Datasheet

LT3837
10
3837fd
OPERATION
The LT3837 is a current mode switcher controller IC
designed specifically for use in an isolated flyback topology
employing synchronous rectification. The LT3837 operation
is similar to traditional current mode switchers. The major
difference is that output voltage feedback is derived via
sensing the output voltage through the transformer. This
precludes the need of an optoisolator in isolated designs
greatly improving dynamic response and reliability. The
LT3837 has a unique feedback amplifier that samples a
transformer winding voltage during the flyback period and
uses that voltage to control output voltage.
The internal blocks are similar to many current mode
controllers. The differences lie in the flyback feedback
amplifier and load compensation circuitry. The logic block
also contains circuitry to control the special dynamic
requirements of flyback control.
See Application Note 19 for more information on the
basics of current mode switcher/controllers and isolated
flyback converters.
Feedback Amplifier—Pseudo DC Theory
For the following discussion refer to the simplified
Flyback Feedback Amplifier diagram. When the primary side
MOSFET switch MP turns off, its drain voltage rises above
the V
IN
rail. Flyback occurs when the primary MOSFET is
off and the synchronous secondary MOSFET is on. Dur-
ing flyback the voltage on nondriven transformer pins is
determined by the secondary voltage. The amplitude of this
flyback pulse as seen on the third winding is given as:
V
FLBK
=
V
OUT
+I
SEC
• ESR +R
DS(ON)
( )
N
SF
R
DS(ON)
= on resistance of the synchronous MOSFET M
S
I
SEC
= transformer secondary current
ESR = impedance of secondary circuit capacitor, winding
and traces
N
SF
= transformer effective secondary-to-feedback winding
turns ratio (i.e., N
S
/N
FLBK
)
The flyback voltage is then scaled by an external resistive
divider R1/R2 and presented at the FB pin. The feedback
amplifier then compares the voltage to the internal bandgap
reference. The feedback amp is actually a transconductance
amplifier whose output is connected to V
C
only during a
period in the flyback time. An external capacitor on the V
C
pin integrates the net feedback amp current to provide the
control voltage to set the current mode trip point.
The regulation voltage at the FB pin is nearly equal to the
bandgap reference V
FB
because of the high gain in the
overall loop. The relationship between V
FLBK
and V
FB
is
expressed as:
V
FLBK
=
R1
+
R2
R2
• V
FB
Combining this with the previous V
FLBK
expression yields
an expression for V
OUT
in terms of the internal reference,
programming resistors and secondary resistances:
V
OUT
=
R1+R2
R2
•V
FB
•N
SF






–I
SEC
• ESR+R
DS(ON)
( )
Rearranging yields the equation for R1.
R1= R2•
V
OUT
+I
SEC
• ESR+R
DS(ON)
( )
N
SF
( )
V
FB
( )
–1
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
The effect of nonzero secondary output impedance is dis-
cussed in further detail; see Load Compensation Theory.
The practical aspects of applying this equation for V
OUT
are found in the Applications Information section.
Feedback Amplifier Dynamic Theory
So far, this has been a pseudo-DC treatment of flyback
feedback amplifier operation. But the flyback signal is a
pulse, not a DC level. Provision must be made to enable
the flyback amplifier only when the flyback pulse is present.
This is accomplished by the “Enable” line in the diagram.
Timing signals are then required to enable and disable the
flyback amplifier. There are several timing signals which
are required for proper LT3837 operation. Please refer to
the Timing Diagram.