Application Information

ADJUSTING LED CURRENT

The White LED current is set using the following equation:

The LED current can be controlled using a PWM signal on

the SHDN pin with frequencies in the range of 100Hz

(greater than visible frequency spectrum) to 1kHz. For con-

trolling LED currents down to the µA levels, it is best to use

a PWM signal frequency between 200-500Hz. The LM3500

LED current can be controlled with PWM signal frequencies

above 1kHz but the controllable current decreases with

higher frequency. The maximum LED current would be

achieved using the equation above with 100% duty cycle, ie.

the SHDN pin always high.

LED-DRIVE CAPABILITY

The maximum number of LEDs that can be driven by the

LM3500 is limited by the output voltage capability of the

LM3500. When using the LM3500 in the typical application

configuration, with LEDs stacked in series between the V

OUT

and FB pins, the maximum number of LEDs that can be

placed in series (N

forward voltage (V

back pin (V

voltage protection level of the chosen LM3500 option

(LM3500-16: OVP

For the circuit to function properly, the following inequality

must be met:

(N

≤ OVP

MIN

When inserting a value for maximim LED V

voltage variation over the operating temperature range

should be considered. The table below provides maximum

LED voltage numbers for the LM3500-16 and LM3500-21 in

the typical application circuit configuration (with 3, 4, 5, 6, or

7 LEDs placed in series between the V

# of LEDs

(in series)

Maximum LED V

F

LM3500-16

LM3500-21

3

4.82V

6.49V

4

3.61V

4.86V

5

2.89V

3.89V

6

X

3.24V

7

X

2.78V

For the LM3500 to operate properly, the output voltage must

be kept above the input voltage during operation. For most

applications, this requires a minimum of 2 LEDs (total of 6V

or more) between the FB and V

OUTPUT OVERVOLTAGE PROTECTION

The LM3500 contains dedicated circuitry for monitoring the

output voltage. In the event that the primary LED network is

disconnected from the LM3500-16, the output voltage will

increase and be limited to 15.5V (typ.). There is a 900mV

hysteresis associated with this circuitry which will cause the

output to fluctuate between 15.5V and 14.6V (typ.) if the

primary network is disconnected. In the event that the net-

work is reconnected regulation will begin at the appropriate

output voltage. The 15.5V limit allows the use of 16V 1µF

ceramic output capacitors creating an overall small solution

for white LED applications.

In the event that the primary LED network is disconnected

from the LM3500-21, the output voltage will increase and be

limited to 20.5V (typ.). There is a 1V hysteresis associated

with this circuitry which will cause the output to fluctuate

between 20.5V and 19.5V (typ.) if the primary network is

disconnected. In the event that the network is reconnected

regulation will begin at the appropriate output voltage. The

20.5V limit allows the use of 25V 1µF ceramic output capaci-

tors.

RELIABILITY AND THERMAL SHUTDOWN

The maximum continuous pin current for the 8 pin thin micro

SMD package is 535mA. When driving the device near its

power output limits the V

than 535mA (see INDUCTOR SELECTION section for aver-

age switch current). To preserve the long term reliability of

the device the average switch current should not exceed

535mA.

The LM3500 has an internal thermal shutdown function to

protect the die from excessive temperatures. The thermal

shutdown trip point is typically 150˚C. There is a hysteresis

of typically 35˚C so the die temperature must decrease to

approximately 115˚C before the LM3500 will return to normal

operation.

INDUCTOR SELECTION

The inductor used with the LM3500 must have a saturation

current greater than the cycle by cycle peak inductor current

(see Typical Peak Inductor Currents table below). Choosing

inductors with low DCR decreases power losses and in-

creases efficiency.

The minimum inductor value required for the LM3500-16 can

be calculated using the following equation:

The minimum inductor value required for the LM3500-21 can

be calculated using the following equation:

For both equations above, L is in µH, V

of the chip in Volts, R

power switch found in the Typical Performance Characteris-

tics section in ohms and D is the duty cycle of the switching

regulator. The above equation is only valid for D greater than

or equal to 0.5. For applications where the minimum duty

cycle is less than 0.5, a 22µH inductor is the typical recom-

mendation for use with most applications. Bench-level veri-

fication of circuit performance is required in these special

cases, however. The duty cycle, D, is given by the following

equation:

where V

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