LTC3803ES6-5#TRM [Linear]
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#TRM |
| 厂家： | 
Linear |
| 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 控制器 |
| 文件： | 总12页 (文件大小：178K) |
| 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
U
FEATURES
DESCRIPTIO
The LTC®3803-5 is a constant frequency current mode
flyback controller optimized for driving 4.5V and 6V-rated
N-channel MOSFETs in high input voltage applications.
The LTC3803-5 operates from inputs as low as 5V. Con-
stant 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
■
4.8V Undervoltage Lockout Threshold
■
Operating Junction Temperature from –40°C to
150°C
■
Adjustable Slope Compensation
■
Internal Soft-Start
■
Constant Frequency 200kHz Operation
■
±1.5% Reference Accuracy
■
Current Mode Operation for Excellent Line and Load
The LTC3803-5 provides ±1.5% output voltage accuracy
and consumes only 240µA of quiescent current. Ground-
referencedcurrentsensingallowsLTC3803-5-basedcon-
verters to accept input supplies beyond the LTC3803-5’s
absolute maximum VCC. For simplicity, the LTC3803-5
can be powered from a high VIN through a resistor, due to
its internal 8V shunt regulator. An internal undervoltage
lockout shuts down the IC when the input voltage falls
below 3.2V, guaranteeing at least 3.2V of gate drive to the
external MOSFET.
Transient Response
■
No Minimum Load Requirement
■
Low Quiescent Current: 240µA
■
Low Profile (1mm) SOT-23 Package
U
APPLICATIO S
■
42V and 12V Automotive Power Supplies
■
Telecom Power Supplies
■
Auxiliary/Housekeeping Power Supplies
■
Power Over Ethernet
The LTC3803-5 is available in a low profile (1mm) 6-lead
SOT-23 (ThinSOTTM) package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
ThinSOT is a trademark of Linear Technology Corporation.
U
TYPICAL APPLICATIO
Efficiency and Power Loss
vs Output Power
Dual Output Wide Input Range Converter
10MQ100N
90
85
80
75
70
65
60
3.0
2.5
2.0
1.5
1.0
0.5
0
VPH5-0155
V
= 8V
V
13V/0.3A
20mA MIN
LOAD
IN
IN
6V TO 50V
1µF
100V
3x
V
V
= 12V
= 24V
IN
22k
PDZ9.1B
7.5k
22µF
10V
MMBTA42
IN
1µF
100V
PHM25NQ10T
10nF
LTC3803-5
1µF
100V
I
/RUN NGATE
TH
GND
V
6.5V/1.2A
CC
4.7k
V
V
= 48V
IN
V
FB
SENSE
B3100
47µF
10V
8.06k
0.012Ω
0.1µF
= 12V
2
IN
57.6k
0
6
8
10
12
4
OUTPUT POWER (W)
ALL CAPACITORS ARE X7R, TDK
38035 TA01
38035 TA01b
38035f
1
LTC3803-5
W W
U W
U
W U
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
VCC to GND (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 Junction Temperature Range (Note 2)
LTC3803E-5 ....................................... – 40°C to 85°C
LTC3803H-5 (Note 3) ....................... – 40°C to 150°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
TOP VIEW
LTC3803HS6-5
LTC3803ES6-5
I
TH
/RUN 1
GND 2
6 NGATE
5 V
CC
V
3
4 SENSE
FB
S6 PART
MARKING
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
TJMAX = 150°C, θJA = 165°C/W
LTBMH
LTBPF
*LTC3803-5 internal clamp circuit self regulates V voltage to 8V.
Consult LTC Marketing for parts specified with wider operating temperature ranges.
CC
ELECTRICAL CHARACTERISTICS
LTC3803E-5: The ● indicates specifications which apply over the full –40°C
to 85°C operating junction temperature range, otherwise specifications are at TJ = 25°C. VCC = 5V, unless otherwise noted. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
MIN
4
TYP
4.8
4
MAX
5.7
UNITS
V
V
V
V
V
V
V
V
V
V
Turn On Voltage
●
●
●
●
●
V
V
V
V
V
TURNON
CC
CC
CC
CC
CC
Turn Off Voltage
3.3
0.05
6.2
6.3
4.9
TURNOFF
HYST
Hysteresis
V
– V
0.8
8
TURNON
TURNOFF
ITH/RUN
Shunt Regulator Voltage
Shunt Regulator Voltage
I
I
= 1mA, V
= 0V
= 0V
9.9
CLAMP1mA
CLAMP25mA
CC
CC
= 25mA, V
8.1
10.3
ITH/RUN
I
Input DC Supply Current
Normal Operation
Undervoltage
(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
0.12
0.07
0.28
0.34
0.45
0.8
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
0.816
V
V
FB
J
–40°C ≤ T ≤ 85°C (Note 5)
●
J
g
Error Amplifier Transconductance
Output Voltage Line Regulation
Output Voltage Load Regulation
I
Pin Load = ±5µA (Note 5)
200
333
0.1
500
µA/V
m
TH/RUN
∆V
∆V
V
< V < V (Note 5)
CLAMP
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
6.5
80
50
230
8.5
90
nA
kHz
%
FB
OSC
Oscillator Frequency
V
V
V
C
C
= 1.3V
170
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
%
ON(MAX)
FB
t
t
= 3000pF
40
ns
RISE
FALL
LOAD
LOAD
Gate Drive Fall Time
= 3000pF
40
ns
V
Peak Current Sense Voltage
Peak Slope Compensation Output Current
Soft-Start Time
R
= 0 (Note 6)
●
90
100
5
115
mV
µA
ms
IMAX
SLMAX
SFST
SL
I
t
(Note 7)
0.7
38035f
2
LTC3803-5
LTC3803H-5: The ● indicates specifications which apply over the full –40°C
ELECTRICAL CHARACTERISTICS
(Notes 2, 3)
to 150°C operating junction temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
3.9
TYP
4.8
4
MAX
5.7
UNITS
V
V
V
V
V
V
V
V
V
V
Turn On Voltage
●
●
●
●
●
V
V
V
V
V
TURNON
CC
CC
CC
CC
CC
Turn Off Voltage
3.2
4.9
TURNOFF
HYST
Hysteresis
V
– V
0.05
6.2
0.8
8
TURNON
TURNOFF
ITH/RUN
Shunt Regulator Voltage
Shunt Regulator Voltage
I
I
= 1mA, V
= 0V
= 0V
10.4
10.7
CLAMP1mA
CLAMP25mA
CC
CC
= 25mA, V
6.3
8.1
ITH/RUN
I
Input DC Supply Current
Normal Operation
Undervoltage
(Note 4)
ITH/RUN
CC
V
V
= 1.3V
240
40
350
100
µA
µA
= V
– 100mV
+ 100mV
●
●
●
CC
TURNON
V
Shutdown Threshold (at I /RUN)
V
V
= V
0.08
0.07
0.28
0.34
0.45
1
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
0.820
V
V
FB
J
–40°C ≤ T ≤ 150°C (Note 5)
●
J
g
Error Amplifier Transconductance
Output Voltage Line Regulation
Output Voltage Load Regulation
I
Pin Load = ±5µA (Note 5)
200
333
0.1
500
µA/V
m
TH/RUN
∆V
∆V
V
< V < V (Note 5)
CLAMP
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
6.5
80
50
230
8.5
90
nA
kHz
%
FB
OSC
Oscillator Frequency
V
V
V
C
C
= 1.3V
170
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)
ON(MAX)
= 1.3V, V = 0.8V
%
FB
t
t
= 3000pF
40
ns
RISE
FALL
LOAD
LOAD
Gate Drive Fall Time
= 3000pF
40
ns
V
Peak Current Sense Voltage
Peak Slope Compensation Output Current
Soft-Start Time
R
= 0 (Note 6)
●
85
100
5
115
mV
µA
ms
IMAX
SLMAX
SFST
SL
I
t
(Note 7)
0.7
lifetime at junction temperatures greater than 125°C is derated to 1000
hours.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 2: The LTC3803H-5 is guaranteed to meet specifications from –40°C
to 150°C. The LTC3803E-5 is guaranteed to meet specifications from 0°C
to 85°C with specifications over the –40°C to 85°C temperature range
assured by design, characterization and correlation with statistical process
controls.
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.
Junction temperature (T is calculated from the ambient temperature T
Note 6: Peak current sense voltage is reduced dependent on duty cycle
J)
A
and the power dissipation P in the LTC3803-5 using the formula:
and an optional external resistor in series with the SENSE pin (R ). For
D
SL
details, refer to the programmable slope compensation feature in the
Applications Information section.
Note 7: Guaranteed by design.
T = T + (P • 230°C/W)
Note 3: High junction temperatures degrade operating lifetimes. Operating
J
A
D
38035f
3
LTC3803-5
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage
vs Supply Voltage
Reference Voltage
vs VCC Shunt Regulator Current
Reference Voltage vs Temperature
812
808
804
800
796
792
812
808
804
800
796
792
788
812
808
804
800
796
792
788
T
= 25°C
V
= 5V
T
= 25°C
A
CC
A
V
≤ V
CC
CLAMP1mA
788
4.0
5.0 5.5 6.0
6.5 7.0 7.5
4.5
5
10
20
–50 –30 –10 10 30 50 70 90 110 130 150
0
25
15
(mA)
V
CC
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
I
CC
38035 F02
38035 G01
38035 G03
Oscillator Frequency
vs Temperature
Oscillator Frequency
vs Supply Voltage
Oscillator Frequency
vs VCC Shunt Regulator Current
220
215
210
205
200
195
190
220
215
210
205
200
195
190
185
180
220
215
210
205
200
195
190
185
180
T
A
= 25°C
V
CC
= 5V
T
= 25°C
A
185
180
50 70
–50 –30 –10 10 30
90 110 130 150
4.0 4.5
5.5 6.0
6.5 7.0
7.5
5.0
0
5
15
(mA)
20
25
10
I
TEMPERATURE (°C)
V
CC
SUPPLY VOLTAGE (V)
CC
38035 G04
38035 G05
38035 G06
VCC Undervoltage Lockout
Thresholds vs Temperature
VCC Shunt Regulator Voltage
vs Temperature
ICC Supply Current
vs Temperature
6.0
5.5
5.0
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
300
280
260
240
220
200
V
V
= 5V
CC
ITH/RUN
= 1.3V
V
TURNON
I
= 25mA
CC
4.5
4.0
I
= 1mA
CC
V
TURNOFF
3.5
3.0
–50
30
70 90
–50
30
70 90
–50
30
70 90
–30 –10 10
50
110 130 150
–30 –10 10
50
110 130 150
–30 –10 10
50
110 130 150
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3803 G07
38035 G08
38035 G08
38035f
4
LTC3803-5
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Start-Up ICC Supply Current
vs Temperature
ITH/RUN Shutdown Threshold
vs Temperature
500
450
400
350
ITH/RUN Start-Up Current Source
vs Temperature
70
1000
900
800
700
600
500
V
CC
= V
TURNON
– 0.1V
V
V
= V
+ 0.1V
TURNON
CC
ITH/RUN
= 0V
60
50
40
30
20
10
0
300
250
200
150
100
50
400
300
200
100
0
0
–30 –10 10 30 50
110 130 150
50 70
–50
70 90
50 70
90 110 130 150
–50 –30 –10 10 30
90 110 130 150
–50 –30 –10 10 30
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3803 G11
38035 G10
38035 G12
Peak Current Sense Voltage
vs Temperature
Soft-Start Time vs Temperature
120
115
110
105
100
95
1.4
1.2
V
= 5V
V
= 5V
CC
CC
1.0
0.8
0.6
0.4
0.2
0
90
85
80
30 50
30 50
70 90 110 130 150
–50 –30 –10 10
70 90 110 130 150
–50 –30 –10 10
TEMPERATURE (°C)
TEMPERATURE (°C)
38035 G13
38035 G14
38035f
5
LTC3803-5
U
U
U
PI FU CTIO S
ITH/RUN (Pin 1): This pin performs two functions. It SENSE (Pin 4): This pin performs two functions. It moni-
servesastheerroramplifiercompensationpointaswellas tors switch current by reading the voltage across an
the run/shutdown control input. Nominal voltage range is external current sense resistor to ground. It also injects a
0.7V to 1.9V. Forcing this pin below 0.28V causes the current ramp that develops slope compensation voltage
LTC3803-5 to shut down. In shutdown mode, the NGATE across an optional external programming resistor.
pin is held low.
VCC (Pin5):SupplyPin.MustbecloselydecoupledtoGND
(Pin 2).
GND (Pin 2): Ground Pin.
V
FB (Pin 3): Receives the feedback voltage from an exter- NGATE (Pin 6): Gate Drive for the External N-Channel
nal resistive divider across the output.
MOSFET. This pin swings from 0V to VCC.
W
BLOCK DIAGRA
5
V
CC
SHUTDOWN
COMPARATOR
0.3µA 0.28V
+
–
V
CC
< V
TURNON
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
/RUN
TH
1
38035 BD
38035f
6
LTC3803-5
U
OPERATIO
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
with the input or output voltages of the power supply
circuit of which it is a part, allowing the conversion of
voltageswellbeyondtheLTC3803-5’sabsolutemaximum
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
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.
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.
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
TheLTC3803-5hastwoshutdownmechanismstodisable
and enable operation: an undervoltage lockout on the VCC
supply pin voltage, and a forced shutdown whenever
externalcircuitrydrivestheITH/RUNpinlow.TheLTC3803-
5 transitions into and out of shutdown according to the
state 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 LTC3803-5 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 are controlled by the ITH/RUN voltage.
LTC3803-5
SHUT DOWN
V
> V
ITH/RUN
ITHSHDN
TURNON
V
< V
TURNOFF
V
< V
ITHSHDN
CC
(NOMINALLY 4V)
ITH/RUN
(NOMINALLY 0.28V)
AND V > V
CC
(NOMINALLY 4.8V)
LTC3803-5
ENABLED
38035 F01
Figure 1. Start-Up/Shutdown State Diagram
Since the ITH/RUN voltage is increased by the error ampli-
fier whenever the output voltage is below nominal, and
38035f
7
LTC3803-5
U
OPERATIO
The undervoltage lockout (UVLO) mechanism prevents
the LTC3803-5 from trying to drive a MOSFET with insuf-
ficient VGS. The voltage at the VCC pin must exceed
Powering the LTC3803-5
In the simplest case, the LTC3803-5 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 8V as long as the VCC pin is not forced
to sink more than 25mA. This shunt regulator is always
active, even when the LTC3803-5 is in shutdown, since it
serves the vital function of protecting the VCC pin from
seeing too much voltage.
V
TURNON (nominally 4.8V) at least momentarily to enable
LTC3803-5 operation. The VCC voltage is then allowed to
fall to VTURNOFF (nominally 4V) before undervoltage lock-
out disables the LTC3803-5.
The ITH/RUN pin can be driven below VSHDN (nominally
0.28V)toforcetheLTC3803-5intoshutdown. Aninternal
0.3µA current source always tries to pull this pin towards
VCC. When the ITH/RUN pin voltage is allowed to exceed
VSHDN, and VCC exceeds VTURNON, the LTC3803-5 begins
to 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.
The VCC pin must be bypassed to ground immediately
adjacent to the IC pins with a ceramic or tantalum capaci-
tor. Proper supply bypassing is necessary to supply the
high transient currents required by the MOSFET gate
driver. 10µF is a good starting point.
Adjustable Slope Compensation
The LTC3803-5 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 approximately linear and begins at zero current at 6.5%
duty cycle, reaching peak current at 80% duty cycle.
Additional details are provided in the Applications Infor-
mation section.
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC3803-5 comes out of shutdown. Specifically, the
ITH/RUNvoltageisclampedandispreventedfromreach-
ing maximum until roughly 0.7ms has passed. This
allows the input and output currents of LTC3803-5-
based power supplies to rise in a smooth and controlled
manner on start-up.
38035f
8
LTC3803-5
W U U
APPLICATIO S I FOR ATIO
U
Many LTC3803-5 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 LTC3803-5 successfully. In
addition to the usual list of caveats dealing with high
frequencypowertransformerdesign,thefollowingshould
prove useful.
The LTC3803-5 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. 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.
V
IN
D1
T1
V
OUT
•
R
C
VCC
C
L
L
SEC
OUT
IN PRI
•
5
C
VCC
V
CC
1
2
6
I
/RUN NGATE
LTC3803-5
Q1
TH
C
C
R
SL
4
GND
R1
SENSE
V
FB
R
SENSE
Turns ratio can be chosen on the basis of desired duty
cycle. However, remember that the input supply voltage
plus the secondary-to-primary referred version of the
flyback pulse (including leakage spike) must not exceed
the allowed external MOSFET breakdown rating.
3
R2
38035 F02
Figure 2. Typical LTC3803-5 Application Circuit
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
Leakage Inductance
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:
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.
VERSA-PAC is a trademark of Coiltronics, Inc.
38035f
9
LTC3803-5
W U U
U
APPLICATIO S I FOR ATIO
CURRENT SENSE RESISTOR CONSIDERATIONS
against subharmonic oscillation. The amount of reduction
in the current comparator threshold (∆VSENSE) 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 – 6.5%
∆VSENSE
=
•5µA •RSL
73.5%
Note: LTC3803-5 enforces 6.5% < 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 LTC3803-5 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 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.
VCC SHUNT REGULATOR
An internal shunt regulator allows the LTC3803-5 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). The shunt
regulator can draw up to 25mA through the VCC pin to
GND to drop enough voltage across RVCC to regulate VCC
to around 8V. For applications where VIN is low enough
such that the static power dissipation in RVCC is accept-
able, using the VCC shunt regulator is the simplest way to
power the LTC3803-5.
PROGRAMMABLE SLOPE COMPENSATION
The LTC3803-5 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 LTC3803-5’s
minimum duty cycle of 6.5%. 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
EXTERNAL PREREGULATOR
The circuit in Figure 4 shows another way to power the
LTC3803-5. An external series preregulator consisting of
series pass transistor Q1, Zener diode D1, and bias resis-
tor RB brings VCC above the VCC turn-on threshold, en-
abling the LTC3803-5.
8V TO
75 V
IN
V
IN
Q1
MMBTA42
R
B
LTC3803-5
100k
R
LTC3803-5
VCC
V
CC
V
CC
C
VCC
0.1µF
D1
6.8V
GND
GND
C
VCC
38035 F04
38035 F03
Figure 4. Powering the LTC3803-5
with an External Preregulator
Figure 3. Powering the LTC3803-5
Via the Internal Shunt Regulator
38035f
10
LTC3803-5
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
LTC3803-5
/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
38035 TA03
PRIMARY GROUND
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
(NOTE 4)
2.80 BSC
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
38035f
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.
11
LTC3803-5
U
TYPICAL APPLICATIO S
90% Efficient Synchronous Flyback Converter
Synchronous Flyback 5VOUT
Synchronous Flyback 3.3VOUT
V
3.3V
1.5A
*
OUT
V
IN
36V TO 72V
T1
Q2
D1
•
•
92
91
90
89
88
87
86
85
91
C
C
IN
270k
O
•
1n
33k
1
2
3
6
5
4
90
89
88
I
/RUN
Q1
GATE
LTC3803-5
TH
0.1µF
560
5k
V
GND
CC
8.06k
SENSE
V
FB
25.5k*
38035 TA04a
1µF
10V
R
R
CS
FB
V
OUT
0.5
1.0
1.5
2.0
2.5
0.5
1.5
2.0
1.0
T1: PULSE ENGINEERING PA1006
Q1: FAIRCHILD FDC2512
Q2: VISHAY Si9803
D1: PHILIPS BAS516
R : VISHAY OR IRC, 80mΩ
CS
: TDK 1µF, 100V, X5R *FOR 5V OUTPUT CHANGE
IN
OUTPUT CURRENT (A)
38035 TA04c
OUTPUT CURRENT (A)
38035 TA04b
C
C
: TDK 100µF, 6.3V, X5R
R
TO 42.2k
O
FB
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
LT1952
Current Mode PWM Controller
Current Mode PWM Controller
Controller for Forward Converters from 30W to 300W
Synchronous Controller for Forward Converters from
30W to 500W
LT3420
Photoflash Capacitor Charger with Automatic Refresh
Photoflash Capacitor Charger in 5-Pin SOT-23
Synchronous Flyback Controller
Specialized Flyback Charges High Voltage Photoflash Capacitors
Quickly and Efficiently
LT3468/LT3468-1
LTC3806
Minimal Component Count, Uses Small Transformers;
V
from 2.5V to 16V
IN
High Efficiency (89%); Multiple Output with
Excellent Cross Regulation
LTC4441
6A N-Channel MOSFET Driver
Gate Drive Adjustable from 5V to 8V, Adjustable Blanking
Prevents Ringing, 10-Lead MSSOP Package
Burst Mode is a registered trademark of Linear Technology Corporation. No R
is a trademark of Linear Technology Corporation.
SENSE
38035f
LT/TP 1104 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 2004
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