LM3409 Datasheet, PDF (17/30 Pages) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives

4. OUTPUT CAPACITANCE
A minimum output capacitance (C_O-MIN) may be necessary to
reduce Δi_LED-PP below Δi_L-PP. With the specified Δi_LED-PP and
the known dynamic resistance (r_D) of the LED string, solve for
the required impedance (Z_C) for C_O-MIN:
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:
Solve for C_O-MIN:
To set V_TURN-ON, solve for R_UV1:
5. INPUT CAPACITANCE
Set the input voltage ripple (Δv_IN-PP) by solving for the re-
quired minimum capacitance (C_IN-MIN):
The necessary RMS input current rating (I_IN-RMS) is:
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):
Given a PFET with on-resistance (R_DS-ON), solve for the RMS
transistor current (I_T-RMS) and power dissipation (P_T):
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 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 fol-
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_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:
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):
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.
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