8
LT1425
OPERATIO
N
U
Within the dashed lines in the Block Diagram can be found
the R
REF
, R
FB
and R
OCOMP
resistors. They are external
resistors on the user-programmable LT1425. The capaci-
tor connected to the R
CCOMP
pin is also external.
The LT1425 operates much the same as traditional current
mode switchers, the major difference being a different
type of error amplifier which 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 LTC’s
Application Note 19.
ERROR AMPLIFIER—PSEUDO DC THEORY
Please refer to the simplified diagram of the Flyback Error
Amplifier. Operation is as follows: when output switch Q4
turns off, its collector voltage rises above the V
IN
rail. The
amplitude of this flyback pulse, i.e., the difference between
it and V
IN
, is given as:
V
FLBK
=
V
F
= D1 forward voltage
I
SEC
= Transformer secondary current
ESR = Total impedance of secondary circuit
N
SP
= Transformer effective secondary-to-primary
turns ratio
V
OUT
+ V
F
+ (I
SEC
)(ESR)
N
SP
The flyback voltage is then converted to a current by the
action of R
FB
and Q1. Nearly all of this current flows
through resistor R
REF
to form a ground-referred voltage.
This is then compared to the internal bandgap reference by
the differential transistor pair Q2/Q3. The collector current
from Q2 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.
The relatively high gain in the overall loop will then cause
the voltage at the R
REF
resistor to be nearly equal to the
bandgap reference V
BG
. (V
BG
is not present in final output
voltage setting equation. See Applications Information
section.) The relationship between V
FLBK
and V
BG
may
then be expressed as:
V
FLBK
R
FB
V
FLBK
= V
BG
α
= Ratio of Q1 I
C
to I
E
V
BG
= Internal bandgap reference
α
=
or,
R
FB
R
REF
V
BG
R
REF
)
)
1
α
))
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:
V
OUT
= V
BG
– V
F
– I
SEC
(ESR)
R
FB
R
REF
)
)
N
SP
α
)
)
Additionally, it includes the effect of nonzero secondary
output impedance. See Load Compensation for details.
The practical aspects 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 dashed line connections to the
block labeled “ENABLE.” Timing signals are then required
to enable and disable the flyback amplifier.
ERROR AMPLIFIER—DYNAMIC THEORY
There are several timing signals that are required for
proper LT1425 operation. Please refer to the Timing
Diagram.
Minimum Output Switch ON Time
The LT1425 effects 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
chosen is to require the output switch to be on for an
absolute minimum time per each oscillator cycle. This in
turn establishes a minimum load requirement to maintain
