Application Information

ADJUSTING LED CURRENT

The maximum White LED current is set using the following

equation:

The LED current can be controlled using an external DC

voltage. The recommended operating range for the voltage

on the CNTRL pin is 0V to 2.7V. When CNTRL is 2.7V, FB =

0.515V (typ.) The FB voltage will continue to increase if

CNTRL is brought above 2.7V (not recommended). The

CNTRL to FB voltage relationship is:

The LED current can be controlled using a PWM signal on

the SHDN pin with frequencies in the range of 100 Hz

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

controlling LED currents down to the µA levels, it is best to

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

LM3501 LED current can be controlled with PWM signal

frequencies above 1 kHz but the controllable current de-

creases with higher frequency. The maximum LED current

would be achieved using the equation above with 100% duty

cycle, ie. the SHDN pin always high.

Applying a voltage greater than 125 mV to the CNTRL pin

will begin regulating current to the LEDs. A voltage below 75

mV will prevent application or regulation of the LED current.

LED-DRIVE CAPABILITY

The maximum number of LEDs that can be driven by the

LM3501 is limited by the output voltage capability of the

LM3501. When using the LM3501 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 LM3501 option

(LM3501-16: OVP

For the circuit to function properly, the following inequality

must be met:

(N

≤ OVP

MIN

When inserting a value for maximum LED VF, LED forward

voltage variation over the operating temperature range

should be considered. The table below provides maximum

LED voltage numbers for the LM3501-16 and LM3501-21 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

LM3501-16

LM3501-21

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 LM3501 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 LM3501 contains dedicated circuitry for monitoring the

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

disconnected from the LM3501-16, the output voltage will

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

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 1 µF

ceramic output capacitors creating an overall small solution

for white LED applications.

In the event that the primary LED network is disconnected

from the LM3501-21, 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 1 µF ceramic output

capacitors.

RELIABILITY AND THERMAL SHUTDOWN

The maximum continuous pin current for the 8 pin thin micro

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

power output limits the V

than 535 mA (see INDUCTOR SELECTION section for av-

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

the device the average switch current should not exceed 535

mA.

The LM3501 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 LM3501 will return to normal

operation.

INDUCTOR SELECTION

The inductor used with the LM3501 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.

www.national.com

13