User manual

k250R5

0.1C6

A1 P

LED

L-PP

4Rx

A1 P

444 mA

200 m:

A02.1

248 k:

Design Procedure

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6.10 IADJ Connection Method

The IADJ pin controls the high-side current sense threshold as outlined in the data sheet. The LM3409

demonstration board allows for two methods to be evaluated using the IADJ pin. The desired method is

chosen as follows:

Method #1: Applying an external voltage to the VADJ terminal between 0 and 1.24V linearly scales the

current sense threshold between 0 and 248mV nominally.

Method #2:If no voltage is applied to the VADJ terminal, the internal 5µA current source will bias the

voltage across the external potentiometer (R5). The potentiometer can be used to adjust the current sense

threshold also. It is sized knowing the maximum desired average LED current which is chosen as I

LED

1A:

(31)

The next highest standard potentiometer of 250kΩ is used. A 0.1µF capacitor (C6) is added from the IADJ

pin to GND in order to eliminate unwanted high frequency noise coupling on the IADJ pin.

The chosen components from step 9 are:

(32)

The Section 7Typical Waveforms shows a typical LED current waveform when analog dimming using the

potentiometer. See the Alternate Designs section for two designs that are optimized to improve analog

dimming range by reducing the switching frequency, increasing the inductance, and adding output

capacitance.

AN-1954 LM3409 Demonstration Board SNVA391D–May 2009–Revised May 2013

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