User guide

AN375
AN375REV4 21
Step 8) Calculate Sense Resistor R
Sense
(R19)
Use Equation 16 on page 13 to solve for sense resistance R
Sense
. See Equation 43:
Resistor R19 is selected to be a 2.61 standard value.
Step 9) Determine R
CTRL1
(R13)
Use Equation 17 on page 13 to solve for resistance R
CTRL1
. See Equation 44:
Resistor R13 is selected to be a 42.2k standard value.
Step 10) Determine R
CTRL2
(R17)
Use Equation 18 on page 13 to solve for resistance R
CTRL2
. See Equation 45:
Resistor R17 is selected to be a 71.5k standard value.
Step 11) Determine the Switching Frequency Range
The circuit parameters yield a Frequency Profile A. The minimum switching frequency will be found at the
conduction threshold voltage V
Start
when the reflected voltage V
Reflected
is at its maximum tolerance and
that the maximum switching frequency is found at the peak of the highest line voltage when reflected
voltage V
Reflected
is at its minimum tolerance.
To determine period T1, start by calculating duty cycle

FREQ
which renders either a minimum or maximum
switching frequency at the condition specified by a switching Frequency Profile A. Use Equation 13 on page 11
to calculate the duty cycles

FREQ(min)
and 
FREQ(max)
. See Equations 46 and 47:
Use Equation 14 on page 12 to calculate minimum and maximum peak current I
PKFREQ
at the condition
specified by a switching Frequency Profile A. See Equations 48 and 49:
R
Sense
1.4V
I
PK
------------
1.4V
513.6mA
------------------------
2.73== =
[Eq. 43]
R
CTRL1
1.4V 4M
511 I
IN CC
R
Sense

------------------------------------------------------------
1.4V 4M
511 107mA 2.73
-----------------------------------------------------------
37.5k===
[Eq. 44]
R
CTRL2
1.4V N 4M
1.25 511 R
Sense
I
OUT

------------------------------------------------------------------------
1.4V 5.75 4M
1.25 511 2.73 250mA
----------------------------------------------------------------------------
74k== =
[Eq. 45]

FREQ min
1
T3
TT
-------
–


1
V
Start
V
Reflected min
---------------------------------------
+


--------------------------------------------------------
1
2s
17.6s
------------------
–


1
30V
165.6V
-------------------
+


----------------------------------
0.75===
[Eq. 46]

FREQ max
1
T3
TT
-------
–


1
V
INPK min
V
Reflected max
----------------------------------------
+


----------------------------------------------------------
1
2s
17.6s
------------------
–


1
0.9 2 120V
165.6V
------------------------------------------
+


----------------------------------------------------------
0.46===
[Eq. 47]
I
PKFREQ min
2I
IN CC


FREQ min
-------------------------------
2107mA
0.75
-----------------------------
= 285mA==
[Eq. 48]
I
PKFREQ max
2I
IN CC


FREQ max
--------------------------------
2 107mA
0.46
-----------------------------
= 465mA==
[Eq. 49]