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PDF LTC3803-3 Data sheet ( Hoja de datos )
| Número de pieza | LTC3803-3 | |
| Descripción | Constant Frequency Current Mode Flyback DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ VIN and VOUT Limited Only by External Components
■ Adjustable Slope Compensation
■ Internal Soft-Start
■ –40°C to 125°C Operating Temperature Range
■ Constant Frequency 300kHz Operation
■ ±1.5% Reference Accuracy
■ Current Mode Operation for Excellent Line and Load
Transient Response
■ No Minimum Load Requirement
■ Low Quiescent Current: 240μA
■ Low Profile (1mm) SOT-23 Package
U
APPLICATIO S
■ Telecom Power Supplies
■ 42V and 12V Automotive Power Supplies
■ Auxiliary/Housekeeping Power Supplies
■ Power Over Ethernet Powered Devices
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
LTC3803-3
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
DESCRIPTIO
The LTC®3803-3 is a constant frequency current mode
flyback controller optimized for driving 6V-rated N-channel
MOSFETs in high input voltage applications. Constant
frequency operation is maintained down to very light
loads, resulting in less low frequency noise generation
over a wide range of load currents. Slope compensation
can be programmed with an external resistor.
The LTC3803-3 provides ±1.5% output voltage accuracy
and consumes only 240μA of quiescent current. Ground-
referenced current sensing allows LTC3803-3-based con-
verters to accept input supplies beyond the LTC3803-3’s
absolute maximum VCC. A micropower hysteretic start-up
feature allows efficient operation at high input voltages.
For simplicity, the LTC3803-3 can also be powered from
a high VIN through a resistor, due to its internal 9.4V shunt
regulator. An internal undervoltage lockout shuts down
the LTC3803-3 when the input voltage falls below 4.4V,
guaranteeing at least 4.4V of gate drive to the external
MOSFET.
The LTC3803-3 is available in a low profile (1mm) 6-lead
SOT-23 (ThinSOTTM) package.
TYPICAL APPLICATIO
5V Output Nonisolated Telecom Housekeeping Power Supply
1μF
10V
X5R
82k
470pF
VIN
36V TO 72V
10k
UPS840
T1
1μF •
100V
X5R
•
300μF*
6.3V
X5R
VCC
ITH/RUN NGATE
LTC3803-3
GND SENSE
VFB
4.7k
FDC2512
68mΩ
150pF
200V
220Ω
VOUT
5V
2A MAX
20k 105k
38033 TA01
T1: COOPER CTX02-15242
*THREE 100μF UNITS IN PARALLEL
Efficiency vs Load Current
100
VOUT = 5V
95
90
85
80
75
70
65
60
VIN = 36V
VIN = 48V
55 VIN = 60V
VIN = 72V
50
250 500 750 1000 1250 1500 1750 2000
LOAD CURRENT (mA)
38033 TA02
38033fa
1
1 page  
LTC3803-3
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
Start-Up ICC Supply Current
vs Temperature
www.data7s0heVeCt4C u=.VcToUmRNON – 0.1V
60
50
40
30
20
10
0
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
38033 G10
ITH/RUN Shutdown Threshold
vs Temperature
450
400
350
300
250
200
150
100
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
38033 G11
ITH/RUN Start-Up Current Source
vs Temperature
700
VCC = VTURNON + 0.1V
600 VITH/RUN = 0V
500
400
300
200
100
0
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
38033 G12
Peak Current Sense Voltage
vs Temperature
120 VCC = 8V
115
110
105
100
95
90
85
80
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
38033 G13
Soft-Start Time vs Temperature
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
38033 G14
38033fa
5
5 Page 
LTC3803-3
APPLICATIO S I FOR ATIO
CURRENT SENSE RESISTOR CONSIDERATIONS
The external current sense resistor (RSENSE in Figure 2)
wwwa.dllaotawshseteht4eu.ucsomer to optimize the current limit behavior for
the particular application. As the current sense resistor is
varied from several ohms down to tens of milliohms, peak
switch current goes from a fraction of an ampere to several
amperes. Care must be taken to ensure proper circuit
operation, especially with small current sense resistor
values.
For example, a peak switch current of 5A requires a sense
resistor of 0.020Ω. Note that the instantaneous peak
power in the sense resistor is 0.5W and it must be rated
accordingly. The LTC3803-3 has only a single sense line
to this resistor. Therefore, any parasitic resistance in the
ground side connection of the sense resistor will increase
its apparent value. In the case of a 0.020Ω sense resistor,
one milliohm of parasitic resistance will cause a 5%
reduction in peak switch current. So the resistance of
printed circuit copper traces and vias cannot necessarily
be ignored.
PROGRAMMABLE SLOPE COMPENSATION
The LTC3803-3 injects a ramping current through its
SENSE pin into an external slope compensation resistor
(RSL in Figure 2). This current ramp starts at zero right
after the NGATE pin has been high for the LTC3803-3’s
minimum duty cycle of 8%. The current rises linearly
towards a peak of 5μA at the maximum duty cycle of 80%,
shutting off once the NGATE pin goes low. A series resistor
(RSL) connecting the SENSE pin to the current sense
resistor (RSENSE) thus develops a ramping voltage drop.
From the perspective of the SENSE pin, this ramping
voltage adds to the voltage across the sense resistor,
effectively reducing the current comparator threshold in
proportion to duty cycle. This stabilizes the control loop
against subharmonic oscillation. The amount of reduction
in the current comparator threshold (ΔVSENSE) can be
calculated using the following equation:
ΔVSENSE
=
Duty
Cycle
80%
–
8%
•
5μA
•
RSL
Note: LTC3803-3 enforces 8% < Duty Cycle < 80%.
A good starting value for RSL is 5.9k, which gives a 30mV
drop in current comparator threshold at 80% duty cycle.
Designs not needing slope compensation may replace RSL
with a short circuit.
INTERNAL WIDE HYSTERESIS UNDERVOLTAGE
LOCKOUT
The LTC3803-3 is designed to implement DC/DC convert-
ers operating from input voltages of typically 48V or more.
The standard operating topology employs a third trans-
former winding (LBIAS in Figure 2) on the primary side that
provides power for the LTC3803-3 via its VCC pin. How-
ever, this arrangement is not inherently self-starting.
Start-up is affected by the use of an external “trickle-
charge” resistor (RSTART in Figure 2) and the presence of
an internal wide hysteresis undervoltage lockout circuit
that monitors VCC pin voltage. Operation is as follows:
“Trickle charge” resistor RSTART is connected to VIN and
supplies a small current, typically on the order of 100μA,
to charge CVCC. After some time, the voltage on CVCC
reaches the VCC turn-on threshold. The LTC3803-3 then
turns on abruptly and draws its normal supply current. The
NGATE pin begins switching and the external MOSFET
(Q1) begins to deliver power. The voltage on CVCC begins
to decline as the LTC3803-3 draws its normal supply
current, which exceeds that delivered by RSTART. After
some time, typically tens of milliseconds, the output
voltage approaches its desired value. By this time, the
third transformer winding is providing virtually all the
supply current required by the LTC3803-3.
38033fa
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3803-3.PDF ] | |
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