OPTO GAINS & EXTERNAL
ERROR AMPLIFIER
FLYBACK POWER STAGE
FEEDBACK GAIN
+
-
LM3450A MULTIPLIER &
SENSING GAINS
V
REF
V
OUT
V
COMP
V
FB
I
CONT
V
E
+
-
G
COMP
G
3450
G
VC
H
FB
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Design Information
Figure 18. Control Loop Block Diagram
9.8
Voltage Control Loop
The CRM topology requires a narrow bandwidth voltage control loop to regulate the output voltage. This
loop needs to be compensated to maintain stability over the desired operating range. The flyback topology
is isolated, therefore the LM3450A internal error amplifier is bypassed and an external secondary side
error amplifier is used instead. The control loop shown in
Figure 18
is comprised of the converter control-
to-output transfer function, the compensator transfer function, and all of the other gains in the loop.
The output voltage is sensed with a resistor divider (R81, R72) and regulated to 1.24V using an LMV431:
(25)
The converter control-to-output transfer function can be approximated as a single pole system:
(26)
The feedback gain (H
FB
) is unity due to the control implementation and the LM3450A device and external
gains are defined:
(27)
A standard PI compensator is used on the secondary to stabilize the system. The error amplifier is
implemented with an LMV431 and a series resistor (R77) and capacitor (C35) in the feedback path as
shown in
Figure 19
. The output of the LMV431 is tied to the cathode of the opto photo-diode. A resistor
(R70 = 2k
Ω
) from the anode of the photodiode to the bias rail provides the current path and ultimately the
output voltage swing of the secondary error amplifier. The primary side of the opto is connected directly to
COMP. With the 5k
Ω
internal pull-up resistor, the maximum current through the primary side of the opto
will be 1mA. A higher frequency roll-off pole is placed on the primary in the form of a capacitor (C24) from
COMP to GND. The resistor divided flyback output voltage is regulated to the 1.24V LMV431 internal
reference. Note the additional soft-start circuit using C34, D13, and D14.
19
SNVA485B – June 2011 – Revised May 2013
AN-2150 LM3450A Evaluation Board
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