LT1424-5 Linear Technology, LT1424-5 Datasheet - Page 11

• Manufacturer Part Number: LT1424-5
• Description: Isolated Flyback Switching Regulator with 5V Output
• Manufacturer: Linear Technology

APPLICATIONS
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, two versions of the LT1424
are available. The LT1424-5, described herein, imple-
ments an isolated 5V output power supply using off-the-
shelf 1:1 transformers as shown in the Typical Applica-
tions. The LT1424-9, described in a separate data sheet,
implements an isolated – 9V output LAN supply with
PCMCIA Type II height compatible components.
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 se-
lection 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 V
reference. The LT1424-5, 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, im-
perfect. Its error, both at 25 C and over temperature is al-
ready included in the specifications for Reference
Voltage.
U
INFORMATION
U
OUT
W
and the internal IC
U
Schottky Diode Drop
The LT1424-5 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,
V
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 (N
from its ideal value. This increases the output voltage tar-
get by a similar percentage and has been nominally taken
into account in the design of the LT1424-5. To the extent
that secondary leakage inductance varies from part-to-
part, the output voltage will be affected.
Output Impedance Error
The LT1424-5 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 V
MINIMUM LOAD CONSIDERATIONS
The LT1424-5 generally provides better low load perfor-
mance than previous generation switcher/controllers
utilizing indirect output voltage sensing techniques. Spe-
cifically, it contains circuitry to detect flyback pulse
“collapse,” thereby supporting operation well into dis-
continuous mode. In general, there are two possible
constraints to ultimate low load operation, minimum
switch ON time which sets a minimum level of delivered
power, and minimum flyback enable time, which deals
with the ability of the feedback system to derive valid
output voltage information from the flyback pulse. In the
application for which the LT1424-5 is designed, the
minimum flyback enable time is more restrictive.
The LT1424-5 derives its output voltage information from
the flyback pulse. If the internal minimum enable time
F
, of the rectifier (usually a Schottky diode). Lot-to-lot
OUT
variation with load.
LT1424-5
11
P
/N
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