Datasheet
A C -R M S (M IN )
B U C K (M IN )
V 2 sin
V
# s ta g e s
u T
L -P K (U N D IM )
7 5 0 m V
I
R 3
L
A V E (U N D IM ) L 2 -P K (U N D IM )
i
I I
2
'
L
L 2 -P K A V E
i
i I
2
'
t
t
OFF
t
ON
I
L2
(t)
I
AVE
I
L2-PK
'i
L
I
L2-MIN
LM3448
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SNOSB51C –SEPTEMBER 2011–REVISED MAY 2013
Figure 35. Inductor Current Waveform in CCM
Knowing the desired average LED current (I
AVE
) and the nominal inductor current ripple (Δi
L
), the peak current for
an application running in CCM is defined as follows:
(26)
Or, the maximum (i.e. un-dimmed) LED current would then be,
(27)
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 SW FET off once the peak sensed voltage
reaches 750 mV.
(28)
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 the internal SW FET. 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 180µs.
VALLEY FILL CAPACITORS
The maximum voltage seen by the valley-fill capacitors is,
(29)
This assumes that the capacitors chosen have identical capacitance 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.
The valley-fill capacitors should be sized to supply energy to the buck converter (V
BUCK
) when the input line is
less than its peak divided by the number of stages used in the valley-fill. The capacitance value should be
calculated when the TRIAC is not firing (i.e. when full LED current is being drawn by the LED string). The
maximum power is delivered to the LED string at this time and therefore the most capacitance will be needed.
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