LTC3803ES6-5#TRMPBF [ADI]
LTC3803-5 - Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT; Package: SOT; Pins: 6; Temperature Range: -40°C to 85°C;
| 型号： | LTC3803ES6-5#TRMPBF |
| 厂家： | 
ADI |
| 描述： | LTC3803-5 - 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 |
LTC3803-5
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
FEATURES
DESCRIPTION
The LTC®3803-5 is a constant frequency current mode
flybackcontrolleroptimizedfordrivingN-channelMOSFETs
inhighinputvoltageapplications.TheLTC3803-5operates
from inputs as low as 5V. 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
Limited Only by External Components
IN
OUT
n
n
4.8V Undervoltage Lockout Threshold
Operating Junction Temperature from –55°C to
150°C
n
n
n
n
n
Adjustable Slope Compensation
Internal Soft-Start
Constant Frequency 200kHz Operation
±±.5ꢀ 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 LTC3803-5 provides ±±.5ꢀ output voltage accuracy
and consumes only 240μA of quiescent current. Ground-
referenced current sensing allows LTC3803-5-based con-
verters to accept input supplies beyond the LTC3803-5’s
n
n
n
absolutemaximumV . Forsimplicity, theLTC3803-5can
CC
be powered from a high V through a resistor, due to its
IN
internal shunt regulator. An internal undervoltage lockout
shuts down the IC when the input voltage is too low to
provide sufficient gate drive to the external MOSFET.
APPLICATIONS
n
42V and ±2V Automotive Power Supplies
n
Telecom Power Supplies
n
Auxiliary/Housekeeping Power Supplies
The LTC3803-5 is available in a low profile (±mm) 6-lead
SOT-23 (ThinSOT™) package.
n
Power Over Ethernet
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
trademarks are the property of their respective owners.
TYPICAL APPLICATION
Efficiency and Power Loss
vs Output Power
Dual Output Wide Input Range Converter
±0MQ±00N
VPH5-0±55
V
±3V/0.3A
20mA MIN
LOAD
IN
90
85
80
75
70
65
60
3.0
2.5
2.0
±.5
±.0
0.5
0
V
= 8V
IN
6V TO 50V
±μF
±00V
×3
22k
PDZ6.8B
7.5k
V
V
= ±2V
= 24V
IN
IN
22μF
±0V
MMBTA42
±μF
±00V
PHM25NQ±0T
±0nF
LTC3803-5
±μF
±00V
I
/RUN NGATE
TH
GND
V
CC
6.5V/±.2A
4.7k
V
FB
SENSE
B3±00
V
V
= 48V
IN
IN
47μF
±0V
8.06k
0.0±2Ω
0.±μF
= ±2V
2
57.6k
ALL CAPACITORS ARE X7R, TDK
0
6
8
±0
±2
4
OUTPUT POWER (W)
38035 TA0±
38035 TA0±b
38035fd
1
LTC3803-5
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
V
to GND (Current Fed) ..................... 25mA into V *
CC
CC
NGATE Voltage.......................................... –0.3V to V
CC
TOP VIEW
V , I /RUN Voltages............................... –0.3V to 3.5V
FB TH
I
/RUN ±
GND 2
6 NGATE
5 V
SENSE Voltage............................................ –0.3V to ±V
NGATE Peak Output Current (<±0μs) ......................... ±A
Operating Junction Temperature Range (Notes 2, 3)
LTC3803E-5 .......................................– 40°C to ±25°C
LTC3803I-5 ........................................– 40°C to ±25°C
LTC3803H-5 (Note 3)......................... –40°C to ±50°C
LTC3803MP-5 (Note 3)...................... –55°C to ±50°C
Storage Temperature Range...................–65°C to ±50°C
Lead Temperature (Soldering, ±0 sec) .................. 300°C
TH
CC
V
FB
3
4 SENSE
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
= ±50°C, θ = ±92°C/W
T
JMAX
JA
*LTC3803-5 internal clamp circuit self regulates V voltage to 8.±V.
CC
ORDER INFORMATION
LEAD FREE FINISH
LTC3803ES6-5#PBF
LTC3803IS6-5#PBF
LTC3803HS6-5#PBF
LTC3803MPS6-5#PBF
LEAD BASED FINISH
LTC3803ES6-5
TAPE AND REEL
PART MARKING*
LTBPF
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to ±25°C
–40°C to ±25°C
–40°C to ±50°C
–55°C to ±50°C
TEMPERATURE RANGE
–40°C to ±25°C
–40°C to ±25°C
–40°C to ±50°C
–55°C to ±50°C
LTC3803ES6-5#TRPBF
LTC3803IS6-5#TRPBF
LTC3803HS6-5#TRPBF
LTC3803MPS6-5#TRPBF
TAPE AND REEL
6-Lead Low Profile (±mm) SOT-23
6-Lead Low Profile (±mm) SOT-23
6-Lead Low Profile (±mm) SOT-23
6-Lead Low Profile (±mm) SOT-23
PACKAGE DESCRIPTION
LTBMH
LTBMH
LTBMH
PART MARKING*
LTBPF
LTC3803ES6-5#TR
LTC3803IS6-5#TR
6-Lead Low Profile (±mm) SOT-23
6-Lead Low Profile (±mm) SOT-23
6-Lead Low Profile (±mm) SOT-23
6-Lead Low Profile (±mm) SOT-23
LTC3803IS6-5
LTBMH
LTC3803HS6-5
LTC3803HS6-5#TR
LTC3803MPS6-5#TR
LTBMH
LTC3803MPS6-5
LTBMH
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
junction temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, unless otherwise noted. (Notes 2, 3)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
l
l
V
V
Turn On Voltage
LTC3803E-5
4
4.8
4.8
5.7
5.7
V
V
TURNON
CC
CC
LTC3803I-5, LTC3803H-5, LTC3803MP-5
3.9
l
l
V
V
Turn Off Voltage
LTC3803E-5
LTC3803I-5, LTC3803H-5, LTC3803MP-5
3.3
3.2
4
4
4.9
4.9
V
V
TURNOFF
l
V
V
V
V
Hysteresis
V
– V
TURNOFF
0.05
0.8
V
HYST
CC
TURNON
Shunt Regulator Voltage at ±mA
I
CC
= ±mA, V
= 0V
CLAMP±mA
CC
ITH/RUN
l
l
l
LTC3803E-5
6.2
6.2
5.9
8
8
8
9.9
±0.4
±0.4
V
V
V
LTC3803I-5, LTC3803H-5
LTC3803MP-5
38035fd
2
LTC3803-5
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, unless otherwise noted. (Notes 2, 3)
SYMBOL
PARAMETER
Shunt Regulator Voltage at 25mA
CONDITIONS
= 25mA, V
MIN
TYP
MAX
UNITS
V
V
CC
I
CC
= 0V
ITH/RUN
CLAMP25mA
l
l
l
LTC3803E-5
6.3
6.3
6
8.±
8.±
8.±
±0.3
±0.7
±0.7
V
V
V
LTC3803I-5, LTC3803H-5
LTC3803MP-5
I
I
Input DC Supply Current in
Normal Operation
(Note 4)
ITH/RUN
CC
V
= ±.3V
240
350
μA
Input DC Supply Current in
Undervoltage
V
= V
– ±00mV
CC,(UV)
CC
TURNON
l
l
LTC3803E-5
40
40
90
±00
μA
μA
LTC3803I-5, LTC3803H-5, LTC3803MP-5
V
Shutdown Threshold (at I /RUN)
V
V
> V
, V
Falling
ITHSHDN
TH
CC
TURNON ITH/RUN
l
l
l
LTC3803E-5
0.±2
0.08
0.08
0.28
0.28
0.28
0.45
0.45
0.47
V
V
V
LTC3803I-5, LTC3803H-5
LTC3803MP-5
I
Start-Up Current Source
= 0V
ITH/RUN
LTC3803E-5
LTC3803I-5, LTC3803H-5, LTC3803MP-5
ITHSTART
l
l
0.07
0.07
0.34
0.34
0.8
±
μA
μA
V
FB
Regulated Feedback Voltage
(Note 5)
LTC3803E-5:
0°C ≤ T ≤ 85°C
–40°C ≤ T ≤ 85°C
0.788
0.780
0.800
0.800
0.8±2
0.8±6
V
V
J
l
l
l
l
J
LTC3803I-5:
0°C ≤ T ≤ 85°C
0.788
0.780
0.800
0.800
0.8±2
0.820
V
V
J
–40°C ≤ T ≤ ±25°C
J
LTC3803H-5:
0°C ≤ T ≤ 85°C
0.788
0.780
0.800
0.800
0.8±2
0.820
V
V
J
–40°C ≤ T ≤ ±50°C
J
LTC3803MP-5:
0°C ≤ T ≤ 85°C
0.788
0.780
0.800
0.800
0.8±2
0.820
V
V
J
–55°C ≤ T ≤ ±50°C
J
g
Error Amplifier Transconductance
Output Voltage Line Regulation
Output Voltage Load Regulation
I
Pin Load = ±5μA (Note 5)
200
333
0.±
500
μA/V
m
TH/RUN
ΔV
ΔV
(Note 5)
mV/V
O(LINE)
I
I
Sinking 5μA (Note 5)
Sourcing 5μA (Note 5)
3
3
mV/μA
mV/μA
O(LOAD)
TH/RUN
TH/RUN
I
f
V
Input Current
FB
(Note 5)
±0
200
6.5
80
50
230
8.5
90
nA
kHz
ꢀ
FB
Oscillator Frequency
V
V
V
C
C
= ±.3V
±70
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
ꢀ
FB
t
= 3000pF
40
ns
RISE
FALL
LOAD
LOAD
t
Gate Drive Fall Time
= 3000pF (Note 7)
40
ns
V
IMAX
Peak Current Sense Voltage
R
SL
= 0 (Note 6)
l
l
l
LTC3803E-5
90
85
85
±00
±00
±00
±±5
±±5
±20
mV
mV
mV
LTC3803I-5, LTC3803H-5
LTC3803MP-5
I
t
Peak Slope Compensation Output
Current
(Note 7)
5
μA
SLMAX
Soft-Start Time
0.7
ms
SFST
38035fd
3
LTC3803-5
ELECTRICAL CHARACTERISTICS
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.
Junction temperature (T ) is calculated from the ambient temperature T
J A
and the power dissipation P in the LTC3803-5 using the formula:
D
T = T + (P • 230°C/W)
J
A
D
Note 3: High junction temperatures degrade operating lifetimes; operating
lifetime is derated for junction temperatures greater than ±25°C.
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 2: The LTC3803-5 is tested under pulsed load conditions such
that T ≈ T . The LTC3803E-5 is guaranteed to meet specifications
J
A
from 0°C to 85°C junction temperature. Specifications over the –40°C
to ±25°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3803I-5 is guaranteed over the –40°C to ±25°C operating junction
temperature range, the LTC3803H-5 is guaranteed over the –40°C to
±50°C operating junction temperature range and the LTC3803MP-5 is
tested and guaranteed over the full –55°C to ±50°C operating junction
temperature range. Note that the maximum ambient temperature
consistent with these specifications is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
impedance and other environmental factors.
Note 5: The LTC3803-5 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.
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
820
8±5
8±0
805
800
795
790
785
780
8±2
808
804
800
796
792
788
T
= 25°C
V
= 5V
T
= 25°C
A
CC
A
V
≤ V
CC
CLAMP±mA
788
4.0
5.0 5.5 6.0
6.5 7.0 7.5
4.5
5
±0
I
20
–60 –30
0
30
60
90
±20 ±50
0
25
±5
(mA)
V
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
CC
CC
38035 G02
38035 G0±
38035 G03
Oscillator Frequency
vs Supply Voltage
Oscillator Frequency
vs VCC Shunt Regulator Current
Oscillator Frequency
vs Temperature
240
220
2±5
2±0
205
200
±95
±90
220
2±5
2±0
205
200
±95
±90
±85
±80
T
= 25°C
T = 25°C
A
V
= 5V
A
CC
230
220
2±0
200
±90
±80
±85
±80
60
–60 –30
0
30
90 ±20 ±50
4.0 4.5
5.5 6.0
6.5 7.0
7.5
0
5
±5
(mA)
20
25
5.0
±0
TEMPERATURE (°C)
V
SUPPLY VOLTAGE (V)
I
CC
CC
38035 G04
38035 G05
38035 G06
38035fd
4
LTC3803-5
TYPICAL PERFORMANCE CHARACTERISTICS
VCC Undervoltage Lockout
Thresholds vs Temperature
VCC Shunt Regulator Voltage
vs Temperature
ICC Supply Current
vs Temperature
300
280
260
240
220
200
6.0
5.5
5.0
±0.5
±0.0
9.5
9.0
8.5
8.0
7.5
7.0
V = 5V
CC
V
ITH/RUN
= ±.3V
V
TURNON
I
= 25mA
CC
4.5
4.0
V
TURNOFF
I
CC
= ±mA
3.5
3.0
–60
–30
0
30
TEMPERATURE (°C)
60
90 ±20 ±50
–60
30
90
–60
90
–30
0
60
±20 ±50
–30
0
30
60
±20 ±50
TEMPERATURE (°C)
TEMPERATURE (°C)
38035 G09
38035 G07
38035 G08
ITH/RUN Start-Up Current Source
vs Temperature
Start-Up ICC Supply Current
vs Temperature
ITH/RUN Shutdown Threshold
vs Temperature
500
450
400
350
±000
900
800
700
600
500
70
V
V
= V
+ 0.±V
TURNON
V
= V
– 0.±V
TURNON
CC
ITH/RUN
CC
= 0V
60
50
40
30
20
±0
0
300
250
200
±50
±00
50
400
300
200
±00
0
0
–30
0
30
±20 ±50
60
TEMPERATURE (°C)
–60
60
90
–60 –30
0
30
90 ±20 ±50
60
–60 –30
0
30
90 ±20 ±50
TEMPERATURE (°C)
TEMPERATURE (°C)
38035 G±±
38035 G±2
38035 G±0
Peak Current Sense Voltage
vs Temperature
Soft-Start Time vs Temperature
±20
±±5
±±0
±05
±00
95
±.4
±.2
V
= 5V
V
= 5V
CC
CC
±.0
0.8
0.6
0.4
0.2
0
90
85
80
30
60
–60 –30
0
90 ±20 ±50
30
60
–60 –30
0
90 ±20 ±50
TEMPERATURE (°C)
TEMPERATURE (°C)
38035 G±3
38035 G±4
38035fd
5
LTC3803-5
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 the shutdown
threshold(V
)causestheLTC3803-5toshutdown.
ITHSHDN
In shutdown mode, the NGATE pin is held low.
V
(Pin 5): Supply Pin. Must be closely decoupled to
CC
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
5
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
±
38035 BD
38035fd
6
LTC3803-5
OPERATION
The LTC3803-5 is a constant frequency current mode
controller for flyback, SEPIC and DC/DC boost converter
applications in a tiny ThinSOT package. The LTC3803-5 is
designed so that none of its pins need to come in contact
withtheinputoroutputvoltagesofthepowersupplycircuit
ofwhichitisapart,allowingtheconversionofvoltageswell
beyond the LTC3803-5’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
is referred to the detailed treatment in Application Note
±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 LTC3803-5 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 LTC3803-5
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
LTC3803-5
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 LTC3803-5 turns on the external power
MOSFET when the internal free-running 200kHz oscillator
sets the RS latch. It turns off the MOSFET when the cur-
rent comparator resets the latch or when 80ꢀ 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 4V)
(NOMINALLY 0.28V)
(NOMINALLY 4.8V)
LTC3803-5
ENABLED
38035 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
38035fd
7
LTC3803-5
OPERATION
The undervoltage lockout (UVLO) mechanism prevents
Powering the LTC3803-5
the LTC3803-5 from trying to drive a MOSFET with in-
In the simplest case, the LTC3803-5 can be powered from
a high voltage supply through a resistor. A built-in shunt
sufficient V . The voltage at the V pin must exceed
GS
CC
V
(nominally 4.8V) at least momentarily to enable
LTC3803-5 operation. The V voltage is then allowed to
TURNOFF
disables the LTC3803-5.
TURNON
regulator from the V pin to GND will draw as much
CC
CC
current as needed through this resistor to regulate the
falltoV
(nominally4V)beforeundervoltagelockout
V
voltage to around 8.±V as long as the V pin is not
CC
CC
forced to sink more than 25mA. This shunt regulator is
TheI /RUNpincanbedrivenbelowV
(nominally
ITHSHDN
always active, even when the LTC3803-5 is in shutdown,
TH
0.28V) to force the LTC3803-5 into shutdown. An internal
since it serves the vital function of protecting the V pin
CC
0.3μA current source always tries to pull this pin towards
from seeing too much voltage.
V . When the I /RUN pin voltage is allowed to exceed
CC
TH
The V pin must be bypassed to ground immediately ad-
CC
V
, and V exceeds V
, the LTC3803-5
ITHSHDN
CC
TURNON
jacent to the IC pins with a ceramic or tantalum capacitor.
Proper supply bypassing is necessary to supply the high
transient currents required by the MOSFET gate driver.
±0μF is a good starting point.
begins to operate and an internal clamp immediately
pulls the I /RUN pin up to about 0.7V. In operation, the
TH
I /RUN pin voltage will vary from roughly 0.7V to ±.9V
TH
to represent current comparator thresholds from zero
to maximum.
Adjustable Slope Compensation
TheLTC3803-5injectsa5μApeakcurrentrampoutthrough
itsSENSEpinwhichcanbeusedforslopecompensationin
designs that require it. This current ramp is approximately
linearandbeginsatzerocurrentat6.5ꢀdutycycle, reach-
ing peak current at 80ꢀ duty cycle. Additional details are
provided in the Applications Information section.
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC3803-5 comes out of shutdown. Specifically, the I /
RUN voltage is clamped and is prevented from reaching
maximum until roughly 0.7ms has passed. This allows
theinputandoutputcurrentsofLTC3803-5-basedpower
supplies to rise in a smooth and controlled manner on
start-up.
TH
38035fd
8
LTC3803-5
APPLICATIONS INFORMATION
Many LTC3803-5 application circuits can be derived from
the topology shown in Figure 2.
Choose resistance values for R± and R2 to be as large as
possible in order to minimize any efficiency loss due to
the static current drawn from V , but just small enough
OUT
The LTC3803-5 itself imposes no limits on allowed power
so that when V
is in regulation, the error caused by
OUT
output,inputvoltageV ordesiredregulatedoutputvoltage
IN
the nonzero input current to the V pin is less than ±ꢀ.
FB
V
OUT
;thesearealldeterminedbytheratingsontheexternal
A good rule of thumb is to choose R± to be 80k or less.
power components. The key factors are: Q±’s maximum
drain-source voltage (BV ), on-resistance (R
)
DS(ON)
DSS
TRANSFORMER DESIGN CONSIDERATIONS
and maximum drain current, T±’s saturation flux level and
winding insulation breakdown voltages, C and C ’s
IN
OUT
Transformer specification and design is perhaps the most
critical part of applying the LTC3803-5 successfully. In
addition to the usual list of caveats dealing with high fre-
quency power transformer design, the following should
prove useful.
maximum working voltage, ESR, and maximum ripple
current ratings, and D± and R
’s power ratings.
SENSE
V
IN
D±
T±
V
OUT
•
R
C
VCC
C
L
L
SEC
OUT
Turns Ratios
IN PRI
•
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.
5
C
VCC
V
CC
±
2
6
I
/RUN NGATE
LTC3803-5
Q±
TH
C
C
R
SL
4
GND
R±
SENSE
V
FB
R
SENSE
3
R2
38035 F02
Figure 2. Typical LTC3803-5 Application Circuit
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
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
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.
OUT
VOUT – 0.8V
R2=
•R1
0.8V
38035fd
9
LTC3803-5
APPLICATIONS INFORMATION
Leakage Inductance
For example, a peak switch current of 5A requires a sense
resistorof0.020Ω.Notethattheinstantaneouspeakpower
in the sense resistor is 0.5W and it must be rated accord-
ingly. The LTC3803-5 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 ap-
parent value. In the case of a 0.020Ω sense resistor, one
milliohm of parasitic resistance will cause a 5ꢀ reduction
in peak switch current. So the resistance of printed circuit
copper traces and vias cannot necessarily be ignored.
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.
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.
PROGRAMMABLE SLOPE COMPENSATION
TheLTC3803-5injectsarampingcurrentthroughitsSENSE
pin into an external slope compensation resistor (R in
SL
Figure 2). This current ramp starts at zero right after the
NGATE pin has been high for the LTC3803-5’s minimum
duty cycle of 6.5ꢀ. The current rises linearly towards a
peak of 5μA at the maximum duty cycle of 80ꢀ, shutting
CURRENT SENSE RESISTOR CONSIDERATIONS
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.
off once the NGATE pin goes low. A series resistor (R )
SL
connecting the SENSE pin to the current sense resistor
(R
) thus develops a ramping voltage drop. From
SENSE
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
38035fd
10
LTC3803-5
APPLICATIONS INFORMATION
to duty cycle. This stabilizes the control loop against
subharmonic oscillation. The amount of reduction in the
GND to drop enough voltage across R
CC
to regulate
VCC
V
to around 8.±V. For applications where V is low
IN
enough such that the static power dissipation in R
is
currentcomparatorthreshold(ΔV
using the following equation:
)canbecalculated
VCC
SENSE
acceptable, using the V shunt regulator is the simplest
CC
way to power the LTC3803-5.
Duty Cycle – 6.5%
ΔVSENSE
=
•5μA •RSL
73.5%
EXTERNAL PREREGULATOR
Note: LTC3803-5 enforces 6.5ꢀ < Duty Cycle < 80ꢀ.
The circuit in Figure 4 shows another way to power the
LTC3803-5. An external series preregulator consisting
of series pass transistor Q±, Zener diode D±, and bias
resistor R brings V above the V turn-on threshold,
A good starting value for R is 5.9k, which gives a 30mV
SL
drop in current comparator threshold at 80ꢀ duty cycle.
Designs not needing slope compensation may replace
B
CC
CC
enabling the LTC3803-5.
R
with a short circuit.
SL
8V TO
75 V
IN
V
CC
SHUNT REGULATOR
An internal shunt regulator allows the LTC3803-5 to be
powered through a single dropping resistor from V to
Q±
MMBTA42
R
B
LTC3803-5
±00k
IN
V
CC
V , in conjunction with a bypass capacitor, C , that
CC
VCC
C
VCC
D±
6.8V
GND
0.±μF
closely decouples V to GND (see Figure 3). The shunt
CC
regulator can draw up to 25mA through the V pin to
CC
38035 F04
V
IN
Figure 4. Powering the LTC3803-5
with an External Preregulator
R
LTC3803-5
VCC
V
CC
GND
C
VCC
38035 F03
Figure 3. Powering the LTC3803-5
Via the Internal Shunt Regulator
38035fd
11
LTC3803-5
TYPICAL APPLICATIONS
2W Isolated Housekeeping Telecom Converter
BAS5±6
PRIMARY SIDE
±0V, ±00mA
OUTPUT
T±
220k
MMBTA42
•
2.2μF
PDZ6.8B
BAS5±6
±30Ω
±μF
BAS5±6
V
IN
36V TO 75V
•
SECONDARY SIDE
±0V, ±00mA
OUTPUT
2.2μF
9.09k
±k
•
SECONDARY
SIDE GROUND
±nF
LTC3803-5
/RUN NGATE
22k
±
I
6
FDC25±2
TH
2
5
4
V
GND
CC
T±: PULSE ENGINEERING PA0648
OR TYCO TTI8698
787Ω
5.6k
±μF
3
V
SENSE
FB
0.±Ω
38035 TA03
PRIMARY GROUND
38035fd
12
LTC3803-5
TYPICAL APPLICATIONS
4:1 Input Range 3.3V Output Isolated Flyback DC/DC Converter
T±
+
V
3.3V
3A
+
PA±277NL
OUT
V
IN
±8 V TO 72V
±00μF
6.3V
× 3
•
2.2μF
220k
–
MMBTA42
V
PDS±040
IN
•
GND
BAS5±6
68Ω
±50pF
PDZ6.8B
±30Ω
V
CC
±0Ω
BAS5±6
22Ω
680Ω
•
0.±μF
±
2
3
6
5
4
FDC25±2
I
/RUN
GATE
LTC3803-5
TH
+
V
OUT
V
CC
GND
4.7k
BAT760
SENSE
V
FB
0.±μF
0.040Ω
270Ω
V
CC
6.8k
+
V
±
2
3
6
5
4
OUT
V
OPTO
COMP
FB
BAS5±6
IN
PS280±-±
47pF
±00k
2.2nF
22.±k
0.±μF
±
2
LT4430
56k
GND
0.33μF
OC
BAS5±6
38035 TA05
Efficiency vs Load Current
84
82
80
78
76
74
72
70
V
V
= 48V
= 24V
IN
IN
0
±
2
3
4
38035 TA05a
I
(A)
OUT
38035fd
13
LTC3803-5
PACKAGE DESCRIPTION
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-±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 REV B
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
38035fd
14
LTC3803-5
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
38035fd
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
LTC3803-5
TYPICAL APPLICATION
Synchronous Flyback Converter
V
IN
36V TO 72V
V
3.3V
±.5A
*
OUT
T±
220k
MMBTA42
C
IN
Q2
D±
•
•
C
O
PDZ6.8B
•
±30Ω
±n
33k
±
6
I
/RUN
Q±
GATE
TH
LTC3803-5
0.±μF
560
5k
2
3
5
4
V
GND
CC
8.06k
SENSE
V
FB
25.5k*
FB
38035 TA04
±μF
±0V
R
R
CS
V
OUT
T±: PULSE ENGINEERING PA±006
Q±: FAIRCHILD FDC25±2
Q2: VISHAY Si9803
D±: PHILIPS BAS5±6
R : VISHAY OR IRC, 80mΩ
CS
C
: TDK ±μF, ±00V, X5R *FOR 5V OUTPUT CHANGE
IN
C : TDK ±00μF, 6.3V, X5R
R
FB
TO 42.2k
O
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3573
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
LTC3805/
LTC3805-5
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/
LTC3873-5
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
LT3757
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
LT3758
Boost, Flyback, SEPIC and Inverting Controller
5.5V ≤ 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, 2.5V ≤ V ≤ 36V, Burst Mode®
SENSE
IN
LTC±87±-7
Flyback, Boost and SEPIC Controller
Operation at Light Load, MSOP-±0
38035fd
LT 0610 REV D • PRINTED IN USA
LinearTechnology Corporation
±630 McCarthy Blvd., Milpitas, CA 95035-74±7
16
●
●
© LINEAR TECHNOLOGY CORPORATION 2004
(408) 432-±900 FAX: (408) 434-0507 www.linear.com
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