LT3955 datasheet(19/30 Pages) LINER

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LT3955

3955fb

For more information www.linear.com/LT3955

applicaTions inFormaTion

best reliability, this capacitor should have low ESR and

ESL and have an adequate ripple current rating.

Table 3. 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 X7R type ceramic capacitors 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. Connect a capacitor from the DIM/SS

pin to GND to use this feature. The soft-start interval is

set by the soft-start capacitor selection according to the

equation:

TSS = CSS •

1.2V

12µA

= CSS •

100µs

provided there is no additional current supplied to the

DIM/SS pin for programming the duty cycle of the PWM

dimming signal generator. A typical value for the soft-start

capacitor is 10nF which gives a 1ms start-up interval. The

soft-start pin reduces the oscillator frequency and the

maximum current in the switch.

The soft-start capacitor discharges if one of the follow-

ing events occurs: the EN/UVLO falls below its threshold;

output overcurrent is detected at the ISP/ISN pins; IC

overtemperature; or INTVCC undervoltage. During start-

up with EN/UVLO, charging of the soft-start capacitor is

enabled after the first PWM high period. In the start-up

sequence,afterswitchingisenabledbyPWMtheswitching

continues until VISP-ISN > 25mV or DIM/SS > 1V. PWM

pin negative edges during this start-up interval are not

processed until one of these two conditions are met so

that the regulator can reach steady state operation shortly

after PWM dimming commences.

Schottky Rectifier Selection

The power Schottky diode conducts current during the

interval when the switch is turned off. Select a diode rated

for the maximum SW voltage of the application and the

RMS diode current. If using the PWM feature for dimming,

it may be important to consider diode leakage, which in-

creases with the temperature, from the output during the

PWM low interval. Therefore, choose the Schottky diode

with sufficiently low leakage current. Table 4 has some

recommended component vendors. The diode current

and VF should be considered when selecting the diode

to be sure that power dissipation does not exceed the

rating of the diode. The power dissipated by the diode in

a converter is:

PD = ID • VF • (1-DMAX)

Itisprudenttomeasurethediodetemperatureinsteadystate

toensurethatitsabsolutemaximumratingsarenotexceeded.

Table 4. Schottky Rectifier Manufacturers

MANUFACTURER

WEB

On Semiconductor

www.onsemi.com

Central Semiconductor

www.centralsemi.com

Diodes, Inc.

www.diodes.com

Inductor Selection

TheinductorusedwiththeLT3955shouldhaveasaturation

current rating appropriate to the maximum switch current

of 4.9A. Choose an inductor value based on operating

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