LINER LTC3803ES6-3 Constant frequency current mode flyback dc/dc controller in thinsot Datasheet

LTC3803-3
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
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FEATURES
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DESCRIPTIO
The LTC®3803-3 is a constant frequency current mode
flyback controller optimized for driving 6V-rated N-channel
MOSFETs in high input voltage applications. Constant
frequency operation is maintained down to very light
loads, resulting in less low frequency noise generation
over a wide range of load currents. Slope compensation
can be programmed with an external resistor.
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
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. Groundreferenced current sensing allows LTC3803-3-based converters 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.
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APPLICATIO S
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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.
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TYPICAL APPLICATIO
5V Output Nonisolated Telecom Housekeeping Power Supply
100
UPS840
T1
10k
1μF
10V
X5R
1μF
100V
X5R
470pF
300μF*
6.3V
X5R
•
VCC
ITH/RUN NGATE
82k
•
VOUT
5V
2A MAX
LTC3803-3
GND
68mΩ
90
85
80
75
70
VIN = 36V
VIN = 48V
VIN = 60V
VIN = 72V
60
220Ω
55
105k
38033 TA01
T1: COOPER CTX02-15242
*THREE 100μF UNITS IN PARALLEL
VOUT = 5V
65
150pF
200V
SENSE
VFB
20k
FDC2512
4.7k
95
EFFICIENCY (%)
VIN
36V TO 72V
Efficiency vs Load Current
50
250
500
750 1000 1250 1500 1750 2000
LOAD CURRENT (mA)
38033 TA02
38033fa
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LTC3803-3
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
VCC to GND
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)
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
*LTC3803-3 internal clamp circuit self regulates VCC voltage to 9.5V.
TOP VIEW
ITH/RUN 1
6 NGATE
GND 2
5 VCC
VFB 3
4 SENSE
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
TJMAX = 125°C, θJA = 230°C/W
ORDER PART NUMBER
LTC3803ES6-3
LTC3803IS6-3
S6 PART MARKING
LTCJS
LTCJT
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
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
VTURNON
VCC Turn On Voltage
VTURNOFF
VCC Turn Off Voltage
VHYST
VCC Hysteresis (VTURNON – VTURN0FF)
●
1
3
VCLAMP1mA
VCC Shunt Regulator Voltage
ICC = 1mA, VITH/RUN = 0V
●
8.3
9.4
10.3
V
VCLAMP25mA
VCC Shunt Regulator Voltage
ICC = 25mA, VITH/RUN = 0V
●
8.4
9.5
10.5
V
VMARGIN
VCLAMP1mA – VTURNON Margin
●
0.05
0.6
ICC
Input DC Supply Current
Normal Operation
Start-Up
(Note 4)
VITH/RUN = 1.3V
VCC = VTURNON – 100mV
Shutdown Threshold (at ITH/RUN)
VCC = VTURNON – 100mV
LTC3803E-3
LTC3803I-3
VITHSHDN
CONDITIONS
LTC3803E-3
LTC3803I-3
MIN
TYP
MAX
UNITS
●
7.6
8.7
9.2
V
●
●
4.6
4.4
5.7
5.7
7
7
V
V
μA
V
V
V
100
100
115
115
V
V
333
500
RSL = 0 (Note 6)
LTC3803E-3
LTC3803I-3
●
●
90
85
ITH/RUN Pin Load = ±5μA (Note 5)
IFB
VFB Input Current
(Note 5)
fOSC
Oscillator Frequency
VITH/RUN = 1.3V
V
V
0.4
●
●
VTURNOFF < VCC < VCLAMP (Note 5)
0.45
0.45
0.812
0.812
0.820
0.788
0.780
0.780
Output Voltage Line Regulation
0.28
0.28
0.3
0°C ≤ TA ≤ 85°C
LTC3803E-3: –40°C ≤ TA ≤ 85°C
LTC3803I-3: –40°C ≤ TA ≤ 125°C
Error Amplifier Transconductance
μA
μA
0.800
0.800
0.800
VITH/RUN = 0V
Regulated Feedback Voltage (Note 5)
ΔVO(LINE)
350
90
0.2
Start-Up Current Source
gm
240
40
0.15
0.10
VFB
Peak Current Sense Voltage
V
●
●
IITHSTART
VIMAX
V
200
0.05
270
μA/V
mV/V
10
50
nA
300
330
kHz
38033fa
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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
DCON(MIN)
Minimum Switch On Duty Cycle
VITH/RUN = 1.3V, VFB = 0.8V
DCON(MAX)
Maximum Switch On Duty Cycle
VITH/RUN = 1.3V, VFB = 0.8V
tRISE
Gate Drive Rise Time
CLOAD = 3000pF
40
ns
tFALL
Gate Drive Fall Time
CLOAD = 3000pF (Note 7)
40
ns
ISLMAX
Peak Slope Compensation Output Current
(Note 7)
tSFST
Soft-Start Time
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 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.
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
TJ = TA + (PD • 230°C/W).
MIN
70
TYP
MAX
UNITS
8
9.6
%
80
90
%
5
μA
1.4
ms
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5: The LTC3803-3 is tested in a feedback loop that servos VFB to the
output of the error amplifier while maintaing ITH/RUN at the midpoint of
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 (RSL). For
details, refer to the programmable slope compensation feature in the
Applications Information section.
Note 7: Guaranteed by design.
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LTC3803-3
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TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage
vs Supply Voltage
Reference Voltage vs Temperature
801.0
VCC = 8V
803
800.6
VFB VOLTAGE (mV)
VFB VOLTAGE (mV)
804
VCC ≤ VCLAMP1mA
800.8
808
Reference Voltage
vs VCC Shunt Regulator Current
804
800
796
802
800.4
VFB VOLTAGE (mV)
812
TA = 25°C unless otherwise noted.
800.2
800.0
799.8
799.6
799.0
6
6.5
8
7.5
8.5
VCC SUPPLY VOLTAGE (V)
7
9
Oscillator Frequency
vs Temperature
OSCILLATOR FREQUENCY (kHz)
290
280
270
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
330
330
320
320
310
300
290
280
270
6.5
8.5
7
7.5
8
VCC SUPPLY VOLTAGE (V)
38033 G04
9.9
VTURNON
280
9
0
5
10
15
20
ICC (mA)
25
30
35
38033 G06
ICC Supply Current
vs Temperature
350
325
VCC = 8V
VITH/RUN = 1.3V
9.8
8.5
9.7
8.0
VCC (V)
VCC UNDERVOLTAGE LOCKOUT (V)
10.0
9.5
7.5
7.0
6.5
5.5
290
VCC Shunt Regulator Voltage
vs Temperature
10.0
25
300
38033 G05
VCC Undervoltage Lockout
Thresholds vs Temperature
6.0
20
15
ICC (mA)
310
270
6
9.6
SUPPLY CURRENT (μA)
OSCILLATOR FREQUENCY (kHz)
300
10
Oscillator Frequency
vs VCC Shunt Regulator Current
OSCILLATOR FREQUENCY (kHz)
VCC = 8V
310
5
0
38033 G03
Oscillator Frequency
vs Supply Voltage
320
9.0
796
9.5
38033 F02
38033 G01
330
799
797
799.2
788
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
800
798
799.4
792
801
ICC = 25mA
9.5
9.4
ICC = 1mA
9.3
VTURNOFF
9.2
300
275
250
225
200
5.0
9.1
175
4.5
–50 –30 –10 10 30 50 80 90 110 130
TEMPERATURE (°C)
9.0
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
150
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
38033 G07
38033 G08
38033 G09
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LTC3803-3
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TYPICAL PERFOR A CE CHARACTERISTICS
Start-Up ICC Supply Current
vs Temperature
ITH/RUN Shutdown Threshold
vs Temperature
ITH/RUN Start-Up Current Source
vs Temperature
700
SHUTDOWN THRESHOLD (mV)
60
50
40
30
20
10
ITH/RUN PIN CURRENT SOURCE (nA)
450
VCC = VTURNON – 0.1V
400
350
300
250
200
150
0
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
100
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
38033 G10
120
600
VCC = VTURNON + 0.1V
VITH/RUN = 0V
500
400
300
200
100
0
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
38033 G12
38033 G11
Peak Current Sense Voltage
vs Temperature
Soft-Start Time vs Temperature
3.5
VCC = 8V
115
3.0
110
SOFT-START TIME (ms)
SENSE PIN VOLTAGE (mV)
START-UP SUPPLY CURRENT (μA)
70
TA = 25°C unless otherwise noted.
105
100
95
90
2.5
2.0
1.5
1.0
0.5
85
80
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
38033 G13
0
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
38033 G14
38033fa
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LTC3803-3
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PI FU CTIO S
SENSE (Pin 4): This pin performs two functions. It monitors 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
serves as the error amplifier compensation point as well as
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.
GND (Pin 2): Ground Pin.
VCC (Pin 5): Supply Pin. Must be closely decoupled to GND
(Pin 2).
VFB (Pin 3): Receives the feedback voltage from an external resistive divider across the output.
NGATE (Pin 6): Gate Drive for the External N-Channel
MOSFET. This pin swings from 0V to VCC.
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BLOCK DIAGRA
5
VCC
0.3μA 0.28V
800mV
REFERENCE
VCC
SHUNT
REGULATOR
+
SHUTDOWN
COMPARATOR
VCC < VTURNON
–
SHUTDOWN
SOFTSTART
CLAMP
+
–
ERROR
AMPLIFIER
CURRENT
COMPARATOR
3
GND
2
VCC
R
+
VFB
UNDERVOLTAGE
LOCKOUT
Q
S
–
20mV
1.2V
300kHz
OSCILLATOR
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
GATE
DRIVER NGATE
SLOPE
COMP
CURRENT
RAMP
SENSE
1
6
4
ITH/RUN
38033 BD
38033fa
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LTC3803-3
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OPERATIO
The LTC3803-3 is a constant frequency current mode
controller for flyback and DC/DC boost converter applications in a tiny ThinSOT package. The LTC3803-3 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 LTC3803-3’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 Switching Power Supply Design.
Please refer to the Block Diagram and the Typical Application on the front page of this data sheet. An external
resistive voltage divider presents a fraction of the output
voltage to the VFB pin. The divider must be designed so that
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 voltage at the ITH/RUN pin commands the pulse-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
sets the RS latch. It turns off the MOSFET when the current
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.
Since the ITH/RUN voltage is increased by the error amplifier 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, bringing the output voltage back up.
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 impedance 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 considerable
influence over the dynamics of the voltage regulation loop.
38033fa
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LTC3803-3
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OPERATIO
Start-Up/Shutdown
The LTC3803-3 has two shutdown mechanisms to disable
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 according to the state diagram (Figure 1).
LTC3803-3
SHUT DOWN
VCC < VTURNOFF
(NOMINALLY 5.7V)
> VITHSHDN
V
VITH/RUN < VITHSHDN ITH/RUN
AND VCC > VTURNON
(NOMINALLY 0.28V)
(NOMINALLY 8.7V)
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 hysteresis range supports the use of a bias winding on the
flyback transformer to power the LTC3803-3—see the
section Powering the LTC3803-3.
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
VSHDN, and VCC exceeds VTURNON, the LTC3803-3 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 represent current comparator thresholds from zero to maximum.
Internal Soft-Start
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 insufficient VGS. The voltage at the VCC pin must exceed
VTURNON (nominally 8.7V) at least momentarily to enable
An internal soft-start feature is enabled whenever the
LTC3803-3 comes out of shutdown. Specifically, the
ITH/RUN voltage is clamped and is prevented from reaching maximum until roughly 1.4ms has passed. This
allows the input and output currents of LTC3803-3based power supplies to rise in a smooth and controlled
manner on start-up.
38033fa
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LTC3803-3
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OPERATIO
Powering the LTC3803-3
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
current as needed through this resistor to regulate the VCC
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.
For higher efficiency or for wide VIN range applications,
flyback controllers are typically powered through a separate 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.
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 necessary to supply the high transient currents required by the
MOSFET gate driver.
Adjustable Slope Compensation
The LTC3803-3 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 8% duty cycle, reaching peak current at 80% duty cycle. Additional details are
provided in the Applications Information section.
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LTC3803-3
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APPLICATIO S I FOR ATIO
Many LTC3803-3 application circuits can be derived from
the topology shown in Figure 2.
The LTC3803-3 itself imposes no limits on allowed power
output, input voltage VIN or desired regulated output voltage 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)) and maximum drain current, T1’s saturation flux
level and winding insulation breakdown voltages, CIN and
COUT’s maximum working voltage, ESR, and maximum
ripple current ratings, and D1 and RSENSE’s power ratings.
T1
LBIAS
D2
R3
•
VIN
CIN
RSTART
D1
VOUT
•
LPRI
LSEC
COUT
•
5
CVCC
1
CC
2
VCC
ITH/RUN NGATE
LTC3803-3
GND
SENSE
6
4
VFB
R1
3
Q1
RSL
RSENSE
R2
38033 F02
Figure 2. Typical LTC3803-3 Application Circuit
TRANSFORMER DESIGN CONSIDERATIONS
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 frequency power transformer design, the following should
prove useful.
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 application. Simple ratios of small integers, e.g., 1:1, 2:1, 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” configurable transformers 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
parallel on the secondary, a 3:1 turns ratio will be achieved.
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.
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 prominent 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.
R2 =
VOUT – 0.8 V
• R1
0.8 V
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.
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LTC3803-3
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APPLICATIO S I FOR ATIO
CURRENT SENSE RESISTOR CONSIDERATIONS
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
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.
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.
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%,
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 reducing the current comparator threshold in
proportion to duty cycle. This stabilizes the control loop
against subharmonic oscillation. The amount of reduction
in the current comparator threshold (ΔVSENSE) can be
calculated using the following equation:
ΔVSENSE =
Duty Cycle – 8%
• 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.
Designs not needing slope compensation may replace RSL
with a short circuit.
INTERNAL WIDE HYSTERESIS UNDERVOLTAGE
LOCKOUT
The LTC3803-3 is designed to implement DC/DC converters operating from input voltages of typically 48V or more.
The standard operating topology employs a third transformer winding (LBIAS in Figure 2) on the primary side that
provides power for the LTC3803-3 via its VCC pin. However, this arrangement is not inherently self-starting.
Start-up is affected by the use of an external “tricklecharge” 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:
“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
turns on abruptly and draws its normal supply current. 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
U
W
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 relaxation oscillation at the VCC node.
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. It should be made large enough to yield a worstcase maximum charging current less than the minimum
rated LTC3803-3 supply current, so that in operation, most
of the LTC3803-3’s supply current is delivered through the
third winding. This results in the highest possible efficiency.
Capacitor CVCC should then be made large enough to avoid
the relaxation oscillation behavior described above. This is
complicated to determine theoretically as it depends on
the particulars of the secondary circuit and load behavior.
Empirical testing is recommended.
The 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 threshold. 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 LTC3803-3’s
VCC shunt regulator, needlessly wasting power.
VCC SHUNT REGULATOR
In applications including a third transformer winding, the
internal VCC shunt regulator serves to protect the
LTC3803-3 from overvoltage transients as the third
winding is powering up.
In applications where a third transformer winding is undesirable 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
pin to GND to drop enough voltage across RVCC to regulate
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.
VIN
RVCC
LTC3803-3
VCC
GND
CVCC
38033 F03
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 resistor 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 LTC3803-3.
VIN
RB
Q1
RSTART
LTC3803-3
VCC
D1
8.2V
CVCC
GND
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
VIN
36V TO 75V
•
22k
806Ω
2.2μF
BAS516
9.2k
1nF
BAS516
1k
1
LTC3803-3
6
ITH/RUN NGATE
2
5
3
GND
VFB
VCC
SENSE
4
220k
•
SECONDARY SIDE
10V, 100mA
OUTPUT
SECONDARY
SIDE GROUND
FDC2512
T1: PULSE ENGINEERING PA0648
OR TYCO TTI8698
5.6k
1μF
PRIMARY GROUND
0.1Ω
38033 TA03
38033fa
13
LTC3803-3
U
TYPICAL APPLICATIO S
4:1 Input Range 3.3V Output Isolated Flyback DC/DC Converter
T1
PA1277NL
VIN+
18 V TO 72V
VIN–
•
2.2μF
220k
MMBTA42
100k
GND
BAS516
68Ω
PDZ6.8B
100μF
6.3V
×3
PDS1040
•
VOUT+
3.3V
3A
150pF
VCC
10Ω
22Ω BAS516
680Ω
•
1
ITH/RUN
GATE
6
0.1μF
FDC2512
LTC3803-3
2
5
VIN
GND
3
VFB
SENSE
4
VOUT+
4.7k
BAT760
0.1μF
0.040Ω
270Ω
VCC
1
6.8k
BAS516
PS2801-1
0.1μF
1
2
0.33μF
BAS516
2
3
VIN
OPTO
LT4430
GND
OC
COMP
FB
VOUT+
6
2.2nF
5
56k
47pF
100k
4
22.1k
38033 TA05
Efficiency vs Load Current
84
82
EFFICIENCY (%)
80
78
76
74
72
70
VIN = 48V
VIN = 24V
0
1
2
IOUT (A)
3
4
38033 TA05a
38033fa
14
LTC3803-3
U
PACKAGE DESCRIPTIO
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
0.62
MAX
2.90 BSC
(NOTE 4)
0.95
REF
1.22 REF
3.85 MAX 2.62 REF
1.4 MIN
2.80 BSC
1.50 – 1.75
(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
0.01 – 0.10
1.00 MAX
DATUM ‘A’
0.30 – 0.50 REF
0.09 – 0.20
(NOTE 3)
1.90 BSC
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3803-3
U
TYPICAL APPLICATIO S
Efficiency vs Load
100
VOUT = 3.3V
EFFICIENCY (%)
95
90% Efficient Synchronous Flyback Converter
VIN
36V TO 72V
VOUT*
3.3V
1.5A
T1
•
Q2
CIN
220k
85
80
VIN = 36V
VIN = 48V
VIN = 60V
VIN = 72V
75
CO
•
70
500
750
D1
33k 1
ITH/RUN
GATE
6
LTC3803-3
2
5
VCC
GND
8.06k
3
VFB = 0.8V SENSE
4
Q1
•
100
4.7k
10μF
10V
VOUT = 5V*
RCS
VOUT
T1: PULSE ENGINEERING PA1006
Q1: FAIRCHILD FDC2512
Q2: VISHAY Si9803
D1: PHILIPS BAS516
2000
38083 TA04b
95
38033 TA04a
25.5k*
RFB
1750
1000 1250 1500
LOAD CURRENT (mA)
Efficiency vs Load
0.1μF
560Ω
EFFICIENCY (%)
1n
90
CIN: TDK 1μF, 100V, X5R
CO: TDK 100μF, 6.3V, X5R
RCS: VISHAY OR IRC, 80mΩ
*FOR 5V OUTPUT CHANGE RFB TO 42.2k
90
85
80
VIN = 36V
VIN = 48V
VIN = 60V
VIN = 72V
75
70
500
750
1000 1250 1500
LOAD CURRENT (mA)
1750
2000
38083 TA04c
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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Isolated Flyback Switching Regulator
with No External Power Devices
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LTC1871
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LT3420
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Specialized Flyback Charges High Voltage Photoflash Capacitors
Quickly and Efficiently
LT3468/LT3468-1
Photoflash Capacitor Charger in 5-Pin SOT-23
Minimal Component Count, Uses Small Transformers;
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LTC3803
Constant Frequency Flyback Controller
200kHz Switching Frequency, Low Profile (1mm) ThinSOT Package
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Constant Frequency Flyback Controller
200kHz Switching Frequency, 4.8V Turn-On Voltage
LTC3806
Synchronous Flyback Controller
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®
Burst Mode is a registered trademark of Linear Technology Corporation. No RSENSE is a trademark of Linear Technology Corporation.
38033fa
16
Linear Technology Corporation
LT 0407 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
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