Datasheet
2
-I
LED
=
R5
SNS
x
V
ADJ
i
PPL-
'
-
R
1UV
=
RV24.1
2UV
x
V24.1V
ONTURN-
R
2UV
=
V
HYS
PA
22
DDD
VIP x
=
D
I (1- D) x
=
LED
I
DSON
2
RMSTT
RIP x
=
-
I
LED
x
=
1D
2
x
+
x
¸
¸
¸
¹
·
¸
¸
¹
·
¨
¨
©
§
¨
¨
¨
©
§
12
1
i
L-PP
'
I
LED
I
RMST-
LEDT
II D x
=
LM3409, LM3409HV, LM3409-Q1
SNVS602J –MARCH 2009–REVISED MAY 2013
www.ti.com
6. PFET
The PFET voltage rating should be at least 15% higher than the maximum input voltage (V
IN-MAX
) and current
rating should be at least 10% higher than the average PFET current (I
T
):
(36)
Given a PFET with on-resistance (R
DS-ON
), solve for the RMS transistor current (I
T-RMS
) and power dissipation
(P
T
):
(37)
(38)
7. DIODE
The Schottky diode needs a voltage rating similar to the PFET. Higher current diodes with a lower forward
voltage are suggested. Given a diode with forward voltage (V
D
), solve for the average diode current (I
D
) and
power dissipation (P
D
):
(39)
(40)
8. INPUT UVLO
Input UVLO is set with the turn-on threshold voltage (V
TURN-ON
) and the desired hysteresis (V
HYS
). To set V
HYS
,
solve for R
UV2
:
(41)
To set V
TURN-ON
, solve for R
UV1
:
(42)
9. IADJ CONNECTION METHOD
The IADJ pin controls the high-side current sense threshold in three ways outlined in the Theory of Operation
section.
Method #1: Leave IADJ pin open and I
LED
is calculated as in the 3. AVERAGE LED CURRENT section of the
Design Guide.
Method #2: Apply an external voltage (V
ADJ
) to the IADJ pin between 0 and 1.24V to analog dim or to reduce
I
LED
as follows:
(43)
Keep in mind that analog dimming will eventually push the converter in to DCM and the inductor current ripple
will no longer be constant causing a divergence from linear dimming at low levels.
A 0.1µF capacitor connected from the IADJ pin to GND is recommended when using this method. It may also be
necessary to have a 1kΩ series resistor with the capacitor to create an RC filter. The filter will help remove high
frequency noise created by other connected circuitry.
Method #3: Connect an external resistor or potentiometer to GND (R
EXT
) and the internal 5µA current source will
set the voltage. Again, a 0.1µF capacitor connected from the IADJ pin to GND is recommended. To set I
LED
,
solve for R
EXT
:
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