be chosen so that its current rating is greater than:

ID1 = IO x (1-D)

(22)

The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin.

To improve efficiency choose a Schottky diode with a low forward voltage drop.

BOOST DIODE

A standard diode such as the 1N4148 type is recommended. For VBOOST circuits derived from voltages less than

3.3V, a small-signal Schottky diode is recommended for greater efficiency. A good choice is the BAT54 small

signal diode.

BOOST CAPACITOR

A ceramic 0.01µF capacitor with a voltage rating of at least 6.3V is sufficient. The X7R and X5R MLCCs provide

the best performance.

OUTPUT VOLTAGE

The output voltage is set using the following equation where R2 is connected between the FB pin and GND, and

R1 is connected between VO and the FB pin. A good value for R2 is 10kΩ.

(23)

PCB Layout Considerations

When planning layout there are a few things to consider when trying to achieve a clean, regulated output. The

most important consideration when completing the layout is the close coupling of the GND connections of the CIN

capacitor and the catch diode D1. These ground ends should be close to one another and be connected to the

GND plane with at least two through-holes. Place these components as close to the IC as possible. Next in

importance is the location of the GND connection of the COUT capacitor, which should be near the GND

connections of CIN and D1.

There should be a continuous ground plane on the bottom layer of a two-layer board except under the switching

node island.

The FB pin is a high impedance node and care should be taken to make the FB trace short to avoid noise pickup

and inaccurate regulation. The feedback resistors should be placed as close as possible to the IC, with the GND

of R2 placed as close as possible to the GND of the IC. The VOUT trace to R1 should be routed away from the

inductor and any other traces that are switching.

High AC currents flow through the VIN, SW and VOUT traces, so they should be as short and wide as possible.

However, making the traces wide increases radiated noise, so the designer must make this trade-off. Radiated

noise can be decreased by choosing a shielded inductor.

The remaining components should also be placed as close as possible to the IC. Please see Application Note

AN-1229 SNVA054 for further considerations and the LM2736 demo board as an example of a four-layer layout.

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