LT1424-9 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1424-9' PDF : 16 Pages View PDF
U
OPERATION
) ∆VRCCOMP
∆ISW
= Data sheet value for RCCOMP pin
action vs switch current
RFB = External “feedback” resistor value
ROUT = Uncompensated output impedance
LT1424-9
) ) ∆VOUT
∆IOUT
= K1
∆VRCCOMP
∆ISW
RFB
ROCOMP
Nominal output impedance cancellation is obtained by
equating this expression with ROUT.
APPLICATIONS INFORMATION
The LT1424-X is an application-specific 8-pin part which
implements an isolated flyback switcher/controller. Three
on-chip thin-film resistors are used to “program” the part
for a specific application including mainly desired output
voltage, transformer turns ratio and secondary circuit ESR
behavior. As of Initial Release, the LT1424-9 is available
which implements the “– 9V PCMCIA II Isolated LAN
Supply” as described in the Typical Application section.
Potential users with a high volume requirement for other
applications are advised as follows: general experimenta-
tion/breadboarding may be done with the LT1425. This is
a general purpose 16-pin part whose functionality is
similar to the LT1424-X, with the exception that the three
application resistors are external user-supplied compo-
nents. Application information relating to the proper
selection of these resistor values is contained within the
LT1425 data sheet. Once technical feasibility is demon-
strated, the potential user may discuss the possibility of an
additional LT1424-X version with the factory.
OUTPUT VOLTAGE ERROR SOURCES
Conventional nonisolated switching power supply ICs
typically have only two substantial sources of output
voltage error— the internal or external resistor divider
network that connects to VOUT and the internal IC refer-
ence. The LT1424-9, which senses the output voltage in
both a dynamic and an isolated manner, exhibits addi-
tional potential error sources to contend with. Some of
these errors are proportional to output voltage, others are
fixed in an absolute millivolt sense. Here is a list of possible
error sources and their effective contribution:
Internal Voltage Reference
The internal bandgap voltage reference is, of course,
imperfect. Its error, both at 25°C and over temperature is
already included in the specifications for Reference
Voltage.
Schottky Diode Drop
The LT1424-9 senses the output voltage from the trans-
former primary side during the flyback portion of the cycle.
This sensed voltage therefore includes the forward drop,
VF, of the rectifier (usually a Schottky diode). Lot-to-lot
and ambient temperature variations 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
(NP/NS) from its ideal value. This increases the output
voltage target by a similar percentage and has been
nominally taken into account in the design of the
LT1424-9. To the extent that secondary leakage induc-
tance varies from part-to-part, the output voltage will be
affected.
Output Impedance Error
The LT1424-9 contains a load compensation function to
provide a nominal, first-order cancellation of the effects
of secondary circuit ESR. Unit-to-unit variation plus
some inherent nonlinearity in the cancellation results in
some residual VOUT variation with load.
sn14249 14249fs
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