Texas Instruments LM3445 Evaluation Board LM3445-230VFLBK/NOPB LM3445-230VFLBK/NOPB Datenbogen
Produktcode
LM3445-230VFLBK/NOPB
V
VF-CAP
=
2
V
AC-RMS
3
+
+
D3
C7
C9
C10
D4
D8
D9
V
BUCK
V+
+
-
+
-
V
BUCK
V
BUCK
V
VF-CAP
=
2
V
AC-RMS
2
SNVS570L – JANUARY 2009 – REVISED MAY 2013
The peak voltage of a two stage valley-fill capacitor is:
(1)
As the AC line decreases from its peak value every cycle, there will be a point where the voltage magnitude of
the AC line is equal to the voltage that each capacitor is charged. At this point diode D3 becomes reversed
biased, and the capacitors are placed in parallel to each other (
the AC line is equal to the voltage that each capacitor is charged. At this point diode D3 becomes reversed
biased, and the capacitors are placed in parallel to each other (
), and V
BUCK
equals the capacitor
voltage.
Figure 20. Two Stage Valley-Fill Circuit When AC Line is Low
A three stage valley-fill circuit performs exactly the same as two-stage valley-fill circuit except now three
capacitors are now charged in series, and when the line voltage decreases to:
capacitors are now charged in series, and when the line voltage decreases to:
(2)
Diode D3 is reversed biased and three capacitors are in parallel to each other.
The valley-fill circuit can be optimized for power factor, voltage hold up and overall application size and cost. The
LM3445 will operate with a single stage or a three stage valley-fill circuit as well. Resistor R8 functions as a
current limiting resistor during start-up, and during the transition from series to parallel connection. Resistors R6
and R7 are 1 M
LM3445 will operate with a single stage or a three stage valley-fill circuit as well. Resistor R8 functions as a
current limiting resistor during start-up, and during the transition from series to parallel connection. Resistors R6
and R7 are 1 M
Ω
bleeder resistors, and may or may not be necessary for each application.
BUCK CONVERTER
The LM3445 is a buck controller that uses a proprietary constant off-time method to maintain constant current
through a string of LEDs. While transistor Q2 is on, current ramps up through the inductor and LED string. A
resistor R3 senses this current and this voltage is compared to the reference voltage at FLTR2. When this
sensed voltage is equal to the reference voltage, transistor Q2 is turned off and diode D10 conducts the current
through the inductor and LEDs. Capacitor C12 eliminates most of the ripple current seen in the inductor. Resistor
R4, capacitor C11, and transistor Q3 provide a linear current ramp that sets the constant off-time for a given
output voltage.
through a string of LEDs. While transistor Q2 is on, current ramps up through the inductor and LED string. A
resistor R3 senses this current and this voltage is compared to the reference voltage at FLTR2. When this
sensed voltage is equal to the reference voltage, transistor Q2 is turned off and diode D10 conducts the current
through the inductor and LEDs. Capacitor C12 eliminates most of the ripple current seen in the inductor. Resistor
R4, capacitor C11, and transistor Q3 provide a linear current ramp that sets the constant off-time for a given
output voltage.
14
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