LM3409EVAL National Semiconductor, LM3409EVAL Datasheet - Page 5

• Manufacturer Part Number: LM3409EVAL
• Description: BOARD EVALUATION FOR LM3409
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3409EVAL

• Current - Output / Channel: 1A
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 15V
• Features: Dimmable
• Voltage - Input: 6 ~ 42 V
• Utilized Ic / Part: LM3409
• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Drives 4 LEDs At Maximum Average LED Current 1A From DC Input Voltage 24V
• Mcu Supported Families: LM3409
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

6. P-CHANNEL MOSFET
Determine minimum Q1 voltage rating and current rating:
A 70V, 5.7A PFET is chosen with R
20nC. Determine I
The chosen component from step 6 is:
7. RE-CIRCULATING DIODE
Determine minimum D1 voltage rating and current rating:
A 60V, 5A diode is chosen with V
The chosen component from step 7 is:
8. INPUT UNDER-VOLTAGE LOCKOUT (UVLO)
Solve for R3:
The closest 1% tolerance resistor is 49.9 kΩ therefore V
is:
T-RMS
and P
T
:
D
= 750mV. Determine P
DS-ON
= 190mΩ and Q
HYS
g
D
=
:
5
Solve for R2:
The closest 1% tolerance resistor is 6.98 kΩ therefore V
ON
The chosen components from step 8 are:
9. IADJ CONNECTION METHOD
The IADJ pin controls the high-side current sense threshold
as outlined in the datasheet. 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 termi-
nal 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
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:
The Typical Waveforms section 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.
is:
LED
= 1A:
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