R

- 1

x R2

r

12

1

)

LM2736

www.ti.com

SNVS316E – SEPTEMBER 2004 – REVISED SEPTEMBER 2011

(20)

When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the

output ripple will be approximately sinusoidal and 90° phase shifted from the switching action. Given the

availability and quality of MLCCs and the expected output voltage of designs using the LM2736, there is really no

need to review any other capacitor technologies. Another benefit of ceramic capacitors is their ability to bypass

high frequency noise. A certain amount of switching edge noise will couple through parasitic capacitances in the

inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not. Since the output

capacitor is one of the two external components that control the stability of the regulator control loop, most

applications will require a minimum at 10 µF of output capacitance. Capacitance can be increased significantly

with little detriment to the regulator stability. Like the input capacitor, recommended multilayer ceramic capacitors

are X7R or X5R. Again, verify actual capacitance at the desired operating voltage and temperature.

Check the RMS current rating of the capacitor. The RMS current rating of the capacitor chosen must also meet

the following condition:

(21)

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:

(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

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

(23)

PCB Layout Considerations

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

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

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

inductor and any other traces that are switching.

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