9
LT1725
1725fa
The LT1725 is a current mode switcher controller IC
designed specifically for the isolated flyback topology. The
Block Diagram shows an overall view of the system. Many
of the blocks are similar to those found in traditional
designs, including: Internal Bias Regulator, Oscillator,
Logic, Current Amplifier and Comparator, Driver and Out-
put Switch. The novel sections include a special Flyback
Error Amplifier and a Load Compensation mechanism.
Also, due to the special dynamic requirements of flyback
control, the Logic system contains additional functionality
not found in conventional designs.
The LT1725 operates much the same as traditional current
mode switchers, the major difference being a different
type of error amplifier that derives its feedback informa-
tion from the flyback pulse. Due to space constraints, this
discussion will not reiterate the basics of current mode
switcher/controllers and isolated flyback converters. A
good source of information on these topics is Application
Note AN19.
ERROR AMPLIFIER—PSEUDO DC THEORY
Please refer to the simplified diagram of the Flyback Error
Amplifier. Operation is as follows: when MOSFET output
switch M1 turns off, its drain voltage rises above the V
IN
rail. The amplitude of this flyback pulse as seen on the third
winding is given as:
V
V V I ESR
N
FLBK
OUT F SEC
ST
=
++
()
•
V
F
= D1 forward voltage
I
SEC
= transformer secondary current
ESR = total impedance of secondary circuit
N
ST
= transformer effective secondary-to-third
winding turns ratio
The flyback voltage is then scaled by external resistor
divider R1/R2 and presented at the FB pin. This is then
compared to the internal bandgap reference by the differ-
ential transistor pair Q1/Q2. The collector current from Q1
is mirrored around and subtracted from fixed current
source I
FXD
at the V
C
pin. An external capacitor integrates
this net current to provide the control voltage to set the
current mode trip point.
OPERATIO
U
The relatively high gain in the overall loop will then cause
the voltage at the FB pin to be nearly equal to the bandgap
reference V
BG
. The relationship between V
FLBK
and V
BG
may then be expressed as:
V
RR
R
V
FLBK BG
=
+
()
12
2
Combination with the previous V
FLBK
expression yields an
expression for V
OUT
in terms of the internal reference,
programming resistors, transformer turns ratio and diode
forward voltage drop:
VV
RR
R
NVIESR
OUT BG ST F SEC
=
+
()
()
12
2
–– •
Additionally, it includes the effect of nonzero secondary
output impedance, which is discussed below in further
detail, see Load Compensation Theory. The practical as-
pects of applying this equation for V
OUT
are found in the
Applications Information section.
So far, this has been a pseudo-DC treatment of flyback
error 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 dotted line connections to the
block labeled “ENAB”. Timing signals are then required to
enable and disable the flyback amplifier.
ERROR AMPLIFIER—DYNAMIC THEORY
There are several timing signals which are required for
proper LT1725 operation. Please refer to the Timing
Diagram.
Minimum Output Switch On Time
The LT1725 affects output voltage regulation via flyback
pulse action. If the output switch is not turned on at all,
there will be no flyback pulse and output voltage informa-
tion is no longer available. This would cause irregular loop
response and start-up/latchup problems. The solution cho-
sen is to require the output switch to be on for an absolute
minimum time per each oscillator cycle. This in turn estab-
lishes a minimum load requirement to maintain regula-
tion. See Applications Information for further details.
