LTC3803ES6-3#TRPBF [Linear]
LTC3803-3 - Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT; Package: SOT; Pins: 6; Temperature Range: -40°C to 85°C;
| 型号： | LTC3803ES6-3#TRPBF |
| 厂家： | 
Linear |
| 描述： | LTC3803-3 - Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT; Package: SOT; Pins: 6; Temperature Range: -40°C to 85°C 开关 光电二极管 |
| 文件： | 总16页 (文件大小：167K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3803-3
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
FEATURES
DESCRIPTION
The LTC®3803-3 is a constant frequency current mode
flybackcontrolleroptimizedfordrivingN-channelMOSFETs
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
ofloadcurrents.Slopecompensationcanbeprogrammed
with an external resistor.
n
V and V
Limited Only by External Components
IN
OUT
n
n
n
n
n
n
Adjustable Slope Compensation
Internal Soft-Start
–55°C to 150°C Operating Temperature Range
Constant Frequency 300kHz Operation
±±.ꢀ5 Reference Accuracy
Current Mode Operation for Excellent Line and Load
Transient Response
No Minimum Load Requirement
Low Quiescent Current: 240μA
Low Profile (±mm) SOT-23 Package
The LTC3803-3 provides ±±.ꢀ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
n
n
n
absolute maximum V . A micropower hysteretic start-up
CC
APPLICATIONS
featureallowsefficientoperationathighinputvoltages.For
simplicity, theLTC3803-3canalsobepoweredfromahigh
n
Telecom Power Supplies
V througharesistor,duetoitsinternalshuntregulator.An
IN
n
42V and ±2V Automotive Power Supplies
internal undervoltage lockout shuts down the LTC3803-3
when the input voltage is too low to provide sufficient gate
drive to the external MOSFET.
n
Auxiliary/Housekeeping Power Supplies
Power over Ethernet Powered Devices
n
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
and ThinSOT and No R
are trademarks of Linear Technology Corporation. All other
SENSE
The LTC3803-3 is available in a low profile (±mm) 6-lead
trademarks are the property of their respective owners.
™
SOT-23 (ThinSOT ) package.
TYPICAL APPLICATION
5V Output Nonisolated Telecom Housekeeping Power Supply
Efficiency vs Load Current
±00
9ꢀ
90
8ꢀ
80
7ꢀ
70
6ꢀ
60
ꢀꢀ
ꢀ0
V
IN
V
= ꢀV
OUT
36V TO 72V
UPS840
V
OUT
ꢀV
T±
•
2A MAX
±μF
±00V
XꢀR
300μF*
6.3V
±0k
XꢀR
±μF
±0V
XꢀR
•
V
CC
I
/RUN NGATE
LTC3803-3
FDC2ꢀ±2
68mΩ
TH
82k
470pF
±ꢀ0pF
200V
4.7k
V
V
V
V
= 36V
= 48V
= 60V
= 72V
IN
IN
IN
IN
GND
SENSE
V
FB
220Ω
20k
±0ꢀk
2ꢀ0
7ꢀ0 ±000 ±2ꢀ0 ±ꢀ00 ±7ꢀ0 2000
LOAD CURRENT (mA)
ꢀ00
38033 TA0±
T±: COOPER CTX02-±ꢀ242
*THREE ±00μF UNITS IN PARALLEL
38033 TA02
38033fd
1
LTC3803-3
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
TOP VIEW
V
to GND
CC
Low Impedance Source .......................... –0.3V to 8V
I
/RUN ±
GND 2
6 NGATE
ꢀ V
TH
Current Fed....................................... 2ꢀmA into V *
CC
CC
V
FB
3
4 SENSE
NGATE Voltage.......................................... –0.3V to VCC
V , I /RUN Voltages............................... –0.3V to 3.ꢀV
FB TH
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
SENSE Voltage............................................ –0.3V to ±V
NGATE Peak Output Current (<±0μs) ......................... ±A
Operating Junction Temperature Range (Notes 2, 3)
LTC3803E-3, LTC3803I-3 ................... –40°C to ±2ꢀ°C
LTC3803H-3....................................... –40°C to ±ꢀ0°C
LTC3803MP-3.................................... –ꢀꢀ°C to ±ꢀ0°C
Storage Temperature Range................... –6ꢀ°C to ±ꢀ0°C
Lead Temperature (Soldering, ±0 sec) .................. 300°C
T
JMAX
= ±ꢀ0°C, θ = ±92°C/W
JA
*LTC3803-3 internal clamp circuit self regulates V voltage to 9.ꢀV.
CC
ORDER INFORMATION
LEAD FREE FINISH
LTC3803ES6-3#PBF
LTC3803IS6-3#PBF
LTC3803HS6-3#PBF
LTC3803MPS6-3#PBF
LEAD BASED FINISH
LTC3803MPS6-3
TAPE AND REEL
PART MARKING*
LTCJS
PACKAGE DESCRIPTION
6-Lead Plastic TSOT-23
6-Lead Plastic TSOT-23
6-Lead Plastic TSOT-23
6-Lead Plastic TSOT-23
PACKAGE DESCRIPTION
6-Lead Plastic TSOT-23
TEMPERATURE RANGE
LTC3803ES6-3#TRPBF
LTC3803IS6-3#TRPBF
LTC3803HS6-3#TRPBF
–40°C to ±2ꢀ°C
–40°C to ±2ꢀ°C
–40°C to ±ꢀ0°C
–ꢀꢀ°C to ±ꢀ0°C
TEMPERATURE RANGE
–ꢀꢀ°C to ±ꢀ0°C
LTCJT
LTCJT
LTC3803MPS6-3#TRPBF LTCJT
TAPE AND REEL
PART MARKING
LTCJT
LTC3803MPS6-3#TR
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C. VCC = 8V, unless otherwise noted. (Notes 2, 3)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
l
l
l
V
V
V
Turn On Voltage
Turn Off Voltage
LTC3803E-3, LTC3803I-3
LTC3803H-3
7.6
7.ꢀ
7.ꢀ
8.7
8.7
8.7
9.2
9.2ꢀ
9.4
V
V
V
TURNON
CC
CC
LTC3803MP-3
l
l
l
l
V
LTC3803E-3
LTC3803I-3
LTC3803H-3
LTC3803MP-3
4.6
4.4
4
ꢀ.7
ꢀ.7
ꢀ.7
ꢀ.7
7
7
7
V
V
V
V
TURNOFF
4
7.2
l
V
V
V
V
Hysteresis (V
– V )
TURN0FF
±
3
V
HYST
CC
CC
TURNON
Shunt Regulator Voltage
I
CC
= ±mA, V
= 0V
CLAMP±mA
ITH/RUN
LTC3803E-3, LTC3803I-3
LTC3803H-3
l
l
l
8.3
8.3
8
9.4
9.4
9.4
±0.3
±0.ꢀ
±0.ꢀ
V
V
V
LTC3803MP-3
38033fd
2
LTC3803-3
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C. VCC = 8V, unless otherwise noted. (Notes 2, 3)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
V
Shunt Regulator Voltage
I
CC
= 2ꢀmA, V
= 0V
ITH/RUN
CLAMP2ꢀmA
CC
l
l
l
LTC3803E-3, LTC3803I-3
LTC3803H-3
8.4
8.4
8.±
9.ꢀ
9.ꢀ
9.ꢀ
±0.ꢀ
±0.7
±0.7
V
V
V
LTC3803MP-3
V
V
– V
Margin
LTC3803E-3, LTC3803I-3,
LTC3803H-3
LTC3803MP-3
MARGIN
CLAMP±mA
TURNON
l
l
0.0ꢀ
0.03
0.6
0.6
V
V
I
CC
Input DC Supply Current
Normal Operation
Start-Up
(Note 4)
V
V
= ±.3V
TURNON
240
40
3ꢀ0
90
μA
μA
ITH/RUN
= V
– ±00mV
CC
V
Shutdown Threshold (at I /RUN)
V
> V
, V
Falling
ITHSHDN
TH
CC
TURNON ITH/RUN
l
l
l
LTC3803E-3
0.±ꢀ
0.±0
0.09
0.28
0.28
0.28
0.4ꢀ
0.4ꢀ
0.4ꢀ
V
V
V
LTC3803I-3, LTC3803H-3
LTC3803MP-3
I
Start-Up Current Source
V
= 0V
0.2
0.3
0.4
μA
ITHSTART
ITH/RUN
V
FB
Regulated Feedback Voltage
(Note ꢀ)
0°C ≤ T ≤ 8ꢀ°C
0.788
0.780
0.780
0.780
0.780
0.800
0.800
0.800
0.800
0.800
0.8±2
0.8±2
0.820
0.820
0.820
V
V
V
V
V
J
l
l
l
l
LTC3803E-3: –40°C ≤ T ≤ 8ꢀ°C
J
LTC3803I-3: –40°C ≤ T ≤ ±2ꢀ°C
J
J
LTC3803H-3: –40°C ≤ T ≤ ±ꢀ0°C
LTC3803MP-3: –ꢀꢀ°C ≤ T ≤ ±ꢀ0°C
J
V
Peak Current Sense Voltage
R
= 0 (Note 6)
IMAX
m
SL
l
l
l
LTC3803E-3
90
8ꢀ
8ꢀ
±00
±00
±00
±±ꢀ
±±ꢀ
±20
mV
mV
mV
LTC3803I-3, LTC3803H-3
LTC3803MP-3
g
Error Amplifier Transconductance
Output Voltage Line Regulation
I
Pin Load = ±ꢀμA (Note ꢀ)
200
333
0.0ꢀ
±0
ꢀ00
μA/V
mV/V
nA
TH/RUN
ΔV
(Note ꢀ)
(Note ꢀ)
O(LINE)
I
f
V
Input Current
FB
ꢀ0
330
9.6
90
FB
OSC
Oscillator Frequency
V
V
V
C
C
= ±.3V
270
70
300
8
kHz
5
ITH/RUN
ITH/RUN
ITH/RUN
DC
DC
Minimum Switch On Duty Cycle
Maximum Switch On Duty Cycle
Gate Drive Rise Time
= ±.3V, V = 0.8V
FB
ON(MIN)
ON(MAX)
= ±.3V, V = 0.8V
80
5
FB
t
t
I
t
= 3000pF
40
ns
RISE
LOAD
LOAD
Gate Drive Fall Time
= 3000pF (Note 7)
40
ns
FALL
Peak Slope Compensation Output Current (Note 7)
Soft-Start Time
ꢀ
μA
SLMAX
SFST
±.4
ms
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
conjunction with board layout, the rated package thermal impedance and
other environmental factors.
Note 3: Junction temperature T is calculated from the ambient
J
temperature T and power dissipation P according to the following
A
D
Note 2: The LTC3803-3 is tested under pulsed load conditions such
formula:
that T ≈ T . The LTC3803E-3 is guaranteed to meet specifications
J
A
T = T + (P • 230°C/W).
J
A
D
from 0°C to 8ꢀ°C junction temperature. Specifications over the –40°C
to ±2ꢀ°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3803I-3 is guaranteed over the –40°C to ±2ꢀ°C operating junction
temperature range, the LTC3803H-3 is guaranteed over the –40°C to
±ꢀ0°C operating junction temperature range and the LTC3803MP-3 is
tested and guaranteed over the full –ꢀꢀ°C to ±ꢀ0°C operating junction
temperature range. High junction temperatures degrade operating
lifetimes; operating lifetime is derated for junction temperatures greater
than ±2ꢀ°C. Note that the maximum ambient temperature consistent with
these specifications is determined by specific operating conditions in
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5: The LTC3803-3 is tested in a feedback loop that servos V to the
FB
output of the error amplifier while maintaining I /RUN at the midpoint of
TH
the current limit range.
Note 6: Peak current sense voltage is reduced dependent on duty cycle
and an optional external resistor in series with the SENSE pin (R ). For
SL
details, refer to the programmable slope compensation feature in the
Applications Information section.
Note 7: Guaranteed by design.
38033fd
3
LTC3803-3
T = 25°C unless otherwise noted.
A
TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage
vs Supply Voltage
Reference Voltage
vs VCC Shunt Regulator Current
Reference Voltage vs Temperature
820
8±ꢀ
8±0
80ꢀ
800
79ꢀ
790
78ꢀ
780
801.0
800.8
800.6
800.4
800.2
800.0
799.8
799.6
799.4
799.2
799.0
804
803
802
80±
V
= 8V
V
b V
CC
CC
CLAMP1mA
800
799
798
797
796
–30
0
30
60
±20 ±ꢀ0
–60
90
8
9.5
6
7
7.5
8.5
9
ꢀ
±0
I
20
6.5
0
2ꢀ
±ꢀ
(mA)
V
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
CC
CC
38033 G0±
38033 F02
38033 G03
Oscillator Frequency
vs Temperature
Oscillator Frequency
vs Supply Voltage
Oscillator Frequency
vs VCC Shunt Regulator Current
330
320
3±0
300
290
280
270
330
320
3±0
300
290
280
270
330
320
3±0
300
290
280
270
V
= 8V
CC
–60 –30
0
30
60
90 ±20 ±ꢀ0
6
7
7.ꢀ
8
8.ꢀ
9
0
±0
±ꢀ
20
2ꢀ
30
3ꢀ
6.ꢀ
ꢀ
TEMPERATURE (°C)
V
SUPPLY VOLTAGE (V)
I
CC
(mA)
CC
38033 G04
38033 G0ꢀ
38033 G06
VCC Undervoltage Lockout
Thresholds vs Temperature
VCC Shunt Regulator Voltage
vs Temperature
ICC Supply Current
vs Temperature
3ꢀ0
32ꢀ
300
27ꢀ
2ꢀ0
22ꢀ
200
±0.0
9.9
9.8
9.7
9.6
9.ꢀ
9.4
9.3
9.2
9.±
9.0
9.0
8.ꢀ
8.0
7.ꢀ
7.0
6.ꢀ
6.0
ꢀ.ꢀ
ꢀ.0
4.ꢀ
4.0
V = 8V
CC
V
ITH/RUN
= ±.3V
V
TURNON
I
= 2ꢀmA
CC
I
= ±mA
V
CC
TURNOFF
–30
0
30
60
±20 ±ꢀ0
–60
90
–30
0
30
60
±20 ±ꢀ0
–60
90
–30
0
30
60
±20 ±ꢀ0
–60
90
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
38033 G09
38033 G08
38033 G07
38033fd
4
LTC3803-3
T = 25°C unless otherwise noted.
A
TYPICAL PERFORMANCE CHARACTERISTICS
Start-Up ICC Supply Current
vs Temperature
ITH/RUN Shutdown Threshold
vs Temperature
ITH/RUN Start-Up Current Source
vs Temperature
800
700
600
ꢀ00
400
300
200
±00
0
4ꢀ0
400
90
80
70
60
ꢀ0
40
30
20
±0
0
V
V
= V
+ 0.±V
TURNON
V
= V
TURNON
– 0.±V
CC
ITH/RUN
CC
= 0V
3ꢀ0
300
2ꢀ0
200
±ꢀ0
±00
–30
0
30
60
±20 ±ꢀ0
–60
90
–30
0
30
60
±20 ±ꢀ0
–30
0
30
60
±20 ±ꢀ0
–60
90
–60
90
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
38033 G±2
38033 G±0
38033 G±±
Peak Current Sense Voltage
vs Temperature
Soft-Start Time vs Temperature
±20
±±ꢀ
±±0
±0ꢀ
±00
9ꢀ
4.0
3.ꢀ
3.0
2.ꢀ
2.0
±.ꢀ
±.0
0.ꢀ
0
V
= 8V
CC
90
8ꢀ
80
–30
0
30
60
±20 ±ꢀ0
–60
90
–30
0
30
60
±20 ±ꢀ0
–60
90
TEMPERATURE (°C)
TEMPERATURE (°C)
38033 G±3
38033 G±4
38033fd
5
LTC3803-3
PIN FUNCTIONS
SENSE(Pin4):Thispinperformstwofunctions.Itmonitors
switch current by reading the voltage across an external
current sense resistor to ground. It also injects a current
ramp that develops slope compensation voltage across
an optional external programming resistor.
I /RUN (Pin 1): This pin performs two functions. It
TH
serves as the error amplifier compensation point as well
as the run/shutdown control input. Nominal voltage range
is 0.7V to ±.9V. Forcing this pin below the shutdown
threshold(V
)causestheLTC3803-3toshutdown.
ITHSHDN
In shutdown mode, the NGATE pin is held low.
V
(Pin 5): Supply Pin. Must be closely decoupled to
CC
GND (Pin 2).
GND (Pin 2): Ground Pin.
NGATE (Pin 6): Gate Drive for the External N-Channel
V (Pin3):Receivesthefeedbackvoltagefromanexternal
FB
MOSFET. This pin swings from 0V to V .
resistive divider across the output.
CC
BLOCK DIAGRAM
ꢀ
V
CC
SHUTDOWN
COMPARATOR
0.3μA 0.28V
+
–
V
< V
TURNON
CC
UNDERVOLTAGE
LOCKOUT
V
CC
SHUNT
800mV
REFERENCE
REGULATOR
SHUTDOWN
SOFT-
START
CLAMP
CURRENT
COMPARATOR
V
–
+
CC
ERROR
AMPLIFIER
GATE
DRIVER
+
–
SWITCHING
R
S
NGATE
LOGIC AND
BLANKING
CIRCUIT
Q
6
V
FB
3
2
SLOPE
COMP
CURRENT
RAMP
20mV
GND
300kHz
OSCILLATOR
±.2V
SENSE
4
I
/RUN
TH
±
38033 BD
38033fd
6
LTC3803-3
OPERATION
TheLTC3803-3isaconstantfrequencycurrentmodecon-
troller for flyback and DC/DC boost converter applications
in a tiny ThinSOT package. The LTC3803-3 is designed so
that none of its pins need to come in contact with the input
or output voltages of the power supply circuit of which it
is a part, allowing the conversion of voltages well beyond
the LTC3803-3’s absolute maximum ratings.
voltage regulation loop is closed. For example, whenever
the load current increases, output voltage will decrease
slightly, and sensing this, the error amplifier raises the
I /RUN voltage by sourcing current into the I /RUN pin,
TH
TH
raising the current comparator threshold, thus increasing
the peak currents through the transformer primary and
secondary. Thisdeliversmorecurrenttotheload,bringing
the output voltage back up.
Main Control Loop
The I /RUN pin serves as the compensation point for
TH
Duetospacelimitations,thebasicsofcurrentmodeDC/DC
conversion will not be discussed here; instead, the reader
isreferredtothedetailedtreatmentinApplicationNote ±9,
or in texts such as Abraham Pressman’s Switching Power
Supply Design.
the control loop. Typically, an external series RC network
is connected from I /RUN to ground and is chosen for
TH
optimalresponsetoloadandlinetransients.Theimpedance
of this RC network converts the output current of the error
amplifier to the I /RUN voltage which sets the current
TH
comparator threshold and commands considerable influ-
Please refer to the Block Diagram and the Typical Ap-
plication on the front page of this data sheet. An external
resistive voltage divider presents a fraction of the output
ence over the dynamics of the voltage regulation loop.
Start-Up/Shutdown
voltage to the V pin. The divider must be designed so
FB
that when the output is at the desired voltage, the V pin
The LTC3803-3 has two shutdown mechanisms to disable
FB
voltage will equal the 800mV from the internal reference.
and enable operation: an undervoltage lockout on the V
CC
If the load current increases, the output voltage will de-
supply pin voltage, and a forced shutdown whenever ex-
crease slightly, causing the V pin voltage to fall below
ternal circuitry drives the I /RUN pin low. The LTC3803-3
FB
TH
800mV. The error amplifier responds by feeding current
transitionsintoandoutofshutdownaccordingtothestate
into the I /RUN pin. If the load current decreases, the
diagram (Figure ±).
TH
V
voltage will rise above 800mV and the error amplifier
will sink current away from the I /RUN pin.
FB
TH
ThevoltageattheI /RUNpincommandsthepulse-width
TH
LTC3803-3
SHUT DOWN
modulator formed by the oscillator, current comparator
and RS latch. Specifically, the voltage at the I /RUN pin
TH
sets the current comparator’s trip threshold. The current
comparator monitors the voltage across a current sense
resistor in series with the source terminal of the external
MOSFET. The LTC3803-3 turns on the external power
MOSFET when the internal free-running 300kHz oscillator
sets the RS latch. It turns off the MOSFET when the cur-
rent comparator resets the latch or when 805 duty cycle
is reached, whichever happens first. In this way, the peak
current levels through the flyback transformer’s primary
V
> V
ITH/RUN
ITHSHDN
TURNON
V
< V
V
< V
CC
TURNOFF
ITH/RUN ITHSHDN
AND V > V
CC
(NOMINALLY ꢀ.7V)
(NOMINALLY 0.28V)
(NOMINALLY 8.7V)
LTC3803-3
ENABLED
38033 F0±
and secondary are controlled by the I /RUN voltage.
TH
Figure 1. Start-Up/Shutdown State Diagram
Since the I /RUN voltage is increased by the error ampli-
TH
fier whenever the output voltage is below nominal, and
decreased whenever output voltage exceeds nominal, the
38033fd
7
LTC3803-3
OPERATION
The undervoltage lockout (UVLO) mechanism prevents
regulator from the V pin to GND will draw as much
CC
the LTC3803-3 from trying to drive a MOSFET with in-
current as needed through this resistor to regulate the
sufficient V . The voltage at the V pin must exceed
V
voltage to around 9.ꢀV as long as the V pin is not
GS
CC
CC CC
V
(nominally 8.7V) at least momentarily to enable
LTC3803-3 operation. The V voltage is then allowed
forced to sink more than 2ꢀmA. This shunt regulator is
always active, even when the LTC3803-3 is in shutdown,
TURNON
CC
to fall to V
(nominally ꢀ.7V) before undervoltage
since it serves the vital function of protecting the V pin
from seeing too much voltage.
TURNOFF
CC
lockoutdisablestheLTC3803-3.ThiswideUVLOhysteresis
range supports the use of a bias winding on the flyback
transformer to power the LTC3803-3—see the section
Powering the LTC3803-3.
For higher efficiency or for wide V range applications,
IN
flybackcontrollersaretypicallypoweredthroughaseparate
bias winding on the flyback transformer. The LTC3803-3
The I /RUN pin can be driven below the shutdown
has the wide UVLO hysteresis (±V min) and small V
CC
TH
threshold (V
) to force the LTC3803-3 into shut-
supply current draw (<90μA when V < V
) that is
ITHSHDN
CC
TURNON
down. An internal 0.3μA current source always tries to
needed to support such bootstrapped hysteretic start-up
pull this pin towards V . When the I /RUN pin voltage
schemes.
CC
TH
is allowed to exceed V
, and V exceeds V
,
ITHSHDN
CC
TURNON
The V pin must be bypassed to ground immediately
CC
the LTC3803-3 begins to operate and an internal clamp
adjacent to the IC pins with a minimum of a ±μF ceramic
or tantalum capacitor. Proper supply bypassing is neces-
sary to supply the high transient currents required by the
MOSFET gate driver.
immediately pulls the I /RUN pin up to about 0.7V. In
TH
operation, the I /RUN pin voltage will vary from roughly
TH
0.7V to ±.9V to represent current comparator thresholds
from zero to maximum.
Adjustable Slope Compensation
Internal Soft-Start
TheLTC3803-3injectsaꢀμApeakcurrentrampoutthrough
itsSENSEpinwhichcanbeusedforslopecompensationin
designs that require it. This current ramp is approximately
linear and begins at zero current at 85 duty cycle, reach-
ing peak current at 805 duty cycle. Additional details are
provided in the Applications Information section.
An internal soft-start feature is enabled whenever the
LTC3803-3 comes out of shutdown. Specifically, the I /
TH
RUN voltage is clamped and is prevented from reaching
maximum until roughly ±.4ms has passed. This allows
the input and output currents of LTC3803-3-based power
supplies to rise in a smooth and controlled manner on
start-up.
Powering the LTC3803-3
In the simplest case, the LTC3803-3 can be powered from
a high voltage supply through a resistor. A built-in shunt
38033fd
8
LTC3803-3
APPLICATIONS INFORMATION
Many LTC3803-3 application circuits can be derived from
TRANSFORMER DESIGN CONSIDERATIONS
the topology shown in Figure 2.
Transformer specification and design is perhaps the most
critical part of applying the LTC3803-3 successfully. In
addition to the usual list of caveats dealing with high fre-
quency power transformer design, the following should
prove useful.
The LTC3803-3 itself imposes no limits on allowed power
output,inputvoltageV ordesiredregulatedoutputvoltage
IN
V
OUT
;thesearealldeterminedbytheratingsontheexternal
power components. The key factors are: Q±’s maximum
drain-source voltage (BV ), on-resistance (R
)
DS(ON)
DSS
Turns Ratios
and maximum drain current, T±’s saturation flux level and
winding insulation breakdown voltages, C and C ’s
IN
OUT
Due to the use of the external feedback resistor divider
ratio to set output voltage, the user has relative freedom
in selecting transformer turns ratio to suit a given appli-
cation. Simple ratios of small integers, e.g., ±:±, 2:±, 3:2,
etc. can be employed which yield more freedom in setting
total turns and mutual inductance. Simple integer turns
ratios also facilitate the use of “off-the-shelf” configu-
maximum working voltage, ESR, and maximum ripple
current ratings, and D± and R
’s power ratings.
SENSE
T±
L
BIAS
•
•
D2
V
IN
D±
™
V
rable transformers such as the Coiltronics VERSA-PAC
OUT
C
IN
series in applications with high input to output voltage
ratios. For example, if a 6-winding VERSA-PAC is used
with three windings in series on the primary and three
windings in parallel on the secondary, a 3:± turns ratio
will be achieved.
R3
R
I
C
START
L
L
SEC
OUT
PRI
•
ꢀ
C
VCC
V
CC
±
2
6
/RUN NGATE
LTC3803-3
Q±
TH
C
C
R
SL
4
GND
R±
SENSE
Turns ratio can be chosen on the basis of desired duty
cycle. However, remember that the input supply voltage
plus the secondary-to-primary referred version of the
flyback pulse (including leakage spike) must not exceed
the allowed external MOSFET breakdown rating.
V
FB
R
SENSE
3
R2
38033 F02
Figure 2. Typical LTC3803-3 Application Circuit
Leakage Inductance
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to occur after the
outputswitch(Q±)turn-off.Thisisincreasinglyprominent
at higher load currents, where more stored energy must
be dissipated. In some cases a snubber circuit will be
required to avoid overvoltage breakdown at the MOSFET’s
drain node. Application Note ±9 is a good reference on
snubber design.
The regulated output voltage is determined by the resistor
divider across V
(R± and R2 in Figure 2). The ratio
OUT
of R2 to R± needed to produce a desired V
calculated:
can be
OUT
VOUT – 0.8V
R2 =
•R1
0.8V
Choose resistance values for R± and R2 to be as large as
possible in order to minimize any efficiency loss due to
A bifilar or similar winding technique is a good way to
minimize troublesome leakage inductances. However,
remember that this will limit the primary-to-second-
ary breakdown voltage, so bifilar winding is not always
practical.
the static current drawn from V , but just small enough
OUT
so that when V
is in regulation, the error caused by
OUT
the nonzero input current to the V pin is less than ±5.
FB
A good rule of thumb is to choose R± to be 80k or less.
38033fd
9
LTC3803-3
APPLICATIONS INFORMATION
CURRENT SENSE RESISTOR CONSIDERATIONS
currentcomparatorthreshold(ΔV
)canbecalculated
SENSE
using the following equation:
The external current sense resistor (R
in Figure 2)
SENSE
allows the user 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.
Duty Cycle – 8%
80%
Note: LTC3803-3 enforces 85 < Duty Cycle < 805.
ΔVSENSE
=
• 5μA •RSL
A good starting value for R is ꢀ.9k, which gives a 30mV
SL
drop in current comparator threshold at 805 duty cycle.
DesignsnotneedingslopecompensationmayreplaceR
with a short circuit.
SL
For example, a peak switch current of ꢀA requires a sense
resistor of 0.020Ω. Note that the instantaneous peak
power in the sense resistor is 0.ꢀW 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 resis-
tor, 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.
INTERNAL WIDE HYSTERESIS UNDERVOLTAGE
LOCKOUT
TheLTC3803-3isdesignedtoimplementDC/DCconverters
operatingfrominputvoltagesoftypically48Vormore.The
standard operating topology employs a third transformer
winding (L
in Figure 2) on the primary side that pro-
BIAS
vides power for the LTC3803-3 via its V pin. However,
CC
this arrangement is not inherently self-starting. Start-up
is affected by the use of an external trickle-charge resistor
(R
in Figure 2) and the presence of an internal wide
START
PROGRAMMABLE SLOPE COMPENSATION
hysteresis undervoltage lockout circuit that monitors V
pin voltage. Operation is as follows:
CC
TheLTC3803-3injectsarampingcurrentthroughitsSENSE
pin into an external slope compensation resistor (R in
Trickle-charge resistor R
is connected to V and
IN
SL
START
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 85. The current rises linearly towards a
peak of ꢀμA at the maximum duty cycle of 805, shutting
supplies a small current, typically on the order of ±00μA,
to charge C . After some time, the voltage on C
VCC
VCC
reaches the V turn-on threshold. The LTC3803-3 then
CC
turns on abruptly and draws its normal supply current.
off once the NGATE pin goes low. A series resistor (R )
TheNGATEpinbeginsswitchingandtheexternalMOSFET
SL
connecting the SENSE pin to the current sense resistor
(Q±) begins to deliver power. The voltage on C
begins
VCC
(R
) thus develops a ramping voltage drop. From
SENSE
to decline as the LTC3803-3 draws its normal supply
current, which exceeds that delivered by R . After
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
START
some time, typically tens of milliseconds, the output volt-
age approaches its desired value. By this time, the third
transformer winding is providing virtually all the supply
current required by the LTC3803-3.
38033fd
10
LTC3803-3
APPLICATIONS INFORMATION
One potential design pitfall is undersizing the value of
In applications where a third transformer winding is
undesirable or unavailable, the shunt regulator allows
the LTC3803-3 to be powered through a single dropping
resistor from V to V , in conjunction with a bypass
capacitor C . In this case, the normal supply current
VCC
drawn by the LTC3803-3 will discharge C
too rapidly;
VCC
before the third winding drive becomes effective, the V
CC
IN
CC
turn-off threshold will be reached. The LTC3803-3 turns
capacitor, C , that closely decouples V to GND (see
VCC
CC
off, and the V node begins to charge via R
back up
Figure 3). This simplicity comes at the expense of reduced
CC
START
to the V turn-on threshold. Depending on the particular
efficiency due to the static power dissipation in the R
CC
VCC
situation, this may result in either several on-off cycles
dropping resistor.
beforeproperoperationisreachedorpermanentrelaxation
The shunt regulator can draw up to 2ꢀmA through the
oscillation at the V node.
CC
V
pin to GND to drop enough voltage across R
to
IN
CC
VCC
Component selection is as follows:
regulate V to around 9.ꢀV. For applications where V
CC
is low enough such that the static power dissipation in
Resistor R
should be made small enough to yield a
START
R
is acceptable, using the V shunt regulator is the
VCC
CC
worst-case minimum charging current greater than the
maximum rated LTC3803-3 start-up current, to ensure
simplest way to power the LTC3803-3.
there is enough current to charge C
to the V turn-
V
IN
VCC
CC
on threshold. It should be made large enough to yield
a worst-case maximum charging current less than the
minimum rated LTC3803-3 supply current, so that in
operation, most of the LTC3803-3’s supply current is
delivered through the third winding. This results in the
highest possible efficiency.
R
LTC3803-3
VCC
V
CC
GND
C
VCC
38033 F03
Figure 3. Powering the LTC3803-3 Via the
Internal Shunt Regulator
CapacitorC shouldthenbemadelargeenoughtoavoid
VCC
the relaxation oscillation behavior described above. This
is complicated to determine theoretically as it depends on
the particulars of the secondary circuit and load behavior.
Empirical testing is recommended.
EXTERNAL PREREGULATOR
The circuit in Figure 4 shows a third way to power the
LTC3803-3. An external series preregulator consisting of
seriespasstransistorQ±,ZenerdiodeD±,andbiasresistor
R brings V to at least 7.6V nominal, well above the V
The third transformer winding should be designed so that
its output voltage, after accounting for the D2’s forward
B
CC
CC
turn-off threshold. Resistor R
momentarily charges
voltagedrop,exceedsthemaximumV turn-offthreshold.
START
CC
the V node up to the V turn-on threshold, enabling
Also, thethirdwinding’snominaloutputvoltageshouldbe
CC
CC
the LTC3803-3.
at least 0.ꢀV below the minimum rated V clamp voltage
CC
to avoid running up against the LTC3803-3’s V shunt
V
CC
IN
regulator, needlessly wasting power.
R
Q±
R
START
LTC3803-3
B
V
CC
SHUNT REGULATOR
V
CC
D±
GND
C
VCC
In applications including a third transformer winding,
8.2V
the internal V shunt regulator serves to protect the
CC
38033 F04
LTC3803-3 from overvoltage transients as the third wind-
ing is powering up.
Figure 4. Powering the LTC3803-3 with an External Preregulator
38033fd
11
LTC3803-3
TYPICAL APPLICATIONS
2W Isolated Housekeeping Telecom Converter
BASꢀ±6
PRIMARY SIDE
±0V, ±00mA
OUTPUT
T±
•
2.2μF
±μF
BASꢀ±6
V
IN
36V TO 7ꢀV
•
SECONDARY SIDE
±0V, ±00mA
OUTPUT
2.2μF
BASꢀ±6
9.2k
±
±k 220k
•
SECONDARY
SIDE GROUND
±nF
LTC3803-3
/RUN NGATE
22k
6
I
FDC2ꢀ±2
TH
2
3
ꢀ
4
V
GND
CC
T±: PULSE ENGINEERING PA0648
OR TYCO TTI8698
806Ω
ꢀ.6k
±μF
V
SENSE
FB
0.±Ω
38033 TA03
PRIMARY GROUND
38033fd
12
LTC3803-3
TYPICAL APPLICATIONS
4:1 Input Range 3.3V Output Isolated Flyback DC/DC Converter
T±
+
V
3.3V
3A
+
PA±277NL
OUT
V
IN
±8V TO 72V
±00μF
6.3V
s3
•
2.2μF
220k
–
V
PDS±040
IN
±00k
•
MMBTA42
GND
BASꢀ±6
68Ω
±ꢀ0pF
PDZ6.8B
V
CC
±0Ω
BASꢀ±6
22Ω
680Ω
•
0.±μF
±
2
3
6
ꢀ
4
I
/RUN
TH
GATE
FDC2ꢀ±2
LTC3803-3
V
CC
GND
+
V
OUT
4.7k
SENSE
V
FB
BAT760
0.±μF
0.040Ω
BASꢀ±6
270Ω
V
CC
6.8k
+
V
OUT
±
6
ꢀ
4
V
OPTO
COMP
FB
IN
LT4430
PS280±-±
47pF
±00k
2.2nF
22.±k
0.±μF
±
2
ꢀ6k
2
3
GND
0.33μF
OC
BASꢀ±6
38033 TA0ꢀ
Efficiency vs Load Current
84
82
80
78
76
74
72
V
IN
V
IN
= 48V
= 24V
70
0
±
2
3
4
38033 TA0ꢀa
I
(A)
OUT
38033fd
13
LTC3803-3
PACKAGE DESCRIPTION
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 0ꢀ-08-±636)
2.90 BSC
(NOTE 4)
0.62
MAX
0.9ꢀ
REF
±.22 REF
±.4 MIN
±.ꢀ0 – ±.7ꢀ
2.80 BSC
3.8ꢀ MAX 2.62 REF
(NOTE 4)
PIN ONE ID
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.30 – 0.4ꢀ
6 PLCS (NOTE 3)
0.9ꢀ BSC
0.80 – 0.90
0.20 BSC
DATUM ‘A’
0.0± – 0.±0
±.00 MAX
0.30 – 0.ꢀ0 REF
±.90 BSC
0.09 – 0.20
(NOTE 3)
S6 TSOT-23 0302
NOTE:
±. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
ꢀ. MOLD FLASH SHALL NOT EXCEED 0.2ꢀ4mm
6. JEDEC PACKAGE REFERENCE IS MO-±93
38033fd
14
LTC3803-3
REVISION HISTORY (Revision history begins at Rev D)
REV
DATE
DESCRIPTION
PAGE NUMBER
D
6/±0
MP-grade part added. Reflected throughout the data sheet.
± to ±6
38033fd
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3803-3
TYPICAL APPLICATION
Efficiency vs Load
±00
9ꢀ
90
8ꢀ
80
7ꢀ
70
V
= 3.3V
OUT
90% Efficient Synchronous Flyback Converter
V
3.3V
±.ꢀA
*
OUT
V
IN
36V TO 72V
T±
Q2
D±
•
•
C
C
O
220k
IN
V
IN
V
IN
V
IN
V
IN
= 36V
= 48V
= 60V
= 72V
•
±nF
33k
±
2
3
6
ꢀ
4
I
/RUN
TH
Q±
ꢀ00
7ꢀ0
±000 ±2ꢀ0 ±ꢀ00 ±7ꢀ0 2000
GATE
LOAD CURRENT (mA)
38033 TA04b
LTC3803-3
0.±μF
ꢀ60Ω
4.7k
V
GND
= 0.8V
CC
8.06k
Efficiency vs Load
SENSE
V
FB
38033 TA04a
±00
9ꢀ
90
8ꢀ
80
7ꢀ
70
2ꢀ.ꢀk*
FB
±0μF
±0V
R
CS
V
= ꢀV*
OUT
R
V
OUT
T±: PULSE ENGINEERING PA±006
Q±: FAIRCHILD FDC2ꢀ±2
Q2: VISHAY Si9803
C : TDK ±μF, ±00V, XꢀR
IN
C : TDK ±00μF, 6.3V, XꢀR
O
R
: VISHAY OR IRC, 80mΩ
CS
*FOR ꢀV OUTPUT CHANGE R TO 42.2k
D±: PHILIPS BASꢀ±6
FB
V
V
V
V
= 36V
= 48V
= 60V
= 72V
IN
IN
IN
IN
ꢀ00
±000 ±2ꢀ0 ±ꢀ00 ±7ꢀ0 2000
7ꢀ0
LOAD CURRENT (mA)
38033 TA04c
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3ꢀ73
Isolated Flyback Switching Regulator with 60V
Integrated Switch
3V ≤ V ≤ 40V, No Opto-Isolator or Third Winding Required, Up to 7W
IN
Output Power, MSOP-±6E
LTC380ꢀ/
LTC380ꢀ-ꢀ
Adjustable Constant Frequency Flyback, Boost, SEPIC
DC/DC Controller
V
and V
Limited Only by External Components, 3mm × 3mm DFN-±0,
IN
OUT
MSOP-±0E Packages
LTC3873/
LTC3873-ꢀ
No R
™ Constant Frequency Flyback, Boost, SEPIC
V
IN
and V Limited Only by External Components, 8-pin ThinSOT or
SENSE
OUT
Controller
2mm × 3mm DFN-8 Packages
LT37ꢀ7
Boost, Flyback, SEPIC and Inverting Controller
2.9V ≤ V ≤ 40V, ±00kHz to ±MHz Programmable Operating Frequency,
IN
3mm × 3mm DFN-±0 and MSOP-±0E Package
LT37ꢀ8
Boost, Flyback, SEPIC and Inverting Controller
ꢀ.ꢀV ≤ V ≤ ±00V, ±00kHz to ±MHz Programmable Operating Frequency,
IN
3mm × 3mm DFN-±0 and MSOP-±0E
LTC±87±/LTC±87±-±/ Wide Input Range, No R
Low Quiescent Current
Programmable Operating Frequency, 2.ꢀV ≤ V ≤ 36V, Burst Mode®
SENSE
IN
LTC±87±-7
Flyback, Boost and SEPIC Controller
Operation at Light Load, MSOP-±0
38033fd
LT 0610 REV D • PRINTED IN USA
LinearTechnology Corporation
±630 McCarthy Blvd., Milpitas, CA 9ꢀ03ꢀ-74±7
16
●
●
© LINEAR TECHNOLOGY CORPORATION 2006
(408) 432-±900 FAX: (408) 434-0ꢀ07 www.linear.com
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