LM20343_15 datasheet(13/29 Pages) TI

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IIN-RMS = IOUT D(1 - D)

VDROOP = 'IOUTSTEP x RESR +

L x 'IOUTSTEP

COUT x (VIN - VOUT)

'VOUT = 'iL x

8 x fSW x COUT

RESR +

LM20343

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SNVS559B – MAY 2008 – REVISED APRIL 2013

where

•

ΔVOUT (V) is the amount of peak to peak voltage ripple at the power supply output

•

RESR (Ω) is the series resistance of the output capacitor

•

fSW(Hz) is the switching frequency

•

COUT (F) is the output capacitance used in the design

(3)

The amount of output ripple that can be tolerated is application specific; however a general recommendation is to

keep the output ripple less than 1% of the rated output voltage. Keep in mind ceramic capacitors are sometimes

preferred because they have very low ESR; however, depending on package and voltage rating of the capacitor

the value of the capacitance can drop significantly with applied voltage. The output capacitor selection will also

affect the output voltage droop during a load transient. The peak droop on the output voltage during a load

transient is dependent on many factors; however, an approximation of the transient droop ignoring loop

bandwidth can be obtained using the following equation:

where

•

COUT (F) is the minimum required output capacitance

•

L (H) is the value of the inductor

•

VDROOP (V) is the output voltage drop ignoring loop bandwidth considerations

•

ΔIOUTSTEP (A) is the load step change

•

RESR (Ω) is the output capacitor ESR

•

VIN (V) is the input voltage

•

VOUT (V) is the set regulator output voltage

(4)

Both the tolerance and voltage coefficient of the capacitor should be examined when designing for a specific

output ripple or transient droop target.

INPUT CAPACITOR SELECTION

Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the

switch current during the on-time. In general it is recommended to use a ceramic capacitor for the input as they

provide both a low impedance and small footprint. One important note is to use a good dielectric for the ceramic

capacitor such as X5R or X7R. These provide better over temperature performance and also minimize the DC

voltage derating that occurs on Y5V capacitors. The input capacitors CIN1 and CIN2 should be placed as close as

possible to the VIN and GND pins on both sides of the device.

Non-ceramic input capacitors should be selected for RMS current rating and minimum ripple voltage. A good

approximation for the required ripple current rating is given by the relationship:

(5)

As indicated by the RMS ripple current equation, highest requirement for RMS current rating occurs at 50% duty

cycle. For this case, the RMS ripple current rating of the input capacitor should be greater than half the output

current. For best performance, low ESR ceramic capacitors should be placed in parallel with higher capacitance

capacitors to provide the best input filtering for the device.

SETTING THE OUTPUT VOLTAGE (RFB1, RFB2)

The resistors RFB1 and RFB2 are selected to set the output voltage for the device. Table 1 provides suggestions

for RFB1 and RFB2 for common output voltages.

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