|
|
PDF LT1725 Data sheet ( Hoja de datos )
| Número de pieza | LT1725 | |
| Descripción | General Purpose Isolated Flyback Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
|
Hay una vista previa y un enlace de descarga de LT1725 (archivo pdf) en la parte inferior de esta página. Total 28 Páginas | ||
|
No Preview Available !
www.DataSheet4U.com
LT1725
General Purpose
Isolated Flyback Controller
FEATURES
s Drives External Power MOSFET with External
ISENSE Resistor
s Application Input Voltage Limited Only by
External Power Components
s Senses Output Voltage Directly from Primary Side
Winding—No Optoisolator Required
s Accurate Regulation Without User Trims
s Regulation Maintained Well into Discontinuous Mode
s Switching Frequency from 50kHz to 250kHz with
External Capacitor
s Optional Load Compensation
s Optional Undervoltage Lockout
s Available in 16-Pin SO and SSOP Packages
U
APPLICATIO S
s Telecom Isolated Converters
s Offline Isolated Power Supplies
s Instrumentation Power Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
DESCRIPTIO
The LT®1725 is a monolithic switching regulator control-
ler specifically designed for the isolated flyback topology.
It drives the gate of an external MOSFET and is generally
powered from a third transformer winding. These features
allow for an application input voltage limited only by
external power path components. The third transformer
winding also provides output voltage feedback informa-
tion, such that an optoisolator is not required. Its gate
drive capability coupled with a suitable external MOSFET
can deliver load power up to tens of watts.
The LT1725 has a number of features not found on most
other switching regulator ICs. By utilizing current mode
switching techniques, it provides excellent AC and DC line
regulation. Its unique control circuitry can maintain regu-
lation well into discontinuous mode in most applications.
Optional load compensation circuitry allows for improved
load regulation. An optional undervoltage lockout pin
halts operation when the application input voltage is too
low. An optional external capacitor implements a soft-
start function. A 3V output is available at up to several mA
for powering primary side application circuitry.
TYPICAL APPLICATIO
48V to Isolated 5V Converter
35.7k
1%
3.01k
1%
LT1725
3VOUT
FB
1nF
47pF
SFST
VC
OSCAP
VCC +
51k
51k
51k
2.7k
0.1µF
tON
ENDLY
UVLO
MENAB
ROCMP GATE
RCMPC ISENSE
SGND PGND
BAS16 VIN 36V TO 72V
CTX02-14989
6
1
470pF
18Ω
22Ω
47k
15µF
1µF
2
68Ω
9
12CWQ06
820k
150pF
100pF 33k
1.5µF
11
10 +
150µF
4 12
IRF620
0.18Ω
1725 TA01a
VOUT = 5V
IOUT = 0 to 2A
51Ω
1W
DataSheet4 U .com
Output Load Regulation
5.25
VIN = 36V VIN = 48V
VIN = 72V
5.00
4.75
0
0.5 1.0 1.5 2.0
ILOAD (A)
1725 F10b
1725f
1
1 page  
www.DataSheet4U.com
LT1725
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum Switch-On Time vs
Temperature
275
RTON = 50k
250
Minimum Enable Time vs
Temperature
275
RMINENAB = 50k
250
Enable Delay Time vs
Temperature
275
250
225 225 225
200 200 200
175 175 175
150 150 150
125
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1725 G10
125
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1725 G11
125
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1725 G12
Feedback Amplifier Output Current
vs FB Pin Voltage
80
60
40
20 TA = 25°C
TA = –55°C
0
–20
TA = 125°C
–40
–60
–80
1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40
FB PIN VOLTAGE (V)
1725 G13
Feedback Amplifier
Transconductance vs Temperature
1600
1400
1200
1000
800
600
400
200
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1725 G14
Soft-Start Charging Current vs
Temperature
60
V(SFST) = 0V
50
Soft-Start Sink Current vs
Temperature
2.5
V(SFST) = 1.5V
2.0
40
1.5
30
1.0
20
0.5
10
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1725 G15
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1725 G16
DataSheet4 U .com
1725f
5
5 Page 
www.DataSheet4U.com
U
OPERATIO
transformer secondary and output capacitor. This has
been represented previously by the expression “ISEC •
ESR.” However, it is generally more useful to convert this
expression to an effective output impedance. Because the
secondary current only flows during the off portion of the
duty cycle, the effective output impedance equals the
lumped secondary impedance times the inverse of the OFF
duty cycle. That is:
ROUT
=
ESR
1
DCOFF
where
ROUT = effective supply output impedance
ESR = lumped secondary impedance
DCOFF = OFF duty cycle
Expressing this in terms of the ON duty cycle, remember-
ing DCOFF = 1 – DC,
ROUT
=
ESR
1
1– DC
DC = ON duty cycle
In less critical applications, or if output load current
remains relatively constant, this output impedance error
may be judged acceptable and the external FB resistor
divider adjusted to compensate for nominal expected
error. In more demanding applications, output impedance
error may be minimized by the use of the load compensa-
tion function.
To implement the load compensation function, a voltage is
developed that is proportional to average output switch
current. This voltage is then impressed across the external
ROCMP resistor, and the resulting current acts to increase
the VBG reference used by the flyback error amplifier. As
output loading increases, average switch current increases
to maintain rough output voltage regulation. This causes
an increase in ROCMP resistor current which effects a
corresponding increase in target output voltage.
Assuming a relatively fixed power supply efficiency, Eff,
Power Out = Eff • Power In
VOUT • IOUT = Eff • VIN • IIN
Average primary side current may be expressed in terms
of output current as follows:
DataSheet4 U .com
LT1725
IIN
=
VOUT
VIN • EFF
• IOUT
combining the efficiency and voltage terms in a single
variable:
IIN = K1 • IOUT, where
K1=
VOUT
VIN • EFF
Switch current is converted to voltage by the external
sense resistor and averaged/lowpass filtered by R3 and
the external capacitor on RCMPC. This voltage is then
impressed across the external ROCMP resistor by op amp
A1 and transistor Q3. This produces a current at the
collector of Q3 which is then mirrored around and then
subtracted from the FB node. This action effectively in-
creases the voltage required at the top of the R1/R2
feedback divider to achieve equilibrium. So the effective
change in VOUT target is:
( )∆VOUT =
K1• ∆IOUT
RSENSE
ROCMP
•
(R1|| R2)
or
∆VOUT
∆IOUT
=
K1
RSENSE
ROCMP
• (R1||R2)
Nominal output impedance cancellation is obtained by
equating this expression with ROUT:
ROUT
=
K1
RSENSE
ROCMP
• (R1||R2)
and
ROCMP
=
K1
RSENSE
ROUT
• (R1||R2)
where
K1 = dimensionless variable related to VIN, VOUT and
efficiency as above
RSENSE = external sense resistor
ROUT = uncompensated output impedance
(R1||R2) = impedance of R1 and R2 in parallel
The practical aspects of applying this equation to deter-
mine an appropriate value for the ROCMP resistor are found
in the Applications Information section.
1725f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LT1725.PDF ] | |
Hoja de datos destacado
| Número de pieza | Descripción | Fabricantes |
| LT1720 | Dual/Quad/ 4.5ns/ Single Supply 3V/5V Comparators with Rail-to-Rail Outputs | Linear Technology |
| LT1720 | 4ns/ 150MHz Dual Comparator with Independent Input/Output Supplies | Linear Technology |
| LT1721 | Dual/Quad/ 4.5ns/ Single Supply 3V/5V Comparators with Rail-to-Rail Outputs | Linear Technology |
| LT1725 | General Purpose Isolated Flyback Controller | Linear Technology |
| Número de pieza | Descripción | Fabricantes |
| SLA6805M | High Voltage 3 phase Motor Driver IC. |
 Sanken |
| SDC1742 | 12- and 14-Bit Hybrid Synchro / Resolver-to-Digital Converters. |
 Analog Devices |
|
DataSheet.es es una pagina web que funciona como un repositorio de manuales o hoja de datos de muchos de los productos más populares, |
| DataSheet.es | 2020 | Privacy Policy | Contacto | Buscar |