LM3409HVEVAL National Semiconductor, LM3409HVEVAL Datasheet - Page 7

• Manufacturer Part Number: LM3409HVEVAL
• Description: BOARD EVALUATION FOR LM3409HV
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3409HVEVAL

• Current - Output / Channel: 1.5A
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 42V
• Features: Dimmable
• Voltage - Input: 6 ~ 75 V
• Utilized Ic / Part: LM3409HV
• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Drives 12 LEDs At Maximum Average LED Current 1.5A From DC Input Voltage 48V
• Mcu Supported Families: LM3409HV
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Shunt FET Circuit Modification
When the shunt FET (Q3) is on, the LM3409 is driving current
into a short, therefore a maximum off-time (typical 300 µs)
occurs followed by a minimum on-time. Maximum off-time
followed by minimum on-time continues until Q3 is turned off.
At low dimming frequencies and depending on the duty cycle,
the inductor current may be at a very low level when the Q3
turns off. This will eliminate the benefits of using the shunt
FET over the EN pin because the inductor will have to slew
the current back to the nominal value anyways.
A simple modification to the external parallel FET dimming
circuit will keep the inductor current close to its nominal value
when Q3 is turned off. This modification will ensure that the
rise time of the LED current is only limited by the turn-off time
of the shunt FET as desired. The following circuit additions
Multiple off-timers for shunt FET dimming circuit
7
allow for two different off-times to occur. When Q3 is off, the
standard off-timer referenced from V
the Q3 is on, a second off-timer referenced to the gate signal
of the Q3 is enabled and a controlled (non-maximum) off-time
is set.
This modification includes 2 extra diodes (i.e. BAT54H) and
one resistor (R
PWM dimming below 10 kHz or so. In general, this second
off-timer should be set to allow the inductor current to fall no
more than 10% of its nominal value. A simple approximation
can be used to find R
OFF2
) and is only relevant when shunt FET
OFF2
:
O
is set. However when
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