2.5

3.0

3.5

4.0

4.5

5.0

5.5

1 PH

3.3 PH

1.5 PH

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

2 LEDs

LM3519

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SNVS394A – AUGUST 2005 – REVISED SEPTEMBER 2005

DIODE SELECTION

that is larger than the output voltage. The following criteria should be followed when choosing a diode:

(Output Voltage)

Table 4. Recommended Diode Manufacturers Include but are not Limited to:

Manufacturer

Description

Vishay

SS12(1A/20V)

SS14(1A/40V)

SS16(1A/60V)

Central Semiconductor

CMSH1- 40M(1A/40V)

ONSemi

MBRS1540T3(1.5A/40V)

PWM DIMMING

The LED current is set internally by the LM3519 to 20mA (typical); dimming control may be realized by applying

a pulse width modulated(PWM) signal to the En pin. For example, a 50% duty cycle waveform will produce an

average current of 10mA. A control signal frequency between 17kHz and 30kHz is suitable for dimming.

Although the LM3519 is capable of operation outside this frequency range, it is not recommended to operate

below 17kHz for the following reasons: 1) frequency below 100Hz is likely to cause visible flicker in the light

emitted by the LED string. 2) frequency below 17kHz may induce audible noise due to combinations of some

capacitance/PCB. A PWM frequency above 30kHz is possible but the current linearity vs duty cycle will be

affected.

If it is not possible to operate the dimming control above 17kHz, audible noise emission may be minimized by

using capacitors with low susceptibility to piezoelectric induced stresses, such as poly film designs. Minimum

audible noise is most likely to occur when the PWM frequency is less than 2kHz. It is recommended that any

application using a PWM control signal below 17kHz be thoroughly evaluated for undesirable audible or visible

noise.

DRIVING 2 LEDs

The LM3519 is optimized to drive up to 4LEDs. When driving 2LEDs, a minimum inductance of 2.2µH is required

to maintain good loop regulation and current accuracy. If a smaller inductor is used, the LED current will have

more variation with input voltage than a typical application. The following curve illustrates the behavior.

Copyright © 2005, Texas Instruments Incorporated

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