LTC3803ES6-3 [Linear]
Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT; 采用ThinSOT封装的恒定频率电流模式反激式DC / DC控制器
| 型号： | LTC3803ES6-3 |
| 厂家： | 
Linear |
| 描述： | Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT |
| 文件： | 总16页 (文件大小：166K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3803-3
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
U
FEATURES
DESCRIPTIO
VIN and VOUT Limited Only by External Components
Adjustable Slope Compensation
Internal Soft-Start
–40°C to 125°C Operating Temperature Range
Constant Frequency 300kHz Operation
1.5% Reference Accuracy
Current Mode Operation for Excellent Line and Load
The LTC®3803-3 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.
■
■
■
■
■
■
■
Transient Response
No Minimum Load Requirement
Low Quiescent Current: 240μA
Low Profile (1mm) SOT-23 Package
The LTC3803-3 provides 1.5% output voltage accuracy
and consumes only 240μA of quiescent current. Ground-
referencedcurrentsensingallowsLTC3803-3-basedcon-
verters to accept input supplies beyond the LTC3803-3’s
absolute maximum VCC. A micropower hysteretic start-up
feature allows efficient operation at high input voltages.
For simplicity, the LTC3803-3 can also be powered from
a high VIN through a resistor, due to its internal 9.4V shunt
regulator. An internal undervoltage lockout shuts down
the LTC3803-3 when the input voltage falls below 4.4V,
guaranteeing at least 4.4V of gate drive to the external
MOSFET.
■
■
■
U
APPLICATIO S
■
Telecom Power Supplies
■
■
■
42V and 12V Automotive Power Supplies
Auxiliary/Housekeeping Power Supplies
Power Over Ethernet Powered Devices
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
The LTC3803-3 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
100
36V TO 72V
V
= 5V
UPS840
OUT
V
95
90
85
80
75
70
65
60
55
50
OUT
5V
T1
•
2A MAX
1μF
300μF*
6.3V
10k
100V
X5R
X5R
1μF
10V
X5R
•
V
CC
I
/RUN NGATE
LTC3803-3
FDC2512
TH
82k
470pF
150pF
200V
4.7k
V
V
V
V
= 36V
= 48V
= 60V
= 72V
IN
IN
IN
IN
GND
SENSE
V
FB
68mΩ
220Ω
20k
105k
250
750 1000 1250 1500 1750 2000
LOAD CURRENT (mA)
500
38033 TA01
T1: COOPER CTX02-15242
*THREE 100μF UNITS IN PARALLEL
38033 TA02
38033fa
1
LTC3803-3
W W U W
U
W U
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
VCC to GND
TOP VIEW
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)
I
/RUN 1
GND 2
6 NGATE
5 V
TH
CC
V
3
4 SENSE
FB
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
TJMAX = 125°C, θJA = 230°C/W
ORDER PART NUMBER
S6 PART MARKING
LTC3803E-3 ....................................... – 40°C to 85°C
LTC3803I-3 ...................................... – 40°C to 125°C
Junction Temperature (Note 3)............................ 125°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
LTC3803ES6-3
LTC3803IS6-3
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
LTCJS
LTCJT
Lead Free Part Marking: http://www.linear.com/leadfree/
*LTC3803-3 internal clamp circuit self regulates VCC voltage to 9.5V.
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
TYP
MAX
UNITS
V
V
V
V
Turn On Voltage
Turn Off Voltage
●
7.6
8.7
9.2
V
TURNON
TURNOFF
CC
CC
LTC3803E-3
LTC3803I-3
●
●
4.6
4.4
5.7
5.7
7
7
V
V
V
V
V
V
V
V
V
V
Hysteresis (V
– V )
TURN0FF
●
●
●
●
1
3
V
V
V
V
HYST
CC
TURNON
Shunt Regulator Voltage
Shunt Regulator Voltage
I
I
= 1mA, V = 0V
ITH/RUN
8.3
8.4
0.05
9.4
9.5
0.6
10.3
10.5
CLAMP1mA
CLAMP25mA
MARGIN
CC
CC
CC
= 25mA, V
= 0V
CC
ITH/RUN
– V
Margin
TURNON
CLAMP1mA
I
Input DC Supply Current
Normal Operation
Start-Up
(Note 4)
CC
V
= 1.3V
240
40
350
90
μA
μA
ITH/RUN
= V
V
CC
– 100mV
TURNON
V
I
Shutdown Threshold (at I /RUN)
V
= V
LTC3803E-3
LTC3803I-3
– 100mV
ITHSHDN
TH
CC
TURNON
●
●
0.15
0.10
0.28
0.28
0.45
0.45
V
V
Start-Up Current Source
V
= 0V
0.2
0.3
0.4
μA
ITHSTART
ITH/RUN
V
Regulated Feedback Voltage (Note 5)
0°C ≤ T ≤ 85°C
0.788
0.780
0.780
0.800
0.800
0.800
0.812
0.812
0.820
V
V
V
FB
A
LTC3803E-3: –40°C ≤ T ≤ 85°C
●
●
A
LTC3803I-3: –40°C ≤ T ≤ 125°C
A
V
g
Peak Current Sense Voltage
R
= 0 (Note 6)
SL
LTC3803E-3
LTC3803I-3
IMAX
●
●
90
85
100
100
115
115
V
V
Error Amplifier Transconductance
Output Voltage Line Regulation
I
Pin Load = 5μA (Note 5)
200
333
0.05
10
500
μA/V
mV/V
nA
m
TH/RUN
ΔV
V
< V < V
(Note 5)
CLAMP
O(LINE)
TURNOFF
CC
I
f
V
Input Current
FB
(Note 5)
50
FB
OSC
Oscillator Frequency
V
= 1.3V
270
300
330
kHz
ITH/RUN
38033fa
2
LTC3803-3
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
TYP
8
MAX
9.6
UNITS
%
DC
ON(MIN)
DC
ON(MAX)
Minimum Switch On Duty Cycle
Maximum Switch On Duty Cycle
Gate Drive Rise Time
V
V
C
C
= 1.3V, V = 0.8V
FB
ITH/RUN
ITH/RUN
= 1.3V, V = 0.8V
70
80
40
40
5
90
%
FB
t
t
I
t
= 3000pF
ns
RISE
LOAD
LOAD
Gate Drive Fall Time
= 3000pF (Note 7)
ns
FALL
Peak Slope Compensation Output Current
Soft-Start Time
(Note 7)
μA
SLMAX
SFST
1.4
ms
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
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.
Note 5: The LTC3803-3 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
Note 2: The LTC3803E-3 is guaranteed to meet specifications from 0°C to
85°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls. The LTC3803I-3 is guaranteed to meet performance
specifications over the –40°C to 125°C operating temperature range.
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
details, refer to the programmable slope compensation feature in the
SL
Applications Information section.
Note 3: T is calculated from the ambient temperature T and power
J
A
Note 7: Guaranteed by design.
dissipation P according to the following formula:
D
T = T + (P • 230°C/W).
J
A
D
38033fa
3
LTC3803-3
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
U W
Reference Voltage
vs Supply Voltage
Reference Voltage
Reference Voltage vs Temperature
vs VCC Shunt Regulator Current
801.0
800.8
800.6
800.4
800.2
800.0
799.8
799.6
799.4
799.2
799.0
812
808
804
800
796
792
788
804
803
802
801
V
≤ V
V
= 8V
CC
CLAMP1mA
CC
800
799
798
797
796
6
7
7.5
8
8.5
9
9.5
6.5
–30 –10 10 30 50
110 130
5
10
20
–50
70 90
0
25
15
(mA)
V
CC
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
I
CC
38033 F02
38033 G01
38033 G03
Oscillator Frequency
vs Temperature
Oscillator Frequency
vs Supply Voltage
Oscillator Frequency
vs VCC Shunt Regulator Current
330
320
310
300
290
280
270
330
320
310
300
290
280
270
330
320
310
300
290
280
270
V
= 8V
CC
0
10
15
I
20
25
30
35
50 70
5
–50 –30 –10 10 30
90 110 130
6
7
7.5
8
8.5
9
6.5
(mA)
TEMPERATURE (°C)
V
SUPPLY VOLTAGE (V)
CC
CC
38033 G06
38033 G04
38033 G05
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
4.5
350
325
300
275
250
225
200
175
150
V
V
= 8V
CC
ITH/RUN
= 1.3V
V
TURNON
I
= 25mA
I
CC
= 1mA
CC
V
TURNOFF
–50
30
70 90
110 130
–30 –10 10
50
30 50
TEMPERATURE (°C)
–50
30
80 90
–50 –30 –10 10
70 90 110 130
–30 –10 10
50
110 130
TEMPERATURE (°C)
TEMPERATURE (°C)
38033 G08
38033 G07
38033 G09
38033fa
4
LTC3803-3
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
U W
Start-Up ICC Supply Current
vs Temperature
ITH/RUN Shutdown Threshold
vs Temperature
ITH/RUN Start-Up Current Source
vs Temperature
70
60
50
40
30
20
10
0
700
600
500
400
300
200
100
0
450
400
V
= V
– 0.1V
V
V
= V
+ 0.1V
TURNON
CC
TURNON
CC
ITH/RUN
= 0V
350
300
250
200
150
100
–50 –30
130
–50 –30
130
–10 10 30 50 70 90 110
–10 10 30 50 70 90 110
–30 –10 10 30 50
110 130
–50
70 90
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
38033 G10
38033 G12
38033 G11
Peak Current Sense Voltage
vs Temperature
Soft-Start Time vs Temperature
120
115
110
105
100
95
3.5
V
= 8V
CC
3.0
2.5
2.0
1.5
1.0
0.5
90
85
0
80
30 50
TEMPERATURE (°C)
–50
10
90
150
110 130
–50 –30 –10 10
70 90 110 130
50 70
–30 –10
30
TEMPERATURE (°C)
38033 G13
38033 G14
38033fa
5
LTC3803-3
U
U
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PI FU CTIO S
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.
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
LTC3803-3 to shut down. In shutdown mode, the NGATE
pin is held low.
V
CC (Pin5):SupplyPin.MustbecloselydecoupledtoGND
(Pin 2).
GND (Pin 2): Ground Pin.
NGATE (Pin 6): Gate Drive for the External N-Channel
MOSFET. This pin swings from 0V to VCC.
V
FB (Pin 3): Receives the feedback voltage from an exter-
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
300kHz
OSCILLATOR
1.2V
SENSE
4
I /RUN
TH
1
38033 BD
38033fa
6
LTC3803-3
U
OPERATIO
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 LTC3803-3 turns on the external power
MOSFET when the internal free-running 300kHz 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 are controlled by the ITH/RUN voltage.
The LTC3803-3 is a constant frequency current mode
controller for flyback and DC/DC boost converter applica-
tions in a tiny ThinSOT package. The LTC3803-3 is de-
signedsothatnoneofitspinsneedtocomeincontactwith
the input or output voltages of the power supply circuit of
which it is a part, allowing the conversion of voltages well
beyond the LTC3803-3’s absolute maximum ratings.
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.
Since the ITH/RUN voltage is increased by the error ampli-
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
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.
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.
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 cur-
rent comparator threshold and commands considerable
influenceoverthedynamicsofthevoltageregulationloop.
38033fa
7
LTC3803-3
U
OPERATIO
Start-Up/Shutdown
LTC3803-3 operation. The VCC voltage is then allowed to
fall to VTURNOFF (nominally 5.7V) before undervoltage
lockout disables the LTC3803-3. This wide UVLO hyster-
esis range supports the use of a bias winding on the
flyback transformer to power the LTC3803-3—see the
section Powering the LTC3803-3.
TheLTC3803-3hastwoshutdownmechanismstodisable
and enable operation: an undervoltage lockout on the VCC
supply pin voltage, and a forced shutdown whenever
external circuitry drives the ITH/RUN pin low. The
LTC3803-3 transitions into and out of shutdown accord-
ing to the state diagram (Figure 1).
The ITH/RUN pin can be driven below VSHDN (nominally
0.28V) to force the LTC3803-3 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
LTC3803-3
SHUT DOWN
V
SHDN, and VCC exceeds VTURNON, the LTC3803-3 begins
to operate and an internal clamp immediately pulls the
ITH/RUNpinuptoabout0.7V.Inoperation,theITH/RUNpin
voltagewillvaryfromroughly0.7Vto1.9Vtorepresentcur-
rent comparator thresholds from zero to maximum.
V
> V
ITH/RUN
ITHSHDN
TURNON
V
< V
TURNOFF
V
< V
ITHSHDN
CC
(NOMINALLY 5.7V)
ITH/RUN
(NOMINALLY 0.28V)
AND V > V
CC
(NOMINALLY 8.7V)
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC3803-3 comes out of shutdown. Specifically, the
ITH/RUNvoltageisclampedandispreventedfromreach-
ing maximum until roughly 1.4ms has passed. This
allows the input and output currents of LTC3803-3-
based power supplies to rise in a smooth and controlled
manner on start-up.
LTC3803-3
ENABLED
38033 F01
Figure 1. Start-Up/Shutdown State Diagram
The undervoltage lockout (UVLO) mechanism prevents
the LTC3803-3 from trying to drive a MOSFET with insuf-
ficient VGS. The voltage at the VCC pin must exceed
VTURNON (nominally 8.7V) at least momentarily to enable
38033fa
8
LTC3803-3
U
OPERATIO
Powering the LTC3803-3
The VCC pin must be bypassed to ground immediately
adjacent to the IC pins with a minimum of a 1μF 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 LTC3803-3 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
currentasneededthroughthisresistortoregulatetheVCC
voltage to around 9.5V as long as the VCC pin is not forced
to sink more than 25mA. This shunt regulator is always
active, even when the LTC3803-3 is in shutdown, since it
serves the vital function of protecting the VCC pin from
seeing too much voltage.
Adjustable Slope Compensation
TheLTC3803-3injectsa5μApeakcurrentrampoutthrough
its SENSE pin which can be used for slope compensation
indesignsthatrequireit.Thiscurrentrampisapproximately
linear and begins at zero current at 8% duty cycle, reach-
ing peak current at 80% duty cycle. Additional details are
provided in the Applications Information section.
For higher efficiency or for wide VIN range applications,
flyback controllers are typically powered through a sepa-
rate bias winding on the flyback transformer. The
LTC3803-3 has a wide UVLO hysteresis (1V min) and
small VCC supply current draw (<90μA when VCC
<
VTURNON) that is needed to support such bootstrapped
hysteretic start-up schemes.
38033fa
9
LTC3803-3
W U U
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APPLICATIO S I FOR ATIO
TRANSFORMER DESIGN CONSIDERATIONS
Many LTC3803-3 application circuits can be derived from
the topology shown in Figure 2.
Transformer specification and design is perhaps the most
critical part of applying the LTC3803-3 successfully. In
addition to the usual list of caveats dealing with high fre-
quency power transformer design, the following should
prove useful.
The LTC3803-3 itself imposes no limits on allowed power
output, input voltage VIN or desired regulated output volt-
age VOUT; these are all determined by the ratings on the
external power components. The key factors are: Q1’s
maximum drain-source voltage (BVDSS), on-resistance
(RDS(ON))andmaximumdraincurrent,T1’ssaturationflux
level and winding insulation breakdown voltages, CIN and
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. Simple ratios 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.
C
OUT’s maximum working voltage, ESR, and maximum
ripple current ratings, and D1 and RSENSE’s power ratings.
T1
L
BIAS
•
•
D2
V
IN
D1
V
OUT
C
IN
R3
R
I
C
OUT
START
L
L
SEC
PRI
•
5
C
VCC
V
CC
1
2
6
/RUN NGATE
LTC3803-3
Q1
TH
C
C
R
SL
4
GND
R1
SENSE
Turns ratio can be chosen on the basis of desired duty
cycle. However, remember that the input supply voltage
plus the secondary-to-primary referred version of the
flyback pulse (including leakage spike) must not exceed
the allowed external MOSFET breakdown rating.
V
FB
R
SENSE
3
R2
38033 F02
Figure 2. Typical LTC3803-3 Application Circuit
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
Leakage Inductance
The regulated output voltage is determined by the resistor
divideracrossVOUT (R1andR2inFigure2). TheratioofR2
to R1 needed to produce a desired VOUT can be calculated:
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.
VOUT – 0.8V
R2 =
•R1
0.8V
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.
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.
38033fa
10
LTC3803-3
W U U
APPLICATIO S I FOR ATIO
U
CURRENT SENSE RESISTOR CONSIDERATIONS
in the current comparator threshold (ΔVSENSE) can be
calculated using the following equation:
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.
Duty Cycle – 8%
ΔVSENSE
=
• 5μA •RSL
80%
Note: LTC3803-3 enforces 8% < Duty Cycle < 80%.
A good starting value for RSL is 5.9k, which gives a 30mV
drop in current comparator threshold at 80% duty cycle.
DesignsnotneedingslopecompensationmayreplaceRSL
with a short circuit.
For example, a peak switch current of 5A requires a sense
resistor of 0.020Ω. Note that the instantaneous peak
power in the sense resistor is 0.5W and it must be rated
accordingly. The LTC3803-3 has only a single sense line
to this resistor. Therefore, any parasitic resistance in the
ground side connection of the sense resistor will increase
its apparent value. In the case of a 0.020Ω sense resistor,
one milliohm of parasitic resistance will cause a 5%
reduction in peak switch current. So the resistance of
printed circuit copper traces and vias cannot necessarily
be ignored.
INTERNAL WIDE HYSTERESIS UNDERVOLTAGE
LOCKOUT
The LTC3803-3 is designed to implement DC/DC convert-
ersoperatingfrominputvoltagesoftypically48Vormore.
The standard operating topology employs a third trans-
former winding (LBIAS in Figure 2) on the primary side that
provides power for the LTC3803-3 via its VCC pin. How-
ever, this arrangement is not inherently self-starting.
Start-up is affected by the use of an external “trickle-
charge” resistor (RSTART in Figure 2) and the presence of
an internal wide hysteresis undervoltage lockout circuit
that monitors VCC pin voltage. Operation is as follows:
PROGRAMMABLE SLOPE COMPENSATION
The LTC3803-3 injects a ramping current through its
SENSE pin into an external slope compensation resistor
(RSL in Figure 2). This current ramp starts at zero right
after the NGATE pin has been high for the LTC3803-3’s
minimum duty cycle of 8%. The current rises linearly
towards a peak of 5μA at the maximum duty cycle of 80%,
shuttingoffoncetheNGATEpingoeslow.Aseriesresistor
(RSL) connecting the SENSE pin to the current sense
resistor (RSENSE) thus develops a ramping voltage drop.
From the perspective of the SENSE pin, this ramping
voltage adds to the voltage across the sense resistor,
effectively reducing the current comparator threshold in
proportion to duty cycle. This stabilizes the control loop
against subharmonic oscillation. The amount of reduction
“Trickle charge” resistor RSTART is connected to VIN and
supplies a small current, typically on the order of 100μA,
to charge CVCC. After some time, the voltage on CVCC
reaches the VCC turn-on threshold. The LTC3803-3 then
turnsonabruptlyanddrawsitsnormalsupplycurrent.The
NGATE pin begins switching and the external MOSFET
(Q1) begins to deliver power. The voltage on CVCC begins
to decline as the LTC3803-3 draws its normal supply
current, which exceeds that delivered by RSTART. After
some time, typically tens of milliseconds, the output
voltage approaches its desired value. By this time, the
third transformer winding is providing virtually all the
supply current required by the LTC3803-3.
38033fa
11
LTC3803-3
W U U
U
APPLICATIO S I FOR ATIO
One potential design pitfall is undersizing the value of
capacitor CVCC. In this case, the normal supply current
drawn by the LTC3803-3 will discharge CVCC too rapidly;
before the third winding drive becomes effective, the VCC
turn-off threshold will be reached. The LTC3803-3 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.
In applications where a third transformer winding is unde-
sirable or unavailable, the shunt regulator allows the
LTC3803-3 to be powered through a single dropping
resistor 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.
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 LTC3803-3.
Component selection is as follows:
Resistor RSTART should be made small enough to yield a
worst-case minimum charging current greater than the
maximum rated LTC3803-3 start-up current, to ensure
there is enough current to charge CVCC to the VCC turn-on
threshold.Itshouldbemadelargeenoughtoyieldaworst-
case maximum charging current less than the minimum
ratedLTC3803-3supplycurrent,sothatinoperation,most
oftheLTC3803-3’ssupplycurrentisdeliveredthroughthe
thirdwinding.Thisresultsinthehighestpossibleefficiency.
V
IN
R
VCC
LTC3803-3
V
CC
GND
C
VCC
38033 F03
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.
Figure 3. Powering the LTC3803-3 Via the
Internal Shunt Regulator
EXTERNAL PREREGULATOR
The circuit in Figure 4 shows a third way to power the
LTC3803-3. 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
START momentarily charges the VCC node up to the VCC
turn-on threshold, enabling the LTC3803-3.
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
clampvoltagetoavoidrunningupagainsttheLTC3803-3’s
VCC shunt regulator, needlessly wasting power.
R
V
IN
VCC SHUNT REGULATOR
R
Q1
R
START
LTC3803-3
B
Inapplicationsincludingathirdtransformerwinding, the
internal VCC shunt regulator serves to protect the
LTC3803-3 from overvoltage transients as the third
winding is powering up.
V
CC
D1
8.2V
GND
C
VCC
38033 F04
Figure 4. Powering the LTC3803-3 with an External Preregulator
38033fa
12
LTC3803-3
U
TYPICAL APPLICATIO S
2W Isolated 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
1
1k 220k
•
SECONDARY
SIDE GROUND
1nF
LTC3803-3
/RUN NGATE
22k
6
I
FDC2512
TH
2
3
5
4
V
GND
CC
T1: PULSE ENGINEERING PA0648
OR TYCO TTI8698
806Ω
5.6k
1μF
V
SENSE
FB
0.1Ω
38033 TA03
PRIMARY GROUND
38033fa
13
LTC3803-3
TYPICAL APPLICATIO S
U
4:1 Input Range 3.3V Output Isolated Flyback DC/DC Converter
T1
+
V
3.3V
3A
+
PA1277NL
OUT
V
IN
18 V TO 72V
100μF
6.3V
× 3
•
2.2μF
220k
–
MMBTA42
V
PDS1040
IN
100k
•
GND
BAS516
68Ω
150pF
PDZ6.8B
V
CC
10Ω
BAS516
22Ω
680Ω
•
0.1μF
1
2
3
6
5
4
FDC2512
I
/RUN
GATE
LTC3803-3
TH
V
IN
GND
+
V
OUT
4.7k
SENSE
V
FB
BAT760
0.1μF
0.040Ω
270Ω
V
CC
6.8k
+
V
1
2
3
6
5
4
OUT
V
OPTO
COMP
FB
BAS516
IN
LT4430
PS2801-1
47pF
100k
2.2nF
22.1k
0.1μF
1
2
56k
GND
0.33μF
OC
BAS516
38033 TA05
Efficiency vs Load Current
84
82
80
78
76
74
72
V
V
= 48V
= 24V
IN
IN
70
0
1
2
3
4
38033 TA05a
I
(A)
OUT
38033fa
14
LTC3803-3
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
1.50 – 1.75
(NOTE 4)
2.80 BSC
1.4 MIN
3.85 MAX 2.62 REF
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
38033fa
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.
15
LTC3803-3
TYPICAL APPLICATIO S
U
Efficiency vs Load
100
95
90
85
80
75
70
V
= 3.3V
OUT
90% Efficient Synchronous Flyback Converter
V
3.3V
1.5A
*
OUT
V
V
V
V
V
= 36V
= 48V
= 60V
= 72V
IN
36V TO 72V
IN
IN
IN
IN
T1
Q2
D1
•
•
C
C
O
IN
220k
•
500
1000 1250 1500 1750 2000
750
LOAD CURRENT (mA)
38083 TA04b
1n
33k
1
2
3
6
5
4
I
/RUN
Q1
GATE
TH
LTC3803-3
Efficiency vs Load
0.1μF
560Ω
V
GND
= 0.8V
CC
100
95
90
85
80
75
70
V
= 5V*
OUT
8.06k
4.7k
SENSE
V
FB
38033 TA04a
25.5k*
10μF
10V
R
CS
R
FB
V
OUT
T1: PULSE ENGINEERING PA1006
Q1: FAIRCHILD FDC2512
Q2: VISHAY Si9803
C : TDK 1μF, 100V, X5R
IN
C : TDK 100μF, 6.3V, X5R
O
R
CS
: VISHAY OR IRC, 80mΩ
D1: PHILIPS BAS516
*FOR 5V OUTPUT CHANGE R TO 42.2k
FB
V
IN
V
IN
V
IN
V
IN
= 36V
= 48V
= 60V
= 72V
500
1000 1250 1500 1750 2000
750
LOAD CURRENT (mA)
38083 TA04c
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
No Optoisolator or “Third Winding” Required, Up to 6W Output
LT®1425
Isolated Flyback Switching Regulator
with No External Power Devices
LT1725
General Purpose Isolated Flyback Controller
No Optoisolator Required, V and V
Limited Only by External
IN
OUT
Power Components
LTC1871
Wide Input Range, No R
Flyback, Boost and SEPIC Controller
TM Current Mode
Adjustable Switching Frequency, Programmable Undervoltage
Lockout, Optional Burst Mode® Operation at Light Load
SENSE
LT1950
LT3420
Current Mode PWM Controller
Controller for Forward Converters from 30W to 300W
Photoflash Capacitor Charger with Automatic Refresh
Photoflash Capacitor Charger in 5-Pin SOT-23
Specialized Flyback Charges High Voltage Photoflash Capacitors
Quickly and Efficiently
LT3468/LT3468-1
Minimal Component Count, Uses Small Transformers;
V
from 2.5V to 16V
IN
LTC3803
LTC3803-5
LTC3806
Constant Frequency Flyback Controller
Constant Frequency Flyback Controller
Synchronous Flyback Controller
200kHz Switching Frequency, Low Profile (1mm) ThinSOT Package
200kHz Switching Frequency, 4.8V Turn-On Voltage
High Efficiency (89%); 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
38033fa
LT 0407 • PRINTED IN THE USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
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