LM3409MY National Semiconductor, LM3409MY Datasheet - Page 17

• Manufacturer Part Number: LM3409MY
• Description: LM3409 LM3409HV PFET Buck Controller for High Power LED Drives; Package: MINI SOIC EXP PAD; No of Pins: 10; Qty per Container: 1000/Reel
• Manufacturer: National Semiconductor

4. OUTPUT CAPACITANCE
A minimum output capacitance (C
reduce Δi
the known dynamic resistance (r
the required impedance (Z
Solve for C
5. INPUT CAPACITANCE
Set the input voltage ripple (Δv
quired minimum capacitance (C
The necessary RMS input current rating (I
6. PFET
The PFET voltage rating should be at least 15% higher than
the maximum input voltage (V
be at least 10% higher than the average PFET current (I
Given a PFET with on-resistance (R
transistor current (I
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
the average diode current (I
LED-PP
O-MIN
:
below Δi
T-RMS
) and power dissipation (P
L-PP
C
) for C
D
. With the specified Δi
) and power dissipation (P
IN-MAX
IN-MIN
D
IN-PP
) of the LED string, solve for
O-MIN
O-MIN
) and current rating should
DS-ON
) by solving for the re-
):
) may be necessary to
:
), solve for the RMS
IN-RMS
) is:
D
), solve for
LED-PP
T
):
D
T
):
and
):
17
8. INPUT UVLO
Input UVLO is set with the turn-on threshold voltage (V
ON
R
To set V
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
the Average LED Current section of the Design Guide.
Method #2: Apply an external voltage (V
between 0 and 1.24V to analog dim or to reduce I
lows:
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 nec-
essary 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
voltage. Again, a 0.1µF capacitor connected from the IADJ
pin to GND is recommended. To set I
10. PWM DIMMING METHOD
There are two methods to PWM dim using the LM3409/09HV:
Method #1: Apply an external PWM signal to the EN terminal.
Method #2: Perform external parallel FET shunt dimming as
detailed in the External Parallel FET PWM Dimming section.
UV2
) and the desired hysteresis (V
:
EXT
TURN-ON
) and the internal 5µA current source will set the
, solve for R
UV1
:
HYS
). To set V
LED
LED
, solve for R
ADJ
is calculated as in
) to the IADJ pin
HYS
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LED
, solve for
EXT
as fol-
TURN-
: