LT3477 datasheet(17/26 Pages) AD

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LT3950

Rev. 0

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converters require a lower value input capacitor than buck

mode converters. Use the following equations to estimate

the value of the input capacitor. If the inductor is selected

according to the directions in the Inductor Selection sec-

tion, use the value 15% for the quantity iL(RIPPLE)/iL(DC)

in the following equation, otherwise use the fraction of

average inductor current representing half of the peak to

peak ripple current:

CIN(BOOST) >

iLEDtSW

iL(RIPPLE)

iL(DC)

VLED(MAX)

VIN(MIN)

ΔVIN(MAX)

For a boost converter switching at 2MHz, a 2.2µF input

capacitor will often suffice.

CIN(BUCK) >

iLEDtSW

ΔVIN(MAX)

For the buck case at 2MHz, a 2.2µF input capacitor would

be appropriate to ensure less than 100mV of input volt-

age ripple with iLED = 0.3A. Additional margin is recom-

mended (e.g., the 1.5µF result of evaluating the above

equation may lead to selection of a 2.2µF input capacitor).

In the buck mode configuration, the input capacitor has

large pulsed currents due to the current returned through

the Schottky diode when the switch is off. In the buck con-

verter case it is important to place the capacitor as close

as possible to the Schottky diode and to the exposed pad

of the IC. It is also important to consider the ripple current

rating of the capacitor. For best reliability, this capacitor

should have low ESR and ESL and have an adequate ripple

current rating. Use the following equation to estimate the

RMS input capacitor current for the buck converter case.

iCIN(RMS) =iLED •

VLED

VIN

1–

VLED

VIN

⎛

⎝⎜

⎞

⎠⎟

The selection of the output capacitor depends on load and

power stage configuration. For example, a boost or buck-

boost mode converter will require a much larger output

capacitor than a buck mode converter for the same condi-

tions. The boost and buck-boost mode configurations will

also require similar low ESR and low ESL capacitors like

APPLICATIONS INFORMATION

the input capacitor of the buck mode case. Capacitor val-

ues will increase proportionally with decreasing switch-

ing frequency for the same ripple voltage. The equivalent

resistance presented by an LED load is typically low, so

larger capacitors may be needed to further reduce voltage

ripple. It is likely that the appropriate output capacitor

value will fall between 2.2µF and 47µF. Use the example

applications as a starting point for output capacitor selec-

tion. Sources of quality ceramic capacitors are listed in

Table 3.

Table 3. Capacitor Manufacturers

MANUFACTURER

WEBSITE

MURATA

www.murata.com

TDK

www.tdk.com

KEMET

www.kemet.com

TAIYO YUDEN

www.t-yuden.com

AVX

www.avx.com

Schottky Rectifier Selection

Choose a Schottky diode with reverse breakdown voltage

rating at or above 60V and with average forward current

rating greater than the programmed LED current with

some margin. It is best to find a rectifier with low equiva-

lent capacitance, around or below 350pF. Large equiva-

lent capacitance and/or poor PCB layout can negatively

interact with certain EMI mitigating features in LT3950.

Pay attention to reverse leakage current if the part is to

be used in low frequency PWM dimming (<200Hz) situa-

tions. The reverse leakage current can discharge the out-

put capacitor. This can lead to lengthy turn-on transient

effects that degrade maximum PWM dimming dynamic

range. Note that reverse leakage current increases with

temperature. For many LT3950 applications, the NXP

PMEG6020 will suffice. Table 4 has some recommend

component vendors.

Table 4. Schottky Rectifier Manufaturers

VENDOR

WEBSITE

ON Semiconductor

www.onsemi.com

Diodes, Inc.

www.diodes.com

Central Semiconductor

www.centralsemi.com

NXP

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