Texas Instruments AN-2150 LM3450A, Руководство пользователя

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V
OUT
V
OP1
LMV431
COMP
OPTO
5V LM3450A
5k
R72
R81
R77
C35
R70
C24
SOFT
START
FB
R38
U10
U9
D13
D14
C34
Design Information
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Figure 19. Secondary Error Amplifier Circuit
The compensator transfer function is defined:
(28)
Where the secondary compensator pole is defined:
(29)
And the compensator zero is defined:
(30)
And the primary roll-off pole is defined:
(31)
The resulting control loop gain is
(32)
The compensator design for this system can be complicated; however with some useful assumptions, it
can be simplified. Looking at the total DC gain (G
3450
xG
C0
xH
FB
), the following can be made relatively
constant over all designs:
• R70 = 2k Ω , the 5k Ω internal pull-up, and the 0.55 multiplier gain.
• The opto CTR, though variable over temperature, given a fixed supply rail and a fixed R70 value.
In several cases, the product of two DC gain terms can also be identified as relatively constant over all
designs if all of the previous LM3450A design methodology is observed:
• V
INPK
and K
V
are almost exactly inversely proportional (given x remains constant when solving V
DET
=
V
IN
/x).
• I
P-PK
and R30||R31 are closely inversely proportional (given current limit is a constant percentage above
I
P-PK
).
Given these relationships and following the complete LM3450A design method, the DC gain should only
vary largely with change in output voltage (directly proportional).
The output pole of the converter on the other hand follows these basic relationships:
20 AN-2150 LM3450A Evaluation Board SNVA485B – June 2011 – Revised May 2013
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