0.191*CNTRL

FB =

ILED = VFB(MAX)/RLED

LM3501

SNVS230C – DECEMBER 2003 – REVISED MAY 2013

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APPLICATION INFORMATION

ADJUSTING LED CURRENT

The maximum White LED current is set using the following equation:

(1)

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:

(2)

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 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.

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 VOUT and FB pins, the maximum number of LEDs that can be placed in series (NMAX) is dependent on the

maximum LED forward voltage (VF-MAX), the voltage of the LM3501 feedback pin (VFB-MAX = 0.545V), and the

minimum output overvoltage protection level of the chosen LM3501 option (LM3501-16: OVPMIN = 15V; LM3501-

21: OVPMIN = 20V). For the circuit to function properly, the following inequality must be met:

(NMAX x VF-MAX) + 0.545V ≤ OVPMIN

(3)

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

VOUT and FB pins).

Maximum LED VF

# of LEDs

(in series)

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 VOUT pins.

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 network 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.

14

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