LM3445 datasheet(19/26 Pages) NSC

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SETTING THE LED CURRENT

The LM3445 constant off-time control loop regulates the peak

inductor current (I

L2). The average inductor current equals the

average LED current (I

AVE). Therefore the average LED cur-

rent is regulated by regulating the peak inductor current.

30060325

FIGURE 18. Inductor Current Waveform in CCM

Knowing the desired average LED current, I

AVE and the nom-

inal inductor current ripple,

Δi

L, the peak current for an appli-

cation running in continuous conduction mode (CCM) is

defined as follows:

Or, the maximum, or "undimmed", LED current would then be,

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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