LT3496 datasheet(16/26 Pages) AD

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LT3756/LT3756-1/LT3756-2

Rev. C

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APPLICATIONS INFORMATION

Therefore, a 4.7µF capacitor is an appropriate selection

for a 400kHz boost regulator with 12V input, 48V output

and 1A load.

WiththesameVINvoltagerippleof100mV,theinputcapaci-

tor for a buck converter can be estimated as follows:

CIN(µF)= ILED (A)• tSW(µs)•

4.7µF

A • µs

A 10µF input capacitor is an appropriate selection for a

400kHz buck mode converter with a 1A load.

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 this buck

convertercaseitisimportanttoplacethecapacitorasclose

as possible to the Schottky diode and to the GND return

of the switch (i.e., 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 rating. The RMS

input current for a buck mode LED driver is:

IIN(RMS) = ILED • 1– D

(

) • D

where D is the switch duty cycle.

Table 2. Recommended Ceramic Capacitor Manufacturers

MANUFACTURER

WEB

TDK

www.tdk.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

Output Capacitor Selection

The selection of the output capacitor depends on the load

and converter configuration, i.e., step-up or step-down

and the operating frequency. For LED applications, the

equivalent resistance of the LED is typically low and the

output filter capacitor should be sized to attenuate the

current ripple. Use of an X7R type ceramic capacitor is

recommended.

To achieve the same LED ripple current, the required filter

capacitor is larger in the boost and buck-boost mode ap-

plications than that in the buck mode applications. Lower

operating frequencies will require proportionately higher

capacitor values.

Soft-Start Capacitor Selection

For many applications, it is important to minimize the

inrush current at start-up. The built-in soft-start circuit

significantly reduces the start-up current spike and output

voltage overshoot. The soft-start interval is set by the soft-

start capacitor selection according to the equation:

TSS = CSS •

10µA

A typical value for the soft-start capacitor is 0.01µF. The

soft-start pin reduces the oscillator frequency and the

maximum current in the switch. The soft-start capacitor

is discharged when SHDN/UVLO falls below its threshold,

during an overtemperature event or during an INTVCC

undervoltage event. During start-up with SHDN/UVLO,

charging of the soft-start capacitor is enabled after the

first PWM high period.

Power MOSFET Selection

Forapplicationsoperatingathighinputoroutputvoltages,

the power NMOS FET switch is typically chosen for drain

voltage VDS rating and low gate charge QG. Consideration

of switch on-resistance, RDS(ON), is usually secondary be-

cause switching losses dominate power loss. The INTVCC

regulator on the LT3756 has a fixed current limit to protect

the IC from excessive power dissipation at high VIN, so the

FET should be chosen so that the product of QG at 7V and

switching frequency does not exceed the INTVCC current

limit. For driving LEDs be careful to choose a switch with

a VDS rating that exceeds the threshold set by the FB pin

in case of an open-load fault. Several MOSFET vendors

are listed in Table 3. The MOSFETs used in the application

circuits in this data sheet have been found to work well

with the LT3756. Consult factory applications for other

recommended MOSFETs.

Table 3. MOSFET Manufacturers

VENDOR

WEB

Vishay Siliconix

www.vishay.com

Fairchild

www.fairchildsemi.com

International Rectifier

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