K =

K =

x R2

R1 =

- 1

LM2831

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SNVS422B – AUGUST 2006 – REVISED JANUARY 2012

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 LM2831, 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 of 22 µF of output capacitance. Capacitance often, but not always, can be

increased significantly with little detriment to the regulator stability. Like the input capacitor, recommended

multilayer ceramic capacitors are X7R or X5R types.

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:

(11)

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.

OUTPUT VOLTAGE

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

(Vo = 0.6V), R1 should be between 0

Ω and 100Ω, and R2 should be equal or greater than 10kΩ.

(12)

(13)

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 is the close coupling of the GND connections of the input 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 output 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

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 for further considerations and the LM2831 demo board as an example of a four-layer

layout.

Calculating Efficiency, and Junction Temperature

The complete LM2831 DC/DC converter efficiency can be calculated in the following manner.

(14)

Or

(15)

Calculations for determining the most significant power losses are shown below. Other losses totaling less than

2% are not discussed.

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