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PDF LT1424-5 Data sheet ( Hoja de datos )
| Número de pieza | LT1424-5 | |
| Descripción | Isolated Flyback Switching Regulator with 5V Output | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1424-5
Isolated Flyback
Switching Regulator
with 5V Output
FEATURES
s No Transformer “Third Winding” or Optoisolator
Required
s Designed for Use with 1:1 Ratio Transformers
s Fixed, Application Specific 5V Output Voltage
s Regulation Maintained Well into Discontinuous
Mode (Light Load)
s Load Compensation Provides Excellent
Load Regulation
s Available in 8-Pin PDIP and SO Packages
s Operating Frequency: 285kHz
U
APPLICATIO S
s Isolated Communication Supplies
s Industrial Automation
s Instrumentation Systems
DESCRIPTIO
The LT®1424-5 is a monolithic high power switching
regulator specifically designed for the isolated flyback
topology. No “third winding” or optoisolator is required;
the integrated circuit senses the isolated output voltage
directly from the primary side flyback waveform. A high
current, high efficiency switch is included on the die along
with all oscillator, control and protection circuitry.
The LT1424-5 operates with input supply voltages from
3V to 20V and draws only 7mA quiescent current. It can
deliver up to 400mA at 5V with no external power devices.
By utilizing current mode switching techniques, it pro-
vides excellent AC and DC line regulation.
The LT1424-5 has a number of features not found on other
switching regulator ICs. Its unique control circuitry can
maintain regulation well into discontinuous mode. Load
compensation circuitry allows for improved load regula-
tion. An externally activated shutdown mode reduces total
supply current to 20µA typical for standby operation.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
5V
+ C1
100µF
10V
INPUT
COM
1000pF
5V Output Isolated Power Supply
0.1µF
18
2 SHDN RCCOMP 7
VC VIN
LT1424-5
36
SYNC
4
VSW 5
SGND PGND
ISOLATION
BARRIER
T1
47Ω
MBRS130LT3
1N5248
47Ω
6
•
MBR0540T4
330pF
7
34
••
12
+
0.1µF
C1, C2: AVX TPS D107M010R0080
T1: DALE LPE-4841-A307
330pF
5V
400mA
C2
100µF
10V
1.8k
OUT
1424-5 TA01 COM
Load Regulation
5.25
5.00
4.75
0
100 200 300
OUTPUT CURRENT (mA)
400
1424-5 TA02
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum Switch On Time
vs Temperature
275
250
225
200
175
150
125
100
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
1424-5 G10
Flyback Enable Delay Time
vs Temperature
250
225
200
175
150
125
100
75
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
1424-5 G11
LT1424-5
Minimum Flyback Enable Time
vs Temperature
275
250
225
200
175
150
125
100
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
1424-5 G12
PIN FUNCTIONS
SHDN (Pin 1): Shutdown. This pin is used to turn off the
regulator and reduce VIN input current to a few tens of
microamperes. The SHDN pin can be left floating when
unused.
VC (Pin 2): Control Voltage. This pin is the output of the
feedback amplifier and the input of the current compara-
tor. Frequency compensation of the overall loop is effected
by placing a capacitor between this node and ground.
SYNC (Pin 3): Pin to synchronize internal oscillator to
external frequency reference. It is directly logic compat-
ible and can be driven with any signal between 10% and
90% duty cycle. If unused, this pin should be tied to
ground.
SGND (Pin 4): Signal Ground. This pin is a clean ground.
The internal reference and feedback amplifier are referred
to it. Keep the ground path connection to the VC compen-
sation capacitor free of large ground currents.
PGND (Pin 5): Power Ground. This pin is the emitter of the
power switch device and has large currents flowing through
it. It should be connected directly to a good quality ground
plane.
VSW (Pin 6): This is the collector node of the output switch
and has large currents flowing through it. Keep the traces
to the switching components as short as possible to
minimize electromagnetic radiation and voltage spikes.
VIN (Pin 7): Supply Voltage. Bypass input supply pin with
10µF or more. The part goes into undervoltage lockout
when VIN drops below 2.8V. Undervoltage lockout stops
switching and pulls the VC pin low.
RCCOMP (Pin 8): Pin for the External Filter Capacitor for
Load Compensation Function. A common 0.1µF
ceramic capacitor will suffice.
5
5 Page 
LT1424-5
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, 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 VOUT and the internal IC
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.
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,
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 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 VOUT variation with load.
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
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT1424-5.PDF ] | |
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