9
R
2
I
LED
x
+
¸
¹
·
¨
©
§
Â
i
L-PP
A
1
é
=
A
1
é
I
LED
+
¨
©
§
150 m
:
2
308 mA
x
¸
¹
·
R
EXT
=
R10
!
796
:
1
=
=
F
0.1
2 kHz
2
S
x
x
é
1
6
C
f
2
S
C
x
x
:
=
:
=
k
9
.
49
R8
k
98
.
6
R7
Design Procedure
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The chosen components from step 8 are:
(31)
6.10 IADJ Connection Method
The IADJ pin controls the high-side current sense threshold in three ways outlined in the datasheet. The
LM3409HV evaluation board allows for all three methods to be evaluated using C6, R10, and the VADJ
terminal.
Method #1: If the VADJ terminal is not connected to the power supply, then the internal Zener diode
biases the pin to 1.24V and the current sense threshold is nominally 248mV.
Method #2: Applying an external voltage to the VADJ terminal between 0 and 1.24V linearly scales the
current sense threshold between 0 and 248mV nominally. It can be necessary to have an RC filter when
using an external power supply in order to remove any high frequency noise or oscillations created by the
power supply and the connecting cables. The filter is chosen by assuming a standard value of C6 = 0.1µF
and solving for a cut-off frequency f
C
< 2kHz:
(32)
Since an exact f
C
is not critical, a standard value of 1k
Ω
is used.
Section 8
shows a typical LED current
waveform when analog dimming using an external voltage source.
Method #3: (This method requires modification of the received evaluation board). The internal 5µA current
source can be used to bias the voltage across an external resistor to ground (R
EXT
) across C6 on the
evaluation board. The resistor is sized knowing the desired average LED current I
LED
(must be < 1.5A
which is default using method #1):
(33)
8
AN-1953 LM3409HV Evaluation Board
SNVA390D – May 2009 – Revised May 2013
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