LM3445-120VSMEV National Semiconductor, LM3445-120VSMEV Datasheet - Page 20

• Manufacturer Part Number: LM3445-120VSMEV
• Description: BOARD, EVALUATION, FOR LM3445
• Manufacturer: National Semiconductor

Specifications of LM3445-120VSMEV

• Kit Contents: LM3445 110V Assembled Evaluation Board, Application Note, LM3445 Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Converts 190VAC To 135VAC Input, Drives 7 Or 8 Series Connected LEDs At
• Mcu Supported Families: LM3445

www.national.com
Determining the capacitance value of the valley-fill ca-
pacitors:
The valley fill capacitors should be sized to supply energy to
the buck converter (V
peak divided by the number of stages used in the valley fill
(t
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.
FIGURE 19. Two Stage Valley-Ffill V
From the above illustration and the equation for current in a
capacitor, i = C x dV/dt, the amount of capacitance needed at
V
At 60Hz, and a valley-fill circuit of two stages, the hold up time
(t
of an AC half cycle is 180° and the total time of a half AC line
cycle is 8.33 ms. When the angle of the AC waveform is at
30° and 150°, the voltage of the AC line is exactly ½ of its
peak. With a two stage valley-fill circuit, this is the point where
the LED string switches from power being derived from AC
line to power being derived from the hold up capacitors (C7
and C9). 60° out of 180° of the cycle or 1/3 of the cycle the
power is derived from the hold up capacitors (1/3 x 8.33 ms
= 2.78 ms). This is equal to the hold up time (dt) from the
above equation, and dv is the amount of voltage the circuit is
allowed to droop. From the next section (“Determining Maxi-
mum Number of Series Connected LEDs Allowed”) we know
the minimum V
135V
the peak voltage is 64V. Therefore with some margin the volt-
age at V
equal to (P
Total capacitance (C7 in parallel with C9) can now be calcu-
lated. See “ Design Example" section for further calculations
of the valley-fill capacitors.
Determining Maximum Number of Series Connected
LEDs Allowed:
The LM3445 is an off-line buck topology LED driver. A buck
converter topology requires that the input voltage (V
the output circuit must be greater than the voltage of the LED
stack (V
the minimum voltage observed by the buck converter will be
before the maximum number of LEDs allowed can be deter-
mined. Two variables will have to be determined in order to
accomplish this.
1.
X
X
BUCK
). The capacitance value should be calculated when the
) required at V
AC line operating voltage. This is usually 90V
135V
AC
will be calculated as follows:
line. At 90V
LED
BUCK
AC
OUT
) for proper regulation. One must determine what
for North America. Although the LM3445 can
can not droop more than about 15V (dv). (i) is
/V
BUCK
BUCK
BUCK
AC
voltage will be about 45V for a 90V
BUCK
), where P
TRIAC Dimming
is calculated as follows. The total angle
low line operating condition input, ½ of
) when the input line is less than its
OUT
is equal to (V
BUCK
Voltage with no
LED
AC
BUCK
to
x I
AC
LED
30060352
) of
to
).
20
2.
In this example the most common valley-fill circuit will be used
(two stages).
Figure 21 show three triac dimmed waveforms. One can eas-
ily see that the peak voltage (V
be:
Once the triac is firing at an angle greater than 90° the peak
voltage will lower and equal to:
The voltage at V
similar to the waveforms of figure 22.
The purpose of the valley fill circuit is to allow the buck con-
verter to pull power directly off of the AC line when the line
voltage is greater than its peak voltage divided by two (two
stage valley fill circuit). During this time, the capacitors within
the valley fill circuit (C7 and C8) 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
voltage V
Example:
Line voltage = 90V
Valley-Fill = two stage
Depending on what type and value of capacitors are used,
some derating should be used for voltage droop when the
operate at much lower and higher input voltages a range
is needed to illustrate the design process.
How many stages are implemented in the valley-fill circuit
(1, 2 or 3).
FIGURE 20. AC Line with Firing Angles
BUCK
will encounter.
DC
BUCK
AC
) will lower. V
to 135V
with a valley fill stage of two will look
AC
PEAK
DC
) from 0° to 90° will always
is the lowest value that
30060354