Texas Instruments LM3448 Evaluation Board LM3448-EDSNEV/NOPB LM3448-EDSNEV/NOPB 데이터 시트
제품 코드
LM3448-EDSNEV/NOPB
t
V
PEAK
V
+
V
DC
t
V
PEAK
V
+
V
DC
V
DC
t
V
PEAK
V
+
T
=
90°
t
T
=
45°
V
PEAK
V
+
V
+
t
V
PEAK
T
=
135°
B U C K ( M IN )
9 0
2
s in 1 3 5
V
4 5 V
2
u
q
A C - R M S ( M IN )
B U C K ( M IN )
V
2
s in
V
# s t a g e s
u
T
SNOSB51C – SEPTEMBER 2011 – REVISED MAY 2013
The voltage at V
BUCK
with a valley-fill stage of two will look similar to the waveforms of
. The purpose of
the valley-fill circuit is to allow the buck converter to pull power directly off of the AC line when the line voltage is
greater than its peak voltage divided by two (for a two stage valley-fill circuit). During this time, the capacitors
within the valley-fill circuit (C7 and C9) are charged up to the peak of the AC line voltage. Once the line drops
below its peak divided by two, the two capacitors are placed in parallel and deliver power to the buck converter.
One can now see that if the peak of the AC line voltage is lowered due to variations in the line voltage, or if the
TRIAC is firing at an angle above 90°, the DC offset (V
greater than its peak voltage divided by two (for a two stage valley-fill circuit). During this time, the capacitors
within the valley-fill circuit (C7 and C9) are charged up to the peak of the AC line voltage. Once the line drops
below its peak divided by two, the two capacitors are placed in parallel and deliver power to the buck converter.
One can now see that if the peak of the AC line voltage is lowered due to variations in the line voltage, or if the
TRIAC is firing at an angle above 90°, the DC offset (V
DC
) will lower. V
DC
is the lowest value that voltage V
BUCK
will encounter.
(33)
Example:
Line voltage = 90VAC to 135VAC
Valley-fill stages = 2
(34)
Depending on what type and value of capacitors are used, some derating should be used for voltage droop when
the capacitors are delivering power to the buck converter. When the TRIAC is firing at 135° the current through
the LED string will be small. Therefore the droop should be small at this point and a 5% voltage droop should be
a sufficient derating. With this derating, the lowest voltage the buck converter will see is about 42.5V in this
example.
the capacitors are delivering power to the buck converter. When the TRIAC is firing at 135° the current through
the LED string will be small. Therefore the droop should be small at this point and a 5% voltage droop should be
a sufficient derating. With this derating, the lowest voltage the buck converter will see is about 42.5V in this
example.
To determine how many LEDs can be driven, take the minimum voltage the buck converter will see (42.5V) and
divide it by the worst case forward voltage drop of a single LED.
divide it by the worst case forward voltage drop of a single LED.
Example:
42.5V/3.7V = 11.5 LEDs (11 LEDs with margin)
Figure 37. V
BUCK
Waveforms with Various TRIAC Firing Angles
Figure 38. Two Stage Valley-Fill V
BUCK
Waveforms with Various TRIAC Firing Angles
Copyright © 2011–2013, Texas Instruments Incorporated
29
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