LT3756 datasheet(19/31 Pages) AD

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LT8355-1

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

For a boost converter switching at 200kHz, a 10µF input

capacitor will often be sufficient. In the buck mode con-

figuration, the input capacitor has large pulsed currents

due to the current returned through the Schottky diode

when the switch is off. The minimum input capacitance

can be estimated as:

CIN(BUCK) =

ILED

ΔVIN • fSW

(9)

For the buck case at 2MHz, a 10µF input capacitor would

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

age ripple with ILED = 1.5A. Additional margin is recom-

mended. In the buck converter case, it is important to

place the capacitor as close as possible to the Schottky

diode, external NMOS switch and the sense resistor. 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 rat-

ing. Use Equation 10 to estimate the RMS input capacitor

current for the buck converter case.

ICIN(RMS,BUCK) = ILED •

VLED

VIN

1–

VLED

VIN

⎛

⎝⎜

⎞

⎠⎟

(10)

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

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 frequently low, so

larger capacitors may be needed to further reduce voltage

ripple. It is likely that the appropriate output capacitor

value will fall between 10µF and 47µF. Use the exam-

ple applications as a starting point for output capacitor

selection.

Schottky Rectifier Selection

Choose a Schottky diode with reverse breakdown voltage

greater than the maximum programmed output voltage

and a current rating greater than the peak inductor cur-

rent. Be sure to set the programmable switch current limit

lower than the maximum forward current of the Schottky

rectifier chosen. It is best to find a rectifier with low equiv-

alent capacitance, around or below 500pF. Pay attention

to reverse leakage current if the part is to be used in low

frequency PWM dimming situations. The reverse leakage

current can discharge the output capacitor. This can lead

to lengthy turn-on transient effects that degrade maxi-

mum PWM dimming dynamic range. Not all applications

will require the combination of high reverse voltage and

forward current of this rectifier.

Inductor Selection

Select an inductor for use with LT8355-1 that has a satu-

ration current greater than the switch current limit set by

the switch current sense resistor as 0.1V/RSENSE. Include

some margin for the saturation current. Choose the induc-

tor value based on desired ripple current given input and

output voltage and switching frequency. Use Equation 11

to select an inductor with around 20% ripple.

LBOOST, BUCK-BOOST =

RSENSE • VIN

0.02V • fSW

1–

VIN

VISP

⎛

⎝⎜

⎞

⎠⎟

LBUCK =

RSENSE • VLED

0.02V • fSW

1–

VLED

VIN

⎛

⎝⎜

⎞

⎠⎟

(11)

For boost configurations, VISP = VLED; and for buck-boost

mode, VISP = VLED + VIN.

Table 3 provides some recommended inductor vendors.

Table 3. Recommended Inductor Vendors

VENDOR

WEB

Sumida

www.sumida.com

Wurth Elektronik

www.we-online.com

Coiltronics

www.cooperet.com

Vishay

www.vishay.com

Coilcraft

www.coilcraft.com

Power PMOS Selection

For the PMOS to be used with PWMTG, select a device

with drain-source voltage rating higher than the external

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