LTC3803ES6#TRMPBF [Linear]
LTC3803 - Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT; Package: SOT; Pins: 6; Temperature Range: -40°C to 85°C;
| 型号： | LTC3803ES6#TRMPBF |
| 厂家： | 
Linear |
| 描述： | LTC3803 - Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT; Package: SOT; Pins: 6; Temperature Range: -40°C to 85°C 控制器 |
| 文件： | 总12页 (文件大小：174K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Final Electrical Specifications
LTC3803
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
August 2003
U
DESCRIPTION
FEATURES
The LTC®3803 is a constant frequency current mode
flybackcontrolleroptimizedfordriving6V-ratedN-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.
■
VIN and VOUT Limited Only by External Components
■
Adjustable Slope Compensation
■
Internal Soft-Start
■
Constant Frequency 200kHz Operation
■
±1.5% Reference Accuracy
■
Current Mode Operation for Excellent Line and Load
Transient Response
■
No Minimum Load Requirement
The LTC3803 provides ±1.5% output voltage accuracy
and consumes only 240µA of quiescent current. Ground-
referenced current sensing allows LTC3803-based con-
verters to accept input supplies beyond the LTC3803’s
absolute maximum VCC. A micropower hysteretic start-up
feature allows efficient operation at high input voltages.
For simplicity, the LTC3803 can also be powered from a
high VIN through a resistor, due to its internal 9.4V shunt
regulator. An internal undervoltage lockout shuts down
the LTC3803 when the input voltage falls below 4.6V,
guaranteeing at least 4.6V of gate drive to the external
MOSFET.
■
Low Quiescent Current: 240µA
■
Low Profile (1mm) SOT-23 Package
U
APPLICATIO S
■
Telecom Power Supplies
■
42V and 12V Automotive Power Supplies
■
Auxiliary/Housekeeping Power Supplies
Power Over Ethernet
■
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
The LTC3803 is available in a low profile (1mm) 6-lead
SOT-23 (ThinSOTTM) package.
U
TYPICAL APPLICATIO
5V Output Nonisolated Telecom Housekeeping Power Supply
Efficiency vs Load Current
V
IN
90
36V TO 72V
UPS840
V
V
IN
= 36V
OUT
5V
80
70
60
50
40
30
20
10
0
T1
•
2A MAX
4.7µF
100V
X5R
300µF*
6.3V
X5R
10k
V
IN
= 48V
10µF
10V
X5R
•
V
= 60V
IN
V
IN
= 72V
V
CC
I
/RUN NGATE
LTC3803
FDC2512
TH
56k
0.0022µF
GND
SENSE
V
FB
68mΩ
20k
105k
1
0.1
10
3803 TA01
I
(A)
OUT
T1: COOPER CTX02-15242
*THREE 100µF UNITS IN PARALLEL
3803 TA02
3803i
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
1
LTC3803
W W U W
W U
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
VCC to GND
ORDER PART
NUMBER
Low Impedance Source .......................... –0.3V to 8V
Current Fed...................................... 25mA into VCC*
NGATE Voltage ......................................... –0.3V to VCC
VFB, ITH/RUN Voltages ..............................–0.3V to 3.5V
SENSE Voltage ........................................... –0.3V to 1V
NGATE Peak Output Current (<10µs)........................ 1A
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Note 3)............................ 150°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
*LTC3803 internal clamp circuit self regulates VCC voltage to 9.5V.
TOP VIEW
LTC3803ES6
I
/RUN 1
GND 2
6 NGATE
5 V
TH
CC
V
FB
3
4 SENSE
S6 PART
MARKING
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
TJMAX = 150°C, θJA = 230°C/W
LTACV
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● indicates 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
7.8
TYP
8.7
5.7
3.0
9.4
9.5
0.6
MAX
9.2
UNITS
V
V
V
V
V
V
V
V
V
V
V
V
Turn On Voltage
●
●
●
●
●
●
V
V
V
V
V
V
TURNON
TURNOFF
HYST
CC
Turn Off Voltage
4.6
6.8
CC
Hysteresis
V
– V
1.5
CC
TURNON
TURNOFF
ITH/RUN
Shunt Regulator Voltage
Shunt Regulator Voltage
I
I
= 1mA, V
= 0V
= 0V
8.3
10.3
10.5
CLAMP1mA
CLAMP25mA
MARGIN
CC
CC
CC
= 25mA, V
8.4
CC
ITH/RUN
– V
Margin
0.05
CLAMP1mA
TURNON
I
Input DC Supply Current
Normal Operation
Start-Up
(Note 4)
ITH/RUN
CC
V
V
= 1.3V
240
40
350
90
µA
µA
= V
– 100mV
+ 100mV
●
●
CC
TURNON
V
Shutdown Threshold (at I /RUN)
V
V
= V
0.15
0.2
0.28
0.3
0.45
0.4
V
ITHSHDN
TH
CC
TURNON
I
Start-Up Current Source
= 0V
µA
ITHSTART
ITH/RUN
V
Regulated Feedback Voltage
0°C ≤ T ≤ 85°C (Note 5)
0.788
0.780
0.800
0.800
0.812
0.812
V
V
FB
A
–40°C ≤ T ≤ 85°CC (Note 5)
●
A
g
Error Amplifier Transconductance
Output Voltage Line Regulation
Output Voltage Load Regulation
I
Pin Load = ±5µA (Note 5)
200
333
500
µA/V
m
TH/RUN
∆V
∆V
V
< V < V (Note 5)
CLAMP
0.05
mV/V
O(LINE)
TURNOFF
CC
I
I
/RUN Sinking 5µA (Note 5)
/RUN Sourcing 5µA (Note 5)
3
3
mV/µA
mV/µA
O(LOAD)
TH
TH
I
f
V
Input Current
FB
(Note 5)
10
200
6
50
240
8
nA
kHz
%
FB
OSC
Oscillator Frequency
V
V
V
C
C
= 1.3V
180
70
ITH/RUN
ITH/RUN
ITH/RUN
DC
DC
Minimum Switch On Duty Cycle
Maximum Switch On Duty Cycle
Gate Drive Rise Time
= 1.3V, V = 0.8V
FB
ON(MIN)
= 1.3V, V = 0.8V
80
40
40
100
5
90
%
ON(MAX)
FB
t
t
= 3000pF
ns
RISE
FALL
LOAD
LOAD
Gate Drive Fall Time
= 3000pF
ns
V
Peak Current Sense Voltage
Peak Slope Compensation Output Current
Soft-Start Time
R
SL
= 0 (Note 6)
●
90
115
mV
µA
ms
IMAX
SLMAX
SFST
I
t
(Note 7)
1.4
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
are assured by design, characterization and correlation with statistical
process controls.
Note 2: The LTC3803E is guaranteed to meet specifications from 0°C to
70°C. Specifications over the –40°C to 85°C operating temperature range
Note 3: T is calculated from the ambient temperature T and power
J
A
dissipation P according to the following formula:
D
3803i
2
LTC3803
ELECTRICAL CHARACTERISTICS
T = T + (P • 230°C/W).
Note 6: Peak current sense voltage is reduced dependent on duty cycle
J
A
D
and an optional external resistor in series with the SENSE pin (R ). For
details, refer to the programmable slope compensation feature in the
Applications Information section.
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
SL
Note 5: The LTC3803 is tested in a feedback loop that servos V to the
FB
Note 7: Guaranteed by design.
output of the error amplifier while maintaing I /RUN at the midpoint of
TH
the current limit range.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage
vs Supply Voltage
Reference Voltage
vs VCC Shunt Regulator Current
Reference Voltage vs Temperature
801.0
800.8
800.6
800.4
800.2
800.0
799.8
799.6
799.4
799.2
799.0
800.5
800.0
804
803
802
801
T
= 25°C
V
= 8V
T
= 25°C
A
CC
A
V
≤ V
CC
CLAMP1mA
799.5
799.0
798.5
798.0
797.5
800
799
798
797
796
797.0
6
7
7.5
8
8.5
9
9.5
6.5
–30 –10 10 30 50
110
5
10
20
–50
70 90
0
25
15
(mA)
V
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
CC
I
CC
3803 F02
3803 G01
3803 G03
Oscillator Frequency
vs Temperature
Oscillator Frequency
vs Supply Voltage
Oscillator Frequency
vs VCC Shunt Regulator Current
210
205
200
195
190
185
180
210
208
206
204
202
200
198
196
194
192
190
210
208
206
204
202
200
198
196
194
192
190
V
= 8V
T
= 25°C
T
= 25°C
A
CC
A
50 70
TEMPERATURE (°C)
–50 –30 –10 10 30
90 110
6
6.5
7.5
8
8.5
9
0
5
15
(mA)
20
25
7
10
V
SUPPLY VOLTAGE (V)
I
CC
CC
3803 G04
3803 G05
3803 G06
3803i
3
LTC3803
TYPICAL PERFOR A CE CHARACTERISTICS
U W
VCC Undervoltage Lockout
Thresholds vs Temperature
VCC Shunt Regulator Voltage
vs Temperature
ICC Supply Current
vs Temperature
10.0
9.9
9.8
9.7
9.6
9.5
9.4
9.3
9.2
9.1
9.0
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
265
260
255
250
245
240
235
230
225
220
215
V = 8V
CC
V
ITH/RUN
= 1.3V
V
TURNON
I
CC
= 25mA
I
CC
= 1mA
V
TURNOFF
–50
30
TEMPERATURE (°C)
70 90
–30 –10 10
50
110
–50
30
80 90
–30 –10 10
50
110
90
–50
70
–30 –10 10
30
TEMPERATURE (°C)
50
110
TEMPERATURE (°C)
3803 G08
3803 G07
3803 G08
Start-Up ICC Supply Current
vs Temperature
ITH/RUN Shutdown Threshold
vs Temperature
ITH/RUN Start-Up Current Source
vs Temperature
450
400
60
50
40
30
20
10
0
600
500
400
300
200
100
0
V
= V
– 0.1V
V
V
= V
+ 0.1V
CC
TURNON
CC
ITH/RUN
TURNON
= 0V
350
300
250
200
150
100
50 70
TEMPERATURE (°C)
–30 –10 10 30 50
110
50 70
TEMPERATURE (°C)
–50 –30 –10 10 30
90 110
–50
70 90
–50 –30 –10 10 30
90 110
TEMPERATURE (°C)
3803 G10
3803 G11
3803 G12
Peak Current Sense Voltage
vs Temperature
Soft-Start Time vs Temperature
120
4.0
V
CC
= 8V
115
110
105
100
95
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
90
85
80
30 50
TEMPERATURE (°C)
30 50
–50 –30 –10 10
TEMPERATURE (°C)
–50 –30 –10 10
70 90 110
70 90 110
3803 G13
3803 G14
3803i
4
LTC3803
U
U
U
PI FU CTIO S
ITH/RUN (Pin 1): This pin performs two functions. It
servesastheerroramplifiercompensationpointaswellas
the run/shutdown control input. Nominal voltage range is
0.7V to 1.9V. Forcing this pin below 0.28V causes the
LTC3803toshutdown. Inshutdownmode, theNGATEpin
is held low.
SENSE (Pin 4): This pin performs two functions. It moni-
tors 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.
V
CC (Pin5):SupplyPin.MustbecloselydecoupledtoGND
GND (Pin 2): Ground Pin.
(Pin 2).
V
FB (Pin 3): Receives the feedback voltage from an exter-
NGATE (Pin 6): Gate Drive for the External N-Channel
MOSFET. This pin swings from 0V to VCC.
nal resistive divider across the output.
W
BLOCK DIAGRA
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
1.2V
SENSE
4
I
TH
/RUN
1
3803 BD
3803i
5
LTC3803
U
OPERATIO
The LTC3803 is a constant frequency current mode con-
troller 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.
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
ITH/RUN voltage by sourcing current into the ITH/RUN pin,
raising the current comparator threshold, thus increasing
the peak currents through the transformer primary and
secondary. This delivers more current to the load, bring-
ing the output voltage back up.
Main Control Loop
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 19, or in texts such as Abraham Pressman’s Switch-
ing Power Supply Design.
The ITH/RUN pin serves as the compensation point for the
control loop. Typically, an external series RC network is
connected from ITH/RUN to ground and is chosen for
optimal response to load and line transients. The imped-
ance of this RC network converts the output current of the
error amplifier to the ITH/RUN voltage which sets the
current comparator threshold and commands consider-
able influence over the dynamics of the voltage regulation
loop.
Please refer to the Block Diagram and the Typical Applica-
tion on the front page of this data sheet. An external
resistive voltage divider presents a fraction of the output
voltagetotheVFB pin.Thedividermustbedesignedsothat
when the output is at the desired voltage, the VFB pin
voltage will equal the 800mV from the internal reference.
If the load current increases, the output voltage will
decrease slightly, causing the VFB pin voltage to fall below
800mV. The error amplifier responds by feeding current
into the ITH/RUN pin. If the load current decreases, the VFB
voltage will rise above 800mV and the error amplifier will
sink current away from the ITH/RUN pin.
Start-Up/Shutdown
The LTC3803 has two shutdown mechanisms to disable
and enable operation: an undervoltage lockout on the VCC
supply pin voltage, and a forced shutdown whenever
externalcircuitrydrivestheITH/RUNpinlow.TheLTC3803
transitionsintoandoutofshutdownaccordingtothestate
diagram (Figure 1).
ThevoltageattheITH/RUNpincommandsthepulse-width
modulator formed by the oscillator, current comparator
and RS latch. Specifically, the voltage at the ITH/RUN pin
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 turns on the external power
MOSFET when the internal free-running 200kHz oscillator
setstheRSlatch. ItturnsofftheMOSFETwhenthecurrent
comparator resets the latch or when 80% duty cycle is
reached, whichever happens first. In this way, the peak
current levels through the flyback transformer’s primary
and secondary is controlled by the ITH/RUN voltage.
LTC3803
SHUT DOWN
V
> V
ITH/RUN
ITHSHDN
TURNON
V
< V
V
< V
CC
TURNOFF
ITH/RUN ITHSHDN
AND V > V
CC
(NOMINALLY 5.7V)
(NOMINALLY 0.28V)
(NOMINALLY 8.7V)
LTC3803
ENABLED
3803 F01
Since the ITH/RUN voltage is increased by the error ampli-
fier whenever the output voltage is below nominal, and
Figure 1. Start-Up/Shutdown State Diagram
3803i
6
LTC3803
U
OPERATIO
The undervoltage lockout (UVLO) mechanism prevents
the LTC3803 from trying to drive a MOSFET with insuffi-
cientVGS. ThevoltageattheVCC pinmustexceedVTURNON
(nominally 8.7V) at least momentarily to enable LTC3803
operation. The VCC voltage is then allowed to fall to
VTURNOFF (nominally 5.7V) before undervoltage lockout
disables the LTC3803. This wide UVLO hysteresis range
supports the use of a bias winding on the flyback trans-
former to power the LTC3803—see the section Powering
the LTC3803.
Powering the LTC3803
In the simplest case, the LTC3803 can be powered from a
high voltage supply through a resistor. A built-in shunt
regulator from the VCC pin to GND will draw as much
current as needed through this resistor to regulate the VCC
voltage to around 9.4V as long as the VCC pin is not forced
to sink more than 25mA. This shunt regulator is always
active, even when the LTC3803 is in shutdown, since it
serves the vital function of protecting the VCC pin from
seeing too much voltage.
The ITH/RUN pin can be driven below VSHDN (nominally
0.28V) to force the LTC3803 into shutdown. An internal
0.3µA current source always tries to pull this pin towards
VCC. When the ITH/RUN pin voltage is allowed to exceed
VSHDN, andVCC exceedsVTURNON, theLTC3803beginsto
operate and an internal clamp immediately pulls the
ITH/RUN pin up to about 0.7V. In operation, the ITH/RUN
pin voltage will vary from roughly 0.7V to 1.9V to repre-
sent current comparator thresholds from zero to maxi-
mum.
For higher efficiency or for wide VIN range applications,
flyback controllers are typically powered through a sepa-
ratebiaswindingontheflybacktransformer.TheLTC3803
has a wide UVLO hysteresis (1.5V min) and small VCC
supply current draw (<90µA when VCC < VTURNON) that is
needed to support such bootstrapped hysteretic start-up
schemes.
The VCC pin must be bypassed to ground immediately
adjacent to the IC pins with a minimum of a 10µF ceramic
or tantalum capacitor. Proper supply bypassing is neces-
sary to supply the high transient currents required by the
MOSFET gate driver.
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC3803 comes out of shutdown. Specifically, the
ITH/RUNvoltageisclampedandispreventedfromreach-
ing maximum until roughly 1.4ms has passed. This
allows the input and output currents of LTC3803-based
powersuppliestoriseinasmoothandcontrolledmanner
on start-up.
Adjustable Slope Compensation
The LTC3803 injects a 5µA peak current ramp out through
its SENSE pin which can be used for slope compensation
in designs that require it. This current ramp is approxi-
mately linear and begins at zero current at 6% duty cycle,
reachingpeakcurrentat80%dutycycle.Additionaldetails
are provided in the Applications Information section.
3803i
7
LTC3803
W U U
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APPLICATIO S I FOR ATIO
Many LTC3803 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 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, input voltage VIN or desired regulated output
voltageVOUT;thesearealldeterminedbytheratingsonthe
external power components. The key factors are: Q1’s
maximum drain-source voltage (BVDSS), on-resistance
(RDS(ON)) and maximum drain current, T1’s saturation
flux level and winding insulation breakdown voltages, CIN
and COUT’s maximum working voltage, ESR, and maxi-
mum ripple current ratings, and D1 and RSENSE’s power
ratings.
Turns Ratios
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 applica-
tion. Simpleratios of small integers, e.g., 1:1, 2:1, 3:2, etc.
canbeemployedwhichyieldmorefreedominsettingtotal
turns and mutual inductance. Simple integer turns ratios
alsofacilitatetheuseof“off-the-shelf”configurabletrans-
formers such as the Coiltronics VERSA-PACTM 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
parallelonthesecondary,a3:1turnsratiowillbeachieved.
T1
L
BIAS
•
•
D2
V
IN
D1
V
OUT
R3
R
I
C
START
C
L
L
SEC
OUT
IN PRI
•
5
C
VCC
V
CC
1
2
6
/RUN NGATE
LTC3803
Q1
TH
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.
C
C
R
SL
4
GND
R1
SENSE
V
FB
R
SENSE
3
R2
3803 F02
Figure 2. Typical LTC3803 Application Circuit
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a voltage spike to occur after the
output switch (Q1) turn-off. This is increasingly promi-
nent 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 19 is a good
reference on snubber design.
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
The regulated output voltage is determined by the resistor
divider across VOUT (R1 and R2 in Figure 2). The ratio of
R2 to R1 needed to produce a desired VOUT can be
calculated:
VOUT – 0.8V
R2 =
•R1
0.8V
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.
Choose resistance values for R1 and R2 to be as large as
possible in order to minimize any efficiency loss due to the
static current drawn from VOUT, but just small enough so
that when VOUT is in regulation, the error caused by the
nonzero input current to the VFB pin is less than 1%. A
good rule of thumb is to choose R1 to be 80k or less.
VERSA-PAC is a trademark of Coiltronics, Inc.
3803i
8
LTC3803
W U U
APPLICATIO S I FOR ATIO
U
CURRENT SENSE RESISTOR CONSIDERATIONS
A good starting value for RSL is 5.9k, which gives a 30mV
drop in current comparator threshold at 80% duty cycle.
DesignsnotneedingslopecompensationmayreplaceRSL
with a short circuit.
The external current sense resistor (RSENSE in Figure 2)
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
switchcurrentgoesfromafractionofanamperetoseveral
amperes. Care must be taken to ensure proper circuit
operation, especially with small current sense resistor
values.
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 (LBIAS in Figure 2) on the primary side that
provides power for the LTC3803 via its VCC pin. However,
thisarrangementisnotinherentlyself-starting. Start-upis
affected by the use of an external “trickle-charge” resistor
(RSTART in Figure 2) and the presence of an internal wide
hysteresis undervoltage lockout circuit that monitors VCC
pin voltage. Operation is as follows:
For example, a peak switch current of 5A requires a sense
resistor of 0.020Ω. Note that the instantaneous peak
power in the sense resistor is 0.5W and it must be rated
accordingly. The LTC3803 has only a single sense line to
this resistor. Therefore, any parasitic resistance in the
ground side connection of the sense resistor will increase
its apparent value. In the case of a 0.020Ω sense resistor,
one milliohm of parasitic resistance will cause a 5%
reduction in peak switch current. So the resistance of
printed circuit copper traces and vias cannot necessarily
be ignored.
“Trickle charge” resistor RSTART is connected to VIN and
supplies a small current, typically on the order of 100µA,
to charge CVCC. After some time, the voltage on CVCC
reaches the VCC turn-on threshold. The LTC3803 then
turnsonabruptlyanddrawsitsnormalsupplycurrent.The
NGATE pin begins switching and the external MOSFET
(Q1) begins to deliver power. The voltage on CVCC begins
to decline as the LTC3803 draws its normal supply cur-
rent, which exceeds that delivered by RSTART. After some
time, typically tens of milliseconds, the output voltage
approaches its desired value. By this time, the third
transformer winding is providing virtually all the supply
current required by the LTC3803.
PROGRAMMABLE SLOPE COMPENSATION
The LTC3803 injects a ramping current through its SENSE
pin into an external slope compensation resistor (RSL in
Figure 2). This current ramp starts at zero right after the
NGATE pin has been high for the LTC3803’s minimum
duty cycle of 6%. The current rises linearly towards a peak
of 5µA at the maximum duty cycle of 80%, shutting off
once the NGATE pin goes low. A series resistor (RSL)
connecting the SENSE pin to the current sense resistor
(RSENSE) thus develops a ramping voltage drop. From the
perspective of the SENSE pin, this ramping voltage adds
to the voltage across the sense resistor, effectively reduc-
ingthecurrentcomparatorthresholdinproportiontoduty
cycle.Thisstabilizesthecontrolloopagainstsubharmonic
oscillation. The amount of reduction in the current com-
parator threshold (∆VSENSE) can be calculated using the
following equation:
One potential design pitfall is undersizing the value of
capacitor CVCC. In this case, the normal supply current
drawn by the LTC3803 will discharge CVCC too rapidly;
before the third winding drive becomes effective, the VCC
turn-off threshold will be reached. The LTC3803 turns off,
and the VCC node begins to charge via RSTART back up to
the VCC turn-on threshold. Depending on the particular
situation, this may result in either several on-off cycles
before proper operation is reached or permanent relax-
ation oscillation at the VCC node.
Duty Cycle – 6%
74%
Note: LTC3803 enforces 6% < Duty Cycle < 80%.
∆VSENSE
=
• 5µA •RSL
3803i
9
LTC3803
W U U
U
APPLICATIO S I FOR ATIO
Component selection is as follows:
from overvoltage transients as the third winding is pow-
ering up.
Resistor RSTART should be made small enough to yield a
worst-case minimum charging current greater than the
maximumratedLTC3803start-upcurrent, toensurethere
isenoughcurrenttochargeCVCC totheVCC 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.
In applications where a third transformer winding is unde-
sirable or unavailable, the shunt regulator allows the
LTC3803tobepoweredthroughasingledroppingresistor
from VIN to VCC, in conjunction with a bypass capacitor,
CVCC, that closely decouples VCC to GND (see Figure 3).
This simplicity comes at the expense of reduced efficiency
due to the static power dissipation in the RVCC dropping
resistor.
CapacitorCVCC shouldthenbemadelargeenoughtoavoid
therelaxationoscillationbehaviordescribedabove.Thisis
complicated to determine theoretically as it depends on
the particulars of the secondary circuit and load behavior.
Empirical testing is recommended.
The shunt regulator can draw up to 25mA through the VCC
pintoGNDtodropenoughvoltageacrossRVCC toregulate
VCC to around 9.5V. For applications where VIN is low
enough such that the static power dissipation in RVCC is
acceptable, using the VCC shunt regulator is the simplest
way to power the LTC3803.
The third transformer winding should be designed so that
its output voltage, after accounting for the D2’s forward
voltage drop, exceeds the maximum VCC turn-off thresh-
old. Also, the third winding’s nominal output voltage
should be at least 0.5V below the minimum rated VCC
clamp voltage to avoid running up against the LTC3803’s
VCC shunt regulator, needlessly wasting power.
EXTERNAL PREREGULATOR
The circuit in Figure 4 shows a third way to power the
LTC3803. An external series preregulator consisting of
series pass transistor Q1, Zener diode D1, and bias resis-
tor RB brings VCC to at least 7.6V nominal, well above the
maximum rated VCC turn-off threshold of 6.8V. Resistor
RSTART momentarily charges the VCC node up to the VCC
turn-on threshold, enabling the LTC3803.
VCC SHUNT REGULATOR
Inapplicationsincludingathirdtransformerwinding, the
internalVCCshuntregulatorservestoprotecttheLTC3803
V
IN
V
IN
R
R
Q1
R
START
LTC3803
LTC3803
VCC
B
V
V
CC
CC
GND
D1
8.2V
GND
C
C
VCC
VCC
3803 F03
3803 F04
Figure 3. Powering the LTC3803 Via the Internal Shunt Regulator
Figure 4. Powering the LTC3803 with an External Preregulator
3803i
10
LTC3803
U
PACKAGE DESCRIPTIO
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
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
3803i
11
LTC3803
U
TYPICAL APPLICATIO
2W Housekeeping Telecom Converter
BAS516
PRIMARY SIDE
10V, 100mA
OUTPUT
T1
•
2.2µF
1µF
BAS516
V
IN
36V TO 75V
•
SECONDARY SIDE
10V, 100mA
OUTPUT
2.2µF
BAS516
9.2k
1k 220k
•
SECONDARY
SIDE GROUND
1nF
LTC3803
/RUN NGATE
22k
1
6
I
TH
FDC2512
2
3
5
4
V
GND
CC
T1: PULSE ENGINEERING PA0648
OR TYCO TT18698
806Ω
5.6k
1µF
V
FB
SENSE
0.1Ω
3803 TA03
PRIMARY GROUND
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT®1425
Isolated Flyback Switching Regulator
with No External Power Devices
No Optoisolator or “Third Winding” Required, Up to 6W Output
LT1725
General Purpose Isolated Flyback Controller
No Optoisolator Required, V and V
Power Components
Limited Only by External
IN
OUT
LTC1772
LTC1871
LTC1872
SOT-23 Constant Frequency Current Mode Step-Down
DC/DC Controller
550kHz Switching Frequency, 2.4V to 9.8V V Range
IN
Wide Input Range, No R
TM Current Mode
Adjustable Switching Frequency, Programmable Undervoltage
Lockout, Optional Burst Mode® Operation at Light Load
SENSE
Flyback, Boost and SEPIC Controller
SOT-23 Constant Frequency Current Mode Boost DC/DC
Controller
550kHz Switching Frequency, 2.4V to 9.8V V Range
IN
LT1950
LT3420
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
Burst Mode is a registered trademark of Linear Technology Corporation. No R
is a trademark of Linear Technology Corporation.
SENSE
3803i
LT/TP 0803 1K • PRINTED IN THE USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
12
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
LINEAR TECHNOLOGY CORPORATION 2003
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