LT1425 Linear Technology, LT1425 Datasheet - Page 14

• Manufacturer Part Number: LT1425
• Description: Isolated Flyback Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
V
calculations. Lot-to-lot and ambient temperature varia-
tions will show up as output voltage shift/drift.
Secondary Leakage Inductance
Leakage inductance on the transformer secondary
reduces the effective primary-to-secondary turns ratio
(N
voltage target by a similar percentage. To the extent that
secondary leakage inductance is constant from part-to-
part, this can be accommodated by adjusting the R
R
Output Impedance Error
An additional error source is caused by transformer sec-
ondary current flow through the real life nonzero imped-
ances of the output rectifier, transformer secondary and
output capacitor. Because the secondary current only
flows during the off portion of the duty cycle, the effective
output impedance equals the “DC” lumped secondary
impedance times the inverse of the off duty cycle. If the
output load current remains relatively constant, or, in less
critical applications, the error may be judged acceptable
and the R
more demanding applications, output impedance error
may be minimized by the use of the load compensation
function (see Load Compensation).
V
The LT1425 determines the size of the flyback pulse by
comparing the V
comparison is not perfect, in the sense that an offset exists
between the sensing mechanism and the actual V
is expressed in the data sheet as V
is fixed in absolute millivolt terms relative to V
exception that it is reflected to V
secondary-to-primary turns ratio).
MINIMUM LOAD CONSIDERATIONS
The LT1425 generally provides better low load perfor-
mance than previous generation switcher/controllers
utilizing indirect output voltage sensing techniques.
Specifically, it contains circuitry to detect flyback pulse
LT1425
14
F
IN
REF
P
of this diode should therefore be included in R
/N
Sense Error
resistor ratio.
S
) from its ideal value. This will increase the output
FB
value adjusted for nominal expected error. In
SW
U
signal to V
INFORMATION
U
IN
IN
sense error. This error
OUT
W
, through R
by any nonunity
OUT
U
(with the
FB
IN
. This
. This
FB
FB
to
“collapse,” thereby supporting operation well into discon-
tinuous mode. Nevertheless, there still remain constraints
to ultimate low load operation. They relate to the minimum
switch ON time and the minimum enable time. Discontinu-
ous mode operation will be assumed in the following
theoretical derivations.
As outlined in the Operation section, the LT1425 utilizes a
minimum output switch ON time, t
combined with expected V
yield an expression for minimum delivered power.
This expression then yields a minimum output current
constraint:
An additional constraint has to do with the minimum
enable time. The LT1425 derives its output voltage infor-
mation from the flyback pulse. If the internal minimum
enable time pulse extends beyond the flyback pulse, loss
of regulation will occur. The onset of this condition can be
determined by setting the width of the flyback pulse equal
to the sum of the flyback enable delay, t
minimum enable time, t
the load is then:
which yields a minimum output constraint:
Min Power =
I
where,
f = Switching frequency (nominally 285kHz)
L
V
V
t
Min Power =
OUT(MIN)
ON
PRI
IN
OUT
= Input voltage
= Output switch minimum ON time
= Transformer primary side inductance
= Output voltage
=
= (V
= (V
1
2
1
2
1
2
OUT
OUT
(L
L
L
PRI
)(I
)(I
PRI
SEC
EN
f
f
OUT
OUT
)(V
. Minimum power delivered to
IN
f
(V
[V
and switching frequency to
OUT
)
)
IN
OUT
• t
)
ON
• (t
ON
(V
. This value can be
IN
)
EN
2
• t
+ t
ON
ED
ED
)
, plus the
2
)]
2