LM3500 datasheet(14/25 Pages) TI1

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ILED = VFB/RLED

LM3500

SNVS231F – MAY 2004 – REVISED APRIL 2007

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Application Information

ADJUSTING LED CURRENT

The White LED current is set using the following equation:

(1)

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 controlling 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 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 LM3500 feedback pin (VFB-MAX = 0.53V), and the

minimum output over-voltage protection level of the chosen LM3500 option (LM3500-16: OVPMIN = 15V;

LM3500-21: OVPMIN = 20V). For the circuit to function properly, the following inequality must be met:

(NMAX × VF-MAX) + 0.53V ≤ OVPMIN

When inserting a value for maximim LED VF, LED forward 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

VOUT and FB pins).

Maximum LED VF

# of LEDs

(in series)

LM3500-16

LM3500-21

4.82V

6.49V

3.61V

4.86V

2.89V

3.89V

3.24V

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 VOUT pins.

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 network 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 capacitors.

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 VSW pin can see a higher DC current than 535mA (see INDUCTOR SELECTION

section for average switch current). To preserve the long term reliability of the device the average switch current

should not exceed 535mA.

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