LM2832 datasheet(13/27 Pages) NSC

13 / 27 page

Design Guide (Continued)

Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and

multilayer ceramic capacitors (MLCC) are all good choices

for both input and output capacitors and have very low ESL.

For MLCCs it is recommended to use X7R or X5R type

capacitors due to their tolerance and temperature character-

istics. Consult capacitor manufacturer datasheets to see

how rated capacitance varies over operating conditions.

OUTPUT CAPACITOR

The output capacitor is selected based upon the desired

output ripple and transient response. The initial current of a

load transient is provided mainly by the output capacitor. The

output ripple of the converter is:

When using MLCCs, the ESR is typically so low that the

capacitive ripple may dominate. When this occurs, the out-

put 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 LM2832, there is really no need to review any other

capacitor technologies. Another benefit of ceramic capaci-

tors 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 capaci-

tor 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 appli-

cations will require a minimum of 22 µF of output capaci-

tance. Capacitance often, but not always, can be increased

significantly with little detriment to the regulator stability. Like

the input capacitor, recommended multilayer ceramic ca-

pacitors 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:

D1 =IOUT x (1-D)

The reverse breakdown rating of the diode must be at least

the maximum input voltage plus appropriate margin. To im-

prove 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 R1 is

connected between V

O and the FB pin. A good value for R2

is 10k

Ω. When designing a unity gain converter (Vo = 0.6V),

R1 should be between 0

Ω and 100Ω, and R2 should be

equal or greater than 10k

Ω.

REF = 0.60V

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 resis-

tors should be placed as close as possible to the IC, with the

GND of R1 placed as close as possible to the GND of the IC.

The V

OUT trace to R2 should be routed away from the

inductor and any other traces that are switching. High AC

currents flow through the V

IN, 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 for further considerations and

the LM2832 demo board as an example of a four-layer

layout.

LM2832 Datasheet(PDF) 13 Page - National Semiconductor (TI)

LM2832 Datasheet(HTML) 13 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Datasheet Download

Link URL