LM3445-120VSMEV National Semiconductor, LM3445-120VSMEV Datasheet - Page 19

• Manufacturer Part Number: LM3445-120VSMEV
• Description: BOARD, EVALUATION, FOR LM3445
• Manufacturer: National Semiconductor

Specifications of LM3445-120VSMEV

• Kit Contents: LM3445 110V Assembled Evaluation Board, Application Note, LM3445 Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Converts 190VAC To 135VAC Input, Drives 7 Or 8 Series Connected LEDs At
• Mcu Supported Families: LM3445

SETTING THE LED CURRENT
The LM3445 constant off-time control loop regulates the peak
inductor current (I
average LED current (I
rent is regulated by regulating the peak inductor current.
Knowing the desired average LED current, I
inal inductor current ripple, Δi
cation running in continuous conduction mode (CCM) is
defined as follows:
Or, the maximum, or "undimmed", LED current would then be,
FIGURE 18. Inductor Current Waveform in CCM
L2
). The average inductor current equals the
AVE
). Therefore the average LED cur-
L
, the peak current for an appli-
AVE
and the nom-
30060325
19
This is important to calculate because this peak current mul-
tiplied by the sense resistor R3 will determine when the
internal comparator is tripped. The internal comparator turns
the control MOSFET off once the peak sensed voltage reach-
es 750 mV.
Current Limit: Under normal circumstances, the trip voltage
on the PWM comparator would be less than or equal to 750
mV, depending on the amount of dimming. However, if there
is a short circuit or an excessive load on the output, higher
than normal switch currents will cause a voltage above 1.27V
on the ISNS pin which will trip the I-LIM comparator. The I-
LIM comparator will reset the RS latch, turning off Q2. It will
also inhibit the Start Pulse Generator and the COFF com-
parator by holding the COFF pin low. A delay circuit will
prevent the start of another cycle for 180 µs.
VALLEY FILL CAPACITORS
Determining voltage rating and capacitance value of the val-
ley-fill capacitors:
The maximum voltage seen by the valley-fill capacitors is:
This is, of course, if the capacitors chosen have identical ca-
pacitance values and split the line voltage equally. Often a
20% difference in capacitance could be observed between
like capacitors. Therefore a voltage rating margin of 25% to
50% should be considered.
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