LM3414 datasheet(12/20 Pages) NSC

12 / 20 page

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FIGURE 1. Switching Frequency vs R

SW (kHz)

FS (kΩ)

250

500

1000

TABLE 1. Examples for f

SW Settings

To ensure accurate current regulation, the LM3414/14HV

should be operated in continuous conduction mode (CCM)

and the on time should not be shorter than 400 ns under all

operation condition.

SETTING LED CURRENT

The LM3414/14HV requires no external current sensing re-

sistor for LED current regulation. The average output current

of the LM3414/14HV is adjustable by varying the resistance

of the resistor, R

IADJ that connects across the IADJ and GND

pins. The IADJ pin is internally biased to 1.255V. The LED

current is then governed by the following equation:

where 350 mA < I

LED < 1A

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FIGURE 2. LED Current vs R

IADJ

OUT (mA)

IADJ (kΩ)

350

8.93

500

6.25

700

4.46

1000

3.13

TABLE 2. Examples for I

OUT Settings

The LED current can be set to any level in the range from 350

mA to 1A. In order to provide accurate LED current, R

IADJ

should be a resistor with no more than 0.5% tolerance. If the

IADJ pin is accidentally shorted to GND (R

IADJ = 0), the output

current will be limited to avoid damaging the circuit. When the

over current protection is activated, current regulation cannot

be maintained until the over-current condition is cleared.

MINIMUM SWITCH ON-TIME

As the LM3414 features a 400 ns minimum ON time, it is es-

sential to make sure the ON time of the internal switch is not

shorter than 400 ns when setting the LED driving current. If

the switching ON time is shorter than 400 ns, the accuracy of

the LED current may not maintain and exceed the rated cur-

rent of the LEDs. The ratio of the LED forward voltage to input

voltage is restricted by the following restriction:

PEAK SWITCH CURRENT LIMIT

The LM3414/14HV features an integrated switch current lim-

iting mechanism that protects the LEDs from being overdriv-

en. The switch current limiter will be triggered when the switch

current is three times exceeding the current level set by

IADJ. Once the current limiter is triggered, the internal power

switch turns OFF for 3.6 µs to allow the inductor to discharge

and cycles repetitively until the over current condition is re-

moved. The current limiting feature is exceptionally important

to avoid permanent damage of the LM3414/14HV application

circuit due to short circuit of LED string.

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