LTC3803ES6-3#TR [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#TR |
| 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：152K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3803
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
FEATURES
DESCRIPTION
The LTC®3803 is a constant frequency current mode fly-
back 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.
n
V and V
Adjustable Slope Compensation
Internal Soft-Start
Constant Frequency 200kHz 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
Limited Only by External Components
IN
OUT
n
n
n
n
n
n
n
n
TheLTC3803provides±±.ꢀ5outputvoltageaccuracyand
consumesonly240μAofquiescentcurrent. Ground-refer-
enced current sensing allows LTC3803-based converters
to accept input supplies beyond the LTC3803’s absolute
maximum V . A micropower hysteretic start-up feature
APPLICATIONS
CC
allows efficient operation at high input voltages. For sim-
n
Telecom Power Supplies
plicity, the LTC3803 can also be powered from a high V
IN
n
42V and ±2V Automotive Power Supplies
through a resistor, due to its internal 9.4V shunt regulator.
An internal undervoltage lockout shuts down the LTC3803
when the input voltage falls below 4.6V, guaranteeing at
least 4.6V of gate drive to the external MOSFET.
n
Auxiliary/Housekeeping Power Supplies
Power Over Ethernet
n
L, 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.
The LTC3803 is available in a low profile (±mm) 6-lead
SOT-23 (ThinSOT™) package.
TYPICAL APPLICATION
5V Output Nonisolated Telecom Housekeeping Power Supply
Efficiency vs Load Current
90
V
IN
36V TO 72V
UPS840
V
IN
= 36V
80
70
60
ꢀ0
40
30
20
±0
0
V
OUT
ꢀV
T±
•
2A MAX
4.7μF
±00V
XꢀR
300μF*
6.3V
XꢀR
V
IN
= 48V
±0k
V
= 60V
IN
±0μF
±0V
XꢀR
•
V
IN
= 72V
V
CC
I
/RUN NGATE
LTC3803
FDC2ꢀ±2
68mΩ
TH
ꢀ6k
0.0022μF
GND
SENSE
V
FB
20k
±0ꢀk
±
0.±
±0
3803 TA0±
I
(A)
OUT
T±: COOPER CTX02-±ꢀ242
*THREE ±00μF UNITS IN PARALLEL
3803 TA02
3803fb
1
LTC3803
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
V
to GND
CC
I
/RUN ±
GND 2
6 NGATE
ꢀ V
Low Impedance Source ...........................–0.3V to 8V
Current Fed........................................2ꢀmA into V *
TH
CC
CC
CC
V
3
4 SENSE
FB
NGATE Voltage............................................–0.3V to V
V , I /RUN Voltages...............................–0.3V to 3.ꢀV
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
= ±ꢀ0°C, θ = 230°C/W
FB TH
SENSE Voltage.............................................–0.3V to ±V
NGATE Peak Output Current (<±0μs) ......................... ±A
Operating Temperature Range (Note 2)
T
JMAX
JA
LTC3803E.............................................–40°C to 8ꢀ°C
LTC3803I............................................–40°C to ±2ꢀ°C
LTC3803H ..........................................–40°C to ±ꢀ0°C
Junction Temperature (Note 3) ............................. ±ꢀ0°C
Storage Temperature Range...................–6ꢀ°C to ±ꢀ0°C
Lead Temperature (Soldering, ±0 sec) .................. 300°C
*LTC3803 internal clamp circuit self regulates V voltage to 9.ꢀV.
CC
ORDER INFORMATION
LEAD FREE FINISH
LTC3803ES6#PBF
LTC3803IS6#PBF
LTC3803HS6#PBF
TAPE AND REEL
PART MARKING*
LTACV
PACKAGE DESCRIPTION
6-Lead Plastic TSOT-23
6-Lead Plastic TSOT-23
6-Lead Plastic TSOT-23
TEMPERATURE RANGE
LTC3803ES6#TRPBF
LTC3803IS6#TRPBF
LTC3803HS6#TRPBF
–40°C to 8ꢀ°C
–40°C to ±2ꢀ°C
–40°C to ±ꢀ0°C
LTBNC
LTBNC
LEAD BASED FINISH
LTC3803ES6
TAPE AND REEL
LTC3803ES6#TR
LTC3803IS6#TR
LTC3803HS6#TR
PART MARKING*
LTACV
PACKAGE DESCRIPTION
6-Lead Plastic TSOT-23
6-Lead Plastic TSOT-23
6-Lead Plastic TSOT-23
TEMPERATURE RANGE
–40°C to 8ꢀ°C
LTC3803IS6
LTBNC
–40°C to ±2ꢀ°C
LTC3803HS6
LTBNC
–40°C to ±ꢀ0°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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
temperature range, otherwise specifications are at TA = 25°C. VCC = 8V, unless otherwise noted. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
l
l
V
V
Turn On Voltage
LTC3803E
7.8
7.7
8.7
8.7
9.2
V
V
TURNON
CC
CC
LTC3803H, LTC3803I
9.2ꢀ
l
l
V
V
Turn Off Voltage
LTC3803E
LTC3803H, LTC3803I
4.6
4
ꢀ.7
ꢀ.7
6.8
6.8
V
V
TURNOFF
l
V
V
V
V
Hysteresis
V
– V
TURNOFF
±.ꢀ
3.0
V
HYST
CC
CC
TURNON
Shunt Regulator Voltage
I
CC
= ±mA, V
= 0V
CLAMP±mA
ITH/RUN
l
l
LTC3803E
LTC3803H, LTC3803I
8.3
8.3
9.4
9.4
±0.3
±0.ꢀ
V
V
V
V
Shunt Regulator Voltage
I
= 2ꢀmA, V
= 0V
CLAMP2ꢀmA
CC
CC
ITH/RUN
l
l
LTC3803E
LTC3803H, LTC3803I
8.4
8.4
9.ꢀ
9.ꢀ
±0.ꢀ
±0.7
V
V
3803fb
2
LTC3803
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 8V, unless otherwise noted. (Note 2)
SYMBOL
PARAMETER
– V Margin
TURNON
CONDITIONS
MIN
TYP
MAX
UNITS
l
l
V
V
LTC3803E
LTC3803H, LTC3803I
0.0ꢀ
0.03
0.6
0.6
V
V
MARGIN
CLAMP±mA
I
CC
Input DC Supply Current in Normal
Operation
(Note 4)
ITH/RUN
V
= ±.3V
240
3ꢀ0
μA
I
Input DC Supply Current in Undervoltage
V
= V
– ±00mV
CC(UV)
CC
TURNON
l
l
LTC3803E
40
40
90
μA
μA
LTC3803H, LTC3803I
V = V + ±00mV
CC
LTC3803E
LTC3803H, LTC3803I
±±0
V
Shutdown Threshold (at I /RUN)
ITHSHDN
TH
TURNON
l
l
0.±ꢀ
0.09
0.28
0.28
0.4ꢀ
0.46
V
V
I
Start-Up Current Source
V
= 0V
0.2
0.3
0.4
μA
ITHSTART
ITH/RUN
V
Regulated Feedback Voltage
(Note ꢀ)
LTC3803E:
0°C ≤ T ≤ 8ꢀ°C
FB
0.788
0.780
0.800
0.800
0.8±2
0.8±6
V
V
J
l
l
l
–40°C ≤ T ≤ 8ꢀ°C
J
LTC3803I:
0°C ≤ T ≤ 8ꢀ°C
0.788
0.780
0.800
0.800
0.8±2
0.820
V
V
J
–40°C ≤ T ≤ ±2ꢀ°C
J
LTC3803H:
0°C ≤ T ≤ 8ꢀ°C
0.788
0.780
0.800
0.800
0.8±2
0.820
V
V
J
–40°C ≤ T ≤ ±ꢀ0°C
J
g
Error Amplifier Transconductance
Output Voltage Line Regulation
Output Voltage Load Regulation
I
Pin Load = ±ꢀμA (Note ꢀ)
200
333
ꢀ00
μA/V
m
TH/RUN
V
< V < V (Note ꢀ)
CLAMP
0.0ꢀ
mV/V
ΔV
ΔV
TURNOFF
CC
O(LINE)
I
TH
I
TH
/RUN Sinking ꢀμA (Note ꢀ)
/RUN Sourcing ꢀμA (Note ꢀ)
3
3
mV/μA
mV/μA
O(LOAD)
I
f
V
Input Current
FB
(Note ꢀ)
±0
200
6
ꢀ0
240
8
nA
kHz
5
FB
Oscillator Frequency
V
V
V
C
C
= ±.3V
±80
70
OSC
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
40
40
90
5
FB
t
= 3000pF
ns
RISE
FALL
LOAD
LOAD
t
Gate Drive Fall Time
= 3000pF
ns
V
IMAX
Peak Current Sense Voltage
R
= 0 (Note 6)
SL
l
l
LTC3803E
90
90
±00
±00
±±ꢀ
±20
mV
mV
LTC3803H, LTC3803I
I
t
Peak Slope Compensation Output Current
Soft-Start Time
(Note 7)
ꢀ
μA
SLMAX
±.4
ms
SFST
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.
dissipation P according to the following formula:
D
T = T + (P • 230°C/W).
J
A
D
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 2: The LTC3803E is guaranteed to meet specifications from 0°C to
8ꢀ°C. Specifications over the –40°C to 8ꢀ°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls. The LTC3803I is guaranteed to meet performance
specifications over the –40°C to ±2ꢀ°C operating temperature range. The
LTC3803H is guaranteed to meet performance specifications over the
–40°C to ±ꢀ0°C operating temperature range.
Note 5: The LTC3803 is tested in a feedback loop that servos V to the
FB
output of the error amplifier while maintaing 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.
Note 3: T is calculated from the ambient temperature T and power
J
A
3803fb
3
LTC3803
TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage
vs Supply Voltage
Reference Voltage
vs VCC Shunt Regulator Current
Reference Voltage vs Temperature
8±2
808
804
800
796
792
788
80±.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
T = 2ꢀ°C
A
CC
T
= 2ꢀ°C
A
V
≤ V
CC
CLAMP±mA
800
799
798
797
796
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–ꢀ0
70 90
6
7
7.ꢀ
8
8.ꢀ
9
9.ꢀ
0
ꢀ
±0
I
20
2ꢀ
6.ꢀ
±ꢀ
(mA)
TEMPERATURE (°C)
V
SUPPLY VOLTAGE (V)
CC
CC
3803 G0±
3803 F02
3803 G03
Oscillator Frequency
vs Temperature
Oscillator Frequency
vs Supply Voltage
Oscillator Frequency
vs VCC Shunt Regulator Current
240
230
220
2±0
200
±90
±80
2±0
208
206
204
202
200
±98
±96
±94
±92
±90
2±0
208
206
204
202
200
±98
±96
±94
±92
±90
T
= 2ꢀ°C
T
= 2ꢀ°C
V
CC
= 8V
A
A
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
6
6.ꢀ
7.ꢀ
8
8.ꢀ
9
–ꢀ0
70 90
7
0
ꢀ
±ꢀ
(mA)
20
2ꢀ
±0
I
TEMPERATURE (°C)
V
CC
SUPPLY VOLTAGE (V)
CC
3803 G04
3803 G0ꢀ
3803 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
9.0
8.ꢀ
8.0
7.ꢀ
7.0
6.ꢀ
6.0
ꢀ.ꢀ
ꢀ.0
4.ꢀ
4.0
±0.0
9.9
9.8
9.7
9.6
9.ꢀ
9.4
9.3
9.2
9.±
9.0
V = 8V
CC
V
ITH/RUN
= ±.3V
V
TURNON
I
= 2ꢀmA
CC
I
= ±mA
CC
V
TURNOFF
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–ꢀ0
70 90
–ꢀ0
70 90
–ꢀ0
70 90
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3803 G07
3803 G08
3803 G08
3803fb
4
LTC3803
TYPICAL PERFORMANCE CHARACTERISTICS
Start-Up ICC Supply Current
vs Temperature
ITH/RUN Shutdown Threshold
vs Temperature
ITH/RUN Start-Up Current Source
vs Temperature
4ꢀ0
400
70
60
ꢀ0
40
30
20
±0
0
800
700
600
ꢀ00
400
300
200
±00
0
V
CC
= V
– 0.±V
V
V
= V
+ 0.±V
TURNON
TURNON
CC
ITH/RUN
= 0V
3ꢀ0
300
2ꢀ0
200
±ꢀ0
±00
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–ꢀ0
70 90
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–ꢀ0
70 90
–ꢀ0
70 90
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3803 G±±
3803 G±2
3803 G±0
Peak Current Sense Voltage
vs Temperature
Soft-Start Time vs Temperature
4.0
3.ꢀ
3.0
2.ꢀ
2.0
±.ꢀ
±.0
0.ꢀ
0
±20
±±ꢀ
±±0
±0ꢀ
±00
9ꢀ
V
CC
= 8V
90
8ꢀ
80
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–30 –±0 ±0 30 ꢀ0
±±0 ±30 ±ꢀ0
–ꢀ0
70 90
–ꢀ0
70 90
TEMPERATURE (°C)
TEMPERATURE (°C)
3803 G±4
3803 G±3
3803fb
5
LTC3803
PIN FUNCTIONS
I /RUN (Pin 1): This pin performs two functions. It
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.
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 0.28V causes the
LTC3803 to shut down. In shutdown mode, the NGATE
pin is held low.
V
CC
(Pin 5): Supply Pin. Must be closely decoupled to
GND (Pin 2): Ground Pin.
GND (Pin 2).
V (Pin3):Receivesthefeedbackvoltagefromanexternal
NGATE (Pin 6): Gate Drive for the External N-Channel
FB
resistive divider across the output.
MOSFET. This pin swings from 0V to V .
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
200kHz
OSCILLATOR
±.2V
SENSE
4
I
/RUN
TH
±
3803 BD
3803fb
6
LTC3803
OPERATION
TheLTC3803isaconstantfrequencycurrentmodecontrol-
ler for flyback and DC/DC boost converter applications in
a tiny ThinSOT package. The LTC3803 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’s absolute maximum ratings.
comparator monitors the voltage across a current sense
resistor in series with the source terminal of the external
MOSFET.TheLTC3803turnsontheexternalpowerMOSFET
when the internal free-running 200kHz oscillator sets
the RS latch. It turns off the MOSFET when the current
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
Main Control Loop
and secondary are controlled by the I /RUN voltage.
TH
Due to space limitations, the basics of current mode
DC/DC conversion will not be discussed here; instead, the
reader is referred to the detailed treatment in Application
Note ±9, or in texts such as Abraham Pressman’s Switch-
ing Power Supply Design.
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
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
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
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.
voltage to the V pin. The divider must be designed so
FB
that when the output is at the desired voltage, the V pin
FB
voltage will equal the 800mV from the internal reference.
If the load current increases, the output voltage will de-
The I /RUN pin serves as the compensation point for
TH
crease slightly, causing the V pin voltage to fall below
the control loop. Typically, an external series RC network
FB
800mV. The error amplifier responds by feeding current
is connected from I /RUN to ground and is chosen for
TH
into the I /RUN pin. If the load current decreases, the
optimalresponsetoloadandlinetransients.Theimpedance
TH
V
voltage will rise above 800mV and the error amplifier
will sink current away from the I /RUN pin.
of this RC network converts the output current of the error
FB
amplifier to the I /RUN voltage which sets the current
TH
TH
comparator threshold and commands considerable influ-
ThevoltageattheI /RUNpincommandsthepulse-width
TH
ence over the dynamics of the voltage regulation loop.
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
3803fb
7
LTC3803
OPERATION
Start-Up/Shutdown
operation.TheV voltageisthenallowedtofalltoV
CC TURNOFF
(nominallyꢀ.7V)beforeundervoltagelockoutdisablesthe
LTC3803. This wide UVLO hysteresis range supports the
use of a bias winding on the flyback transformer to power
the LTC3803—see the section Powering the LTC3803.
The LTC3803 has two shutdown mechanisms to disable
and enable operation: an undervoltage lockout on the
CC
V
supply pin voltage, and a forced shutdown whenever
external circuitry drives the I /RUN pin low. The LTC3803
TH
transitionsintoandoutofshutdownaccordingtothestate
The I /RUN pin can be driven below V
(nominally
SHDN
TH
diagram (Figure ±).
0.28V) to force the LTC3803 into shutdown. An internal
0.3μA current source always tries to pull this pin towards
V . When the I /RUN pin voltage is allowed to exceed
CC
TH
LTC3803
SHUT DOWN
V
, and V exceeds V
, the LTC3803 begins
SHDN
CC
TURNON
to operate and an internal clamp immediately pulls the
I /RUN pin up to about 0.7V. In operation, the I /RUN
TH
TH
pinvoltagewillvaryfromroughly0.7Vto±.9Vtorepresent
current comparator thresholds from zero to maximum.
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)
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC3803 comes out of shutdown. Specifically, the I /
LTC3803
ENABLED
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-based power
supplies to rise in a smooth and controlled manner on
start-up.
3803 F0±
Figure 1. Start-Up/Shutdown State Diagram
Theundervoltagelockout(UVLO)mechanismpreventsthe
LTC3803 from trying to drive a MOSFET with insufficient
V . The voltage at the V pin must exceed V
GS
CC
TURNON
(nominally 8.7V) at least momentarily to enable LTC3803
3803fb
8
LTC3803
OPERATION
Powering the LTC3803
The V pin must be bypassed to ground immediately
CC
adjacent to the IC pins with a minimum of a ±0μF ceramic
or tantalum capacitor. Proper supply bypassing is neces-
sary to supply the high transient currents required by the
MOSFET gate driver.
In the simplest case, the LTC3803 can be powered from
a high voltage supply through a resistor. A built-in shunt
regulator from the V pin to GND will draw as much
CC
current as needed through this resistor to regulate the
V
voltage to around 9.4V as long as the V pin is not
CC
CC
Adjustable Slope Compensation
forced to sink more than 2ꢀmA. This shunt regulator is
The LTC3803 injects a ꢀμA peak current ramp out through
itsSENSEpinwhichcanbeusedforslopecompensationin
designs that require it. This current ramp is approximately
linear and begins at zero current at 65 duty cycle, reach-
ing peak current at 805 duty cycle. Additional details are
provided in the Applications Information section.
always active, even when the LTC3803 is in shutdown,
since it serves the vital function of protecting the V pin
CC
from seeing too much voltage.
For higher efficiency or for wide V range applications,
IN
flybackcontrollersaretypicallypoweredthroughaseparate
bias winding on the flyback transformer. The LTC3803
has a wide UVLO hysteresis (±.ꢀV min) and small V
CC
) that is
supply current draw (<90μA when V < V
CC
TURNON
needed to support such bootstrapped hysteretic start-up
schemes.
3803fb
9
LTC3803
APPLICATIONS INFORMATION
Many LTC3803 application circuits can be derived from
the topology shown in Figure 2.
TRANSFORMER DESIGN CONSIDERATIONS
Transformer specification and design is perhaps the
most critical part of applying the LTC3803 successfully.
In addition to the usual list of caveats dealing with high
frequencypowertransformerdesign,thefollowingshould
prove useful.
The LTC3803 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-
rable transformers such as the Coiltronics VERSA-PAC™
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.
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
OUT
R3
R
I
C
OUT
START
C
L
L
SEC
IN PRI
•
ꢀ
C
VCC
V
CC
±
2
6
/RUN NGATE
LTC3803
Q±
TH
C
C
R
SL
4
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.
GND
R±
SENSE
V
FB
R
SENSE
3
R2
3803 F02
Figure 2. Typical LTC3803 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-secondary
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.
3803fb
10
LTC3803
APPLICATIONS INFORMATION
CURRENT SENSE RESISTOR CONSIDERATIONS
Duty Cycle – 6%
74%
ΔVSENSE
=
•5μA •RSL
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.
Note: LTC3803 enforces 65 < Duty Cycle < 805.
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 0.020ꢁ. Note that the instantaneous
peak power in the sense resistor is 0.ꢀW and it must be
rated accordingly. The LTC3803 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 0.020ꢁ 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.
INTERNAL WIDE HYSTERESIS UNDERVOLTAGE
LOCKOUT
The LTC3803 is designed to implement DC/DC converters
operating from input voltages of typically 48V or more.
The standard operating topology employs a third trans-
former winding (L
in Figure 2) on the primary side that
BIAS
provides power for the LTC3803 via its V pin. However,
CC
thisarrangementisnotinherentlyself-starting. Start-upis
affected by the use of an external “trickle-charge” resistor
(R
in Figure 2) and the presence of an internal wide
START
hysteresis undervoltage lockout circuit that monitors V
pin voltage. Operation is as follows:
CC
PROGRAMMABLE SLOPE COMPENSATION
“Trickle charge” resistor R
is connected to V and
IN
START
The LTC3803 injects a ramping current through its SENSE
supplies a small current, typically on the order of ±00μA,
pin into an external slope compensation resistor (R in
SL
to charge C . After some time, the voltage on C
VCC
VCC
Figure 2). This current ramp starts at zero right after the
NGATE pin has been high for the LTC3803’s minimum
duty cycle of 65. The current rises linearly towards a
peak of ꢀμA at the maximum duty cycle of 805, shutting
reaches the V turn-on threshold. The LTC3803 then
CC
turns on abruptly and draws its normal supply current.
TheNGATEpinbeginsswitchingandtheexternalMOSFET
(Q±) begins to deliver power. The voltage on C
begins
VCC
off once the NGATE pin goes low. A series resistor (R )
SL
to decline as the LTC3803 draws its normal supply cur-
rent, which exceeds that delivered by R . After some
connecting the SENSE pin to the current sense resistor
START
(R
) thus develops a ramping voltage drop. From
SENSE
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.
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
One potential design pitfall is undersizing the value of
capacitor C . In this case, the normal supply current
VCC
currentcomparatorthreshold(ΔV
)canbecalculated
SENSE
drawn by the LTC3803 will discharge C
too rapidly;
VCC
using the following equation:
before the third winding drive becomes effective, the V
CC
turn-off threshold will be reached. The LTC3803 turns off,
3803fb
11
LTC3803
APPLICATIONS INFORMATION
V
and the V node begins to charge via R
back up to
IN
CC
START
the V turn-on threshold. Depending on the particular
CC
R
LTC3803
VCC
situation, this may result in either several on-off cycles
V
CC
beforeproperoperationisreachedorpermanentrelaxation
GND
C
VCC
oscillation at the V node.
CC
3803 F03
Component selection is as follows:
Figure 3. Powering the LTC3803 Via the Internal Shunt Regulator
Resistor R
should be made small enough to yield a
START
worst-case minimum charging current greater than the
maximum rated LTC3803 start-up current, to ensure there
isenoughcurrenttochargeC totheV turn-onthresh-
old. It should be made large enough to yield a worst-case
maximum charging current less than the minimum rated
LTC3803 supply current, so that in operation, most of the
LTC3803’s supply current is delivered through the third
winding. This results in the highest possible efficiency.
The shunt regulator can draw up to 2ꢀmA through the
V
pin to GND to drop enough voltage across R
to
CC
VCC
VCC
CC
regulate V to around 9.ꢀV. For applications where V
CC
IN
is low enough such that the static power dissipation in
R
is acceptable, using the V shunt regulator is the
VCC
CC
simplest way to power the LTC3803.
EXTERNAL PREREGULATOR
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.
The circuit in Figure 4 shows a third way to power the
LTC3803. An external series preregulator consisting of
series pass transistor Q±, Zener diode D±, and bias resis-
tor R brings V to at least 7.6V nominal, well above the
B
CC
maximum rated V turn-off threshold of 6.8V. Resistor
CC
The third transformer winding should be designed so that
its output voltage, after accounting for the D2’s forward
R
START
momentarily charges the V node up to the V
turn-on threshold, enabling the LTC3803.
CC CC
voltagedrop,exceedsthemaximumV turn-offthreshold.
CC
Also, the third winding’s nominal output voltage should
V
IN
be at least 0.ꢀV below the minimum rated V clamp volt-
CC
age to avoid running up against the LTC3803’s V shunt
CC
regulator, needlessly wasting power.
R
Q±
R
START
LTC3803
B
V
CC
D±
8.2V
GND
C
VCC
V
SHUNT REGULATOR
CC
In applications including a third transformer winding,
3803 F04
the internal V shunt regulator serves to protect the
CC
Figure 4. Powering the LTC3803 with an External Preregulator
LTC3803 from overvoltage transients as the third wind-
ing is powering up.
In applications where a third transformer winding is
undesirable or unavailable, the shunt regulator allows
the LTC3803 to be powered through a single dropping
resistor from V to V , in conjunction with a bypass
IN
CC
capacitor, C , that closely decouples V to GND (see
VCC
CC
Figure 3). This simplicity comes at the expense of reduced
efficiency due to the static power dissipation in the R
dropping resistor.
VCC
3803fb
12
LTC3803
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
/RUN NGATE
22k
6
I
TH
FDC2ꢀ±2
2
3
ꢀ
4
V
GND
CC
T±: PULSE ENGINEERING PA0648
OR TYCO TTI8698
806Ω
ꢀ.6k
±μF
V
SENSE
FB
0.±Ω
3803 TA03
PRIMARY GROUND
3803fb
13
LTC3803
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
s 3
•
2.2μF
220k
MMBTA42
–
PDS±040
V
±00k
IN
•
GND
BASꢀ±6
68Ω
±ꢀ0pF
PDZ6.8B
V
CC
±0Ω
BASꢀ±6
22Ω
680Ω
•
0.±μF
±
2
3
6
ꢀ
4
I
/RUN
FDC2ꢀ±2
GATE
TH
V
GND
IN
+
V
OUT
LTC3803
SENSE
4.7k
V
FB
BAT760
0.±μF
0.040Ω
270Ω
V
CC
+
V
±
2
3
6
ꢀ
4
6.8k
OUT
V
OPTO
COMP
FB
BASꢀ±6
IN
PS280±-±
47pF
2.2nF
22.±k
0.±μF
±
2
LT4430
ꢀ6k
GND
0.33μF
±00k
OC
BASꢀ±6
3803 TA0ꢀ
Efficiency vs Load Current
84
82
80
78
76
74
72
V
V
= 48V
= 24V
IN
IN
70
0
±
2
3
4
3803 TA0ꢀa
I
(A)
OUT
3803fb
14
LTC3803
PACKAGE DESCRIPTION
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 0ꢀ-08-±636)
2.90 BSC
(NOTE 4)
0.62
MAX
0.95
REF
1.22 REF
1.4 MIN
1.50 – 1.75
2.80 BSC
3.85 MAX 2.62 REF
(NOTE 4)
PIN ONE ID
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.30 – 0.45
6 PLCS (NOTE 3)
0.95 BSC
0.80 – 0.90
0.20 BSC
DATUM ‘A’
0.01 – 0.10
1.00 MAX
0.30 – 0.50 REF
1.90 BSC
0.09 – 0.20
(NOTE 3)
S6 TSOT-23 0302
NOTE:
1. 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
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
3803fb
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
TYPICAL APPLICATIONS
Synchronous Flyback 3.3VOUT
9±
90
89
90% Efficient Synchronous Flyback Converter
V
3.3V
±.ꢀA
*
OUT
V
IN
T±
36V TO 72V
Q2
D±
•
•
C
C
O
IN
270k
•
88
0.ꢀ
±.ꢀ
±.0
OUTPUT CURRENT (A)
2.0
±n
33k
±
2
3
6
ꢀ
4
3803 TA04b
I
/RUN
Q±
GATE
TH
GND
= 0.8V
LTC3803
0.±μF
ꢀ60R
ꢀk
V
IN
Synchronous Flyback 5VOUT
8.06k
SENSE
V
FB
92
9±
90
89
88
87
86
8ꢀ
3803 TA04a
2ꢀ.ꢀk*
±μF
±0V
R
CS
R
T±: PULSE ENGINEERING PA±006
Q±: FAIRCHILD FDC2ꢀ±2
Q2: VISHAY Si9803
FB
V
OUT
D±: PHILIPS BASꢀ±6
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
FB
0.ꢀ
±.0
±.ꢀ
2.0
2.ꢀ
OUTPUT CURRENT (A)
3803 TA04c
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
No Optoisolator or “Third Winding” Required, Up to 6W Output
LT®±42ꢀ
Isolated Flyback Switching Regulator
with No External Power Devices
LT±72ꢀ
General Purpose Isolated Flyback Controller
No Optoisolator Required, V and V
Limited Only by External
OUT
IN
Power Components
LTC±772
LTC±87±
SOT-23 Constant Frequency Current Mode Step-Down
DC/DC Controller
ꢀꢀ0kHz Switching Frequency, 2.4V to 9.8V V Range
IN
Wide Input Range, No R
™ Current Mode
SENSE
Adjustable Switching Frequency, Programmable Undervoltage
Lockout, Optional Burst Mode® Operation at Light Load
Flyback, Boost and SEPIC Controller
LTC±872
LT±9ꢀ0
LT3420
SOT-23 Constant Frequency Current Mode Boost DC/DC ꢀꢀ0kHz Switching Frequency, 2.4V to 9.8V V Range Controller
IN
Current Mode PWM Controller
Controller for Forward Converters from 30W to 300W
Photoflash Capacitor Charger with Automatic Refresh
Specialized Flyback Charges High Voltage Photoflash Capacitors
Quickly and Efficiently
LT3468/LT3468-±
LTC3806
Photoflash Capacitor Charger in ꢀ-Pin SOT-23
Synchronous Flyback Controller
Minimal Component Count, Uses Small Transformers;
IN
V
from 2.ꢀV to ±6V
High Efficiency (895); Multiple Output with
Excellent Cross Regulation
Burst Mode is a registered trademark of Linear Technology Corporation. No R
is a trademark of Linear Technology Corporation.
SENSE
3803fb
LT 0109 REV B • PRINTED IN USA
LinearTechnology Corporation
±630 McCarthy Blvd., Milpitas, CA 9ꢀ03ꢀ-74±7
16
●
●
© LINEAR TECHNOLOGY CORPORATION 2003
(408) 432-±900 FAX: (408) 434-0ꢀ07 www.linear.com
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