750 mV

R3

2

2

t

L2 =

1

K

1

u

1

K

1

u

LM3445

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SNVS570L – JANUARY 2009 – REVISED MAY 2013

(21)

Finally:

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Refer to DESIGN EXAMPLE to better understand the design process.

SETTING THE LED CURRENT

inductor current.

Figure 28. Inductor Current Waveform in CCM

an application running in continuous conduction mode (CCM) is defined as follows:

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

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This is important to calculate because this peak current multiplied 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 reaches 750 mV.

(25)

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 comparator 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 valley-fill capacitors:

The maximum voltage seen by the valley-fill capacitors is:

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