LM3503 datasheet(17/20 Pages) NSC

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Application Information (Continued)

INPUT CAPACITOR SELECTION

The input capacitor serves as an energy reservoir for the

inductor. In addition to acting as an energy reservoir for the

inductor the input capacitor is necessary for the reduction in

input voltage ripple and noise experienced by the LM3503.

The reduction in input voltage ripple and noise helps ensure

the LM3503’s proper operation, and reduces the effect of the

LM3503 on other devices sharing the same supply voltage.

To ensure low input voltage ripple, the input capacitor must

have an extremely low ESR. As a result of the low input

voltage ripple requirement multilayer ceramic capacitors are

the best choice. A minimum capacitance of 2.0 µF is required

for normal operation, so consult the capacitor manufactur-

er’s data curves to verify whether the minimum capacitance

requirement is going to be achieved for a particular applica-

tion.

OUTPUT CAPACITOR SELECTION

The output capacitor serves as an energy reservoir for the

white LED load when the internal power FET switch (Figure

1: N1) is on or conducting current. The requirements for the

output capacitor must include worst case operation such as

when the load opens up and the LM3503 operates in over-

voltage protection (OVP) mode operation. A minimum ca-

pacitance of 0.5 µF is required to ensure normal operation.

Consult the capacitor manufacturer’s data curves to verify

whether the minimum capacitance requirement is going to

be achieved for a particular application.

Some recommended capacitor manufacturers included but

are not limited to:

Taiyo-

Yuden

GMK212BJ105MD

(0805/35V)

www.t-yuden.com

muRata

GRM40-035X7R105K

(0805/50V)

www.murata.com

TDK

C3216X7R1H105KT

(1206/50V)

www.tdktca.com

C3216X7R1C475K

(1206/16V)

AVX

08053D105MAT

(0805/25V)

www.avxcorp.com

08056D475KAT

(0805/6.3V)

1206ZD475MAT

(1206/10V)

DIODE SELECTION

To maintain high efficiency it is recommended that the aver-

age current rating (I

F or IO) of the selected diode should be

larger than the peak inductor current (I

Lpeak). At the minimum

the average current rating of the diode should be larger than

the maximum LED current. To maintain diode integrity the

peak repetitive forward current (I

FRM) must be greater than

or equal to the peak inductor current (I

Lpeak). Diodes with low

forward voltage ratings (V

F) and low junction capacitance

magnitudes (C

J or CT or CD) are conducive to high efficiency.

The chosen diode must have a reverse breakdown voltage

rating (V

R and/or VRRM) that is larger than the output voltage

OUT). No matter what type of diode is chosen, Schottky or

not, certain selection criteria must be followed:

R and VRRM > VOUT

F or IO

≥ I

LOAD or IOUT

FRM

≥ I

Lpeak

Some recommended diode manufacturers included but are

not limited to:

Vishay

SS12(1A/20V)

www.vishay.com

SS14(1A/40V)

SS16(1A/60V)

Semiconductor

MBRM120E

(1A/20V)

www.onsemi.com

MBRS1540T3

(1.5A/40V)

MBR240LT

(2A/40V)

Central

Semiconductor

CMSH1-40M

(1A/40V)

www.centralsemi.com

SHUTDOWN AND START-UP

On startup, the LM3503 contains special circuitry that limits

the peak inductor current which prevents large current

spikes from loading the battery or power supply. The

LM3503 is shutdown when both En1 and En2 signals are

less than 0.3V. During shutdown the output voltage is a

diode drop below the supply voltage. When shutdown, the

softstart is reset to prevent inrush current at the next startup.

THERMAL SHUTDOWN

The LM3503 stops regulating when the internal semiconduc-

tor junction temperature reaches approximately 140˚C. The

internal thermal shutdown has approximately 20˚C of hyster-

esis which results in the LM3503 turning back on when the

internal semiconductor junction temperature reaches 120˚C.

When the thermal shutdown temperature is reached, the

softstart is reset to prevent inrush current when the die

temperature cools.

UNDER VOLTAGE PROTECTION

The LM3503 contains protection circuitry to prevent opera-

tion for low input supply voltages. When Vin drops below

2.3V, typically, the LM3503 will no longer regulate. In this

mode, the output voltage will be one diode drop below Vin

and the softstart will be reset. When Vin increases above

2.4V, typically, the device will begin regulating again.

OVER VOLTAGE PROTECTION

The LM3503 contains dedicated ciruitry for monitoring the

output voltage. In the event that the LED network is discon-

nected from the LM3503, the output voltage will increase

and be limited to 15.5V(typ.) for the 16V version, 24V(typ.)

for the 25V version, 34V(typ.) for 35V version and 42V(typ.)

for the 44V version. (see electrical table for more details). In

the event that the network is reconnected regulation will

resume at the appropriate output voltage.

LAYOUT CONSIDERATIONS

All components, except for the white LEDs, must be placed

as close as possible to the LM3503. The die attach pad

(DAP) must be soldered to the ground plane.

The input bypass capacitor C

IN, as shown in the Typical

Application Circuit,, must be placed close to the IC and

connect between the V

IN and Pgnd pins. This will reduce

copper trace resistance which effects input voltage ripple of

the IC. For additional input voltage filtering, a 100 nF bypass

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