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PDF LT1105I Data sheet ( Hoja de datos )
| Número de pieza | LT1105I | |
| Descripción | Offline Switching Regulator | |
| Fabricantes | Linear Technology | |
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LT1103/LT1105
Offline Switching Regulator
FEATURES
s ±1% Line and Load Regulation with No Optocoupler
s Switch Frequency Up to 200kHz
s Internal 2A Switch and Current Sense (LT1103)
s Internal 1A Totem-Pole Driver (LT1105)
s Start-Up Mode Draws Only 200µA
s Fully Protected Against Overloads
s Overvoltage Lockout of Main Supply
s Protected Against Underdrive or Overdrive to FET
s Operates in Continuous or Discontinuous Mode
s Ideal for Flyback and Forward Topologies
s Isolated Flyback Mode Has Fully Floating Outputs
U
APPLICATIONS
s Up to 250W Isolated Mains Converter
s Up to 50W Isolated Telecom Converter
s Fully Isolated Multiple Outputs
s Distributed Power Conversion Networks
DESCRIPTION
The LT®1103 Offline Switching Regulator is designed for
high input voltage applications using an external FET
switch whose source is driven by the open collector output
of the LT1103. The LT1103 is optimized for 15W to 100W
applications. For higher power applications or additional
switch current flexibility, the LT1105 is available and its
totem pole output drives the gate of an external FET.
Unique design of the LT1103/LT1105 eliminates the need
for an optocoupler while still providing ±1% load and line
regulation in a magnetic flux-sensed converter. This sig-
nificantly simplifies the design of offline power supplies
and reduces the number of components which must cross
the isolation barrier to one, the transformer.
The LT1103/LT1105 current mode switching techniques
are well suited to transformer isolated flyback and forward
topologies while providing ease of frequency compensa-
tion with a minimum of external components. Low exter-
nal part count for a typical application combines with a
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
Fully Isolated Flyback 100kHz 50W Converter with Load Regulation Compensation
85VAC TO 270VAC
+
+
–
BRIDGE
RECTIFIER
+
LINE
FILTER
220µF
385V
499Ω
1000pF 100Ω
220k
1W
HIGDHANVGOELRT!A!GE!!
1.5KE300A
5W
MUR150
1N4148
BAV21
BAV21
1N4148
13k 39µF +
1% 35V
18.7k
4.75k
1%
0.047µF
VIN
FB
GND
0.047µF
BUK426-800A
VSW
LT1103
10Ω
15V
VC OSC
+
330Ω
0.1µF
390pF
1µF
25V
Danger!! Lethal Voltages Present – See Text
OPTIONAL OUTPUT FILTER
MBR2045
+
10µH
*50V
3600µF
+
5V
10A
50V
470µF
*OUTPUT CAPACITOR IS THREE 1200µF,
50V CAPACITORS IN PARALLEL TO
ACHIEVE REQUIRED RIPPLE CURRENT
RATING AND LOW ESR.
WINDINGS FOR
OPTIONAL
±12VDC OUTPUTS
TRANSFORMER DATA:
COILTRONICS CTX110228-3
L(PRI) = 1.6mH
NPRI:NSEC = 1:0.05
NBIAS:NSEC = 1:0.27
LT1103 TA13
5.25
5.20
5.15
5.10
5.05
5.00
4.95
4.90
4.85
4.80
4.75
01
Load Regulation
220VAC
270VAC 110VAC
85VAC
2 3 4 5 6 7 8 9 10
IOUT (A)
LT1103 TA02
1
1 page  
LT1103/LT1105
ELECTRICAL CHARACTERISTICS
VIN = 20V, VC = 0.85V, OVLO = 0V, VSW Open, TA = 25°C, unless otherwise noted.
SYMBOL
PARAMETER
Switch Output Low Level
(LT1105)
Rise Time (LT1105)
Fall Time (LT1105)
ILIM Threshold Voltage (LT1105)
Low Switch Drive Lockout
Threshold
High Switch Drive Lockout
Threshold
CONDITONS
ISW = 200mA
ISW = 750mA
CL = 1000pF
CL = 1000pF
Duty Cycle = 25% (Note 5)
Measured at VSW (LT1103)
Measured at 15V Gate Bias Reference (LT1105)
Measured at VSW (LT1103)
Measured at 15V Gate Bias Reference (LT1105)
MIN TYP MAX
q 0.25 0.50
q 0.75 1.50
50
20
q 300 375 450
q 9.0 9.5 10.5
q 17.0 18.5 20.0
UNIT
V
V
ns
ns
mV
V
V
The q denotes specifications which apply over the full operating
temperature range.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The OVLO pin is clamped with a 5.5V Zener and can sink a
maximum input current of 1mA.
Note 3: FB input bias current changes as a function of the VC pin voltage.
Rate of change of FB input bias current is 11µA/V of change on VC. By
including a resistor in series with the FB pin, load regulation can be set
to zero.
Note 4: Current limit on VSW is constant for DC < 35% and decreases for
DC > 35% due to internal slope compensation circuity. The LT1103 switch
current limit is given by ILIM = 1.76 (1.536 – DC) above 35% duty cycle.
Note 5: The current limit threshold voltage is constant for DC < 35% and
decreases for DC > 35% due to internal slope compensation circuitry. The
LT1105 switch current limit threshold voltage is given by VLIM = 0.225
(1.7 – DC) above 35% duty cycle.
TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current vs Input Voltage
25
25°C
20
15
10
5
I SHUT
I START
0
0 5 10 15
20
INPUT VOLTAGE (V)
25 30
LT1103 G01
Start-Up Supply Current vs
Input Voltage
500
450
400
350
300
125°C
25°C
250
200
150 –55°C
100
50
0
0 3 6 9 12 15
INPUT VOLTAGE (V)
LT1103 G02
Quiescent Supply Current vs
Input Voltage
22
21
20 125°C
19 25°C
18
17
16 –55°C
15
14
13
12
0 5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
LT1103 G03
5
5 Page 
LT1103/LT1105
PIN FUNCTIONS
LT1103
FB: The Feedback pin is the inverting input to the sampling
error amplifier. The noninverting input is tied to a 4.5V
reference. The FB pin is used for output voltage sensing.
The input bias current is a function of the control pin VC
voltage and can be used for load regulation compensation
by including a resistor in series with the FB pin. The
sampling error amplifier has a typical gm of 0.012 mhos
and the output of the sampling error amplifier has asym-
metrical slew rate to reduce overshoot during start-up
conditions or following the release of an output overload.
VC: The VC control pin is used for frequency compensa-
tion, current limiting and shutdown. It is the high imped-
ance output of the sampling error amplifier and the input
of the current limit comparator.
GND: The Ground pin acts as both the negative sense
point for the internal sampling error amplifier feedback
signal and as the high current path for the 2A switch.
Also, the case of the 7-lead TO-220 is connected to
ground. Proper connections to ground for signal paths
and high current paths must be made in order to insure
good load regulation.
OSC: The Oscillator pin sets the operating frequency of the
regulator with one external capacitor to ground. Maximum
duty cycle can also be adjusted by using an external
resistor to alter the charge/discharge ratio.
VIN: The Input Supply pin is designed to operate with
voltages of 12V to 30V. The supply current is typically
200µA up to the start-up threshold of 16V. Normal oper-
ating supply current is fairly flat at 18mA down to the
shutdown threshold of 7V. Switching is inhibited for VIN
less than 12V due to the gate drive detection circuit.
15V: A 15V reference is used to bias the gate of an external
power FET. The voltage temperature coefficient is typically
3mV/°C and the output can source 30mA. Typical dropout
voltage is 1.5V for VIN less than 17V and 30mA of load
current.
VSW: The Switch Output pin is the collector of the internal
NPN power switch. This pin has a typical ON resistance of
0.4Ω and a minimum breakdown voltage of 50V. This pin
also ties to the FET gate drive detection circuit.
LT1105
All functions on the LT1105 are equivalent to the LT1103
with the exception of the VSW pin and the ILIM pin and the
availability of the OVLO, 5V, and SS functions.
OVLO: The Overvoltage Lockout pin inhibits switching
when the pin is pulled above its threshold voltage of 2.5V.
OVLO is implemented with a resistor divider network from
the rectified DC line and is used to protect the external FET
from an overvoltage condition in the off state. This func-
tion is only available on the 14-lead PDIP.
5V: A 5V reference is available to power primary-side
circuitry. The temperature coefficient is typically
50ppm/°C and the output can source 25mA. This func-
tion is only available on the 14-lead PDIP.
SS: The Soft-Start pin is used to either program start-up
time with a capacitor to ground or to set external current
limit with a resistor divider. The SS pin has a 40µA pull-up
current and is reset to 0V by a 1mA pull-down current
during start-up and shutdown. This function is only avail-
able on the 14-lead PDIP.
VSW: The Switch Output pin is the output of a 1A NPN
totem-pole stage. The VSW pin turns the external FET on by
pulling its gate high. Break-Before-Make action of 200ns
on each switch edge is built in to eliminate cross conduc-
tion currents.
ILIM: The ILIM pin is the input to the current limit amplifier
and requires the use of a noninductive, power sense
resistor from ILIM to ground to set current limit. The typical
current limit threshold voltage is 350mV. The typical input
bias current is 100µA out of the pin.
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LT1105I.PDF ] | |
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