LTC3803-5
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
FEATURES
DESCRIPTION
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The LTC®3803-5 is a constant frequency current mode
flyback controller optimized for driving N-channel MOSFETs
in high input voltage applications. The LTC3803-5 operates
from inputs as low as 5V. Constant frequency operation
is maintained down to very light loads, resulting in less
low frequency noise generation over a wide range of load
currents. Slope compensation can be programmed with
an external resistor.
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VIN and VOUT Limited Only by External Components
4.8V Undervoltage Lockout Threshold
Operating Junction Temperature from –55°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
Transient Response
No Minimum Load Requirement
Low Quiescent Current: 240μA
Low Profile (1mm) SOT-23 Package
APPLICATIONS
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42V and 12V Automotive Power Supplies
Telecom Power Supplies
Auxiliary/Housekeeping Power Supplies
Power Over Ethernet
The LTC3803-5 provides ±1.5% output voltage accuracy
and consumes only 240μA of quiescent current. Groundreferenced current sensing allows LTC3803-5-based converters 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 shunt regulator. An internal undervoltage lockout
shuts down the IC when the input voltage is too low to
provide sufficient gate drive to the external MOSFET.
The LTC3803-5 is available in a low profile (1mm) 6-lead
SOT-23 (ThinSOT™) package.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
and ThinSOT and No RSENSE are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
TYPICAL APPLICATION
Efficiency and Power Loss
vs Output Power
Dual Output Wide Input Range Converter
VPH5-0155
13V/0.3A
20mA MIN
LOAD
1μF
100V
×3
22k
PDZ6.8B
10MQ100N
22μF
10V
7.5k
LTC3803-5
PHM25NQ10T
1μF
100V
ITH/RUN NGATE
GND
8.06k
VFB
VCC
SENSE
1μF
100V
6.5V/1.2A
4.7k
B3100
0.012Ω
3.0
VIN = 8V
85 VIN = 12V
2.5
80
2.0
VIN = 24V
75
1.5
70
1.0
POWER LOSS (W)
MMBTA42
10nF
90
EFFICIENCY (%)
VIN
6V TO 50V
VIN = 48V
47μF
10V
65
0.5
0.1μF
57.6k
60
ALL CAPACITORS ARE X7R, TDK
38035 TA01
VIN = 12V
0
2
6
8
4
OUTPUT POWER (W)
10
0
12
38035 TA01b
38035fd
1
LTC3803-5
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(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 ( VTURNON, VITH/RUN Falling
LTC3803E-5
LTC3803I-5, LTC3803H-5
LTC3803MP-5
l
l
l
0.12
0.08
0.08
0.28
0.28
0.28
0.45
0.45
0.47
V
V
V
VITH/RUN = 0V
LTC3803E-5
LTC3803I-5, LTC3803H-5, LTC3803MP-5
l
l
0.07
0.07
0.34
0.34
0.8
1
μA
μA
l
0.788
0.780
0.800
0.800
0.812
0.816
V
V
l
0.788
0.780
0.800
0.800
0.812
0.820
V
V
l
0.788
0.780
0.800
0.800
0.812
0.820
V
V
l
0.788
0.780
0.800
0.800
0.812
0.820
V
V
200
333
500
Start-Up Current Source
Regulated Feedback Voltage
(Note 5)
LTC3803E-5:
0°C ≤ TJ ≤ 85°C
–40°C ≤ TJ ≤ 85°C
LTC3803I-5:
0°C ≤ TJ ≤ 85°C
–40°C ≤ TJ ≤ 125°C
LTC3803H-5:
0°C ≤ TJ ≤ 85°C
–40°C ≤ TJ ≤ 150°C
LTC3803MP-5:
0°C ≤ TJ ≤ 85°C
–55°C ≤ TJ ≤ 150°C
gm
Error Amplifier Transconductance
ITH/RUN Pin Load = ±5μA (Note 5)
ΔVO(LINE)
Output Voltage Line Regulation
(Note 5)
ΔVO(LOAD)
Output Voltage Load Regulation
ITH/RUN Sinking 5μA (Note 5)
ITH/RUN Sourcing 5μA (Note 5)
μA/V
0.1
mV/V
3
3
mV/μA
mV/μA
IFB
VFB Input Current
(Note 5)
fOSC
Oscillator Frequency
VITH/RUN = 1.3V
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
VIMAX
Peak Current Sense Voltage
RSL = 0 (Note 6)
LTC3803E-5
LTC3803I-5, LTC3803H-5
LTC3803MP-5
ISLMAX
Peak Slope Compensation Output
Current
tSFST
Soft-Start Time
(Note 7)
170
70
l
l
l
90
85
85
10
50
nA
200
230
kHz
6.5
8.5
%
80
90
%
100
100
100
115
115
120
mV
mV
mV
5
μA
0.7
ms
38035fd
3
LTC3803-5
ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3803-5 is tested under pulsed load conditions such
that TJ ≈ TA. The LTC3803E-5 is guaranteed to meet specifications
from 0°C to 85°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3803I-5 is guaranteed over the –40°C to 125°C operating junction
temperature range, the LTC3803H-5 is guaranteed over the –40°C to
150°C operating junction temperature range and the LTC3803MP-5 is
tested and guaranteed over the full –55°C to 150°C operating junction
temperature range. Note that the maximum ambient temperature
consistent with these specifications is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
impedance and other environmental factors.
Junction temperature (TJ) is calculated from the ambient temperature TA
and the power dissipation PD in the LTC3803-5 using the formula:
TJ = TA + (PD • 230°C/W)
Note 3: High junction temperatures degrade operating lifetimes; operating
lifetime is derated for junction temperatures greater than 125°C.
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5: The LTC3803-5 is tested in a feedback loop that servos VFB to the
output of the error amplifier while maintaining 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.
TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage
vs Supply Voltage
Reference Voltage vs Temperature
820
Reference Voltage
vs VCC Shunt Regulator Current
812
VCC = 5V
815
812
TA = 25°C
VCC ≤ VCLAMP1mA
808
TA = 25°C
808
805
800
795
VFB VOLTAGE (mV)
VFB VOLTAGE (mV)
VFB VOLTAGE (mV)
810
804
800
796
804
800
796
790
792
792
785
780
–60
788
–30
0
30
60
90
TEMPERATURE (°C)
120
4.0
150
4.5
5.0 5.5 6.0
6.5 7.0
VCC SUPPLY VOLTAGE (V)
220
210
200
190
220
TA = 25°C
215
210
205
200
195
190
185
120
150
38035 G04
15
ICC (mA)
20
25
TA = 25°C
215
210
205
200
195
190
185
180
180
60
0
30
90
TEMPERATURE (°C)
10
Oscillator Frequency
vs VCC Shunt Regulator Current
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY (kHz)
220
VCC = 5V
230
5
38035 G03
Oscillator Frequency
vs Supply Voltage
Oscillator Frequency
vs Temperature
180
–60 –30
0
38035 G02
38035 G01
240
788
7.5
4.0
4.5
6.5 7.0
5.0 5.5 6.0
VCC SUPPLY VOLTAGE (V)
7.5
38035 G05
0
5
15
10
ICC (mA)
20
25
38035 G06
38035fd
4
LTC3803-5
TYPICAL PERFORMANCE CHARACTERISTICS
VCC Undervoltage Lockout
Thresholds vs Temperature
10.5
5.5
10.0
300
VCC (V)
VTURNON
4.0
9.0
ICC = 25mA
8.5
8.0
VTURNOFF
ICC = 1mA
3.5
260
240
220
7.5
3.0
–60
–30
90
30
0
60
TEMPERATURE (°C)
120
7.0
–60
150
–30
90
60
30
0
TEMPERATURE (°C)
120
38035 G07
30
20
10
ITH/RUN PIN CURRENT SOURCE (nA)
SHUTDOWN THRESHOLD (mV)
40
400
350
300
250
200
150
100
50
60
0
30
90
TEMPERATURE (°C)
120
150
0
–60 –30
0
30
60
90
TEMPERATURE (°C)
120
Peak Current Sense Voltage
vs Temperature
120
150
800
700
600
500
400
300
200
100
0
–60
–30
60
90
0
30
TEMPERATURE (°C)
120
150
38035 G12
Soft-Start Time vs Temperature
1.4
VCC = 5V
115
VCC = 5V
1.2
SOFT-START TIME (ms)
110
105
100
95
90
1.0
0.8
0.6
0.4
0.2
85
80
–60
150
VCC = VTURNON + 0.1V
900 VITH/RUN = 0V
38035 G11
38035 G10
SENSE PIN VOLTAGE (mV)
–30
120
1000
450
50
30
0
60
90
TEMPERATURE (°C)
ITH/RUN Start-Up Current Source
vs Temperature
500
60
–30
38035 G09
ITH/RUN Shutdown Threshold
vs Temperature
VCC = VTURNON – 0.1V
0
–60
200
–60
150
38035 G08
Start-Up ICC Supply Current
vs Temperature
START-UP SUPPLY CURRENT (μA)
VCC = 5V
VITH/RUN = 1.3V
280
9.5
4.5
70
ICC Supply Current
vs Temperature
SUPPLY CURRENT (μA)
6.0
5.0
VOLTS
VCC Shunt Regulator Voltage
vs Temperature
–30
30
60
0
90
TEMPERATURE (°C)
120
150
38035 G13
0
–60
–30
30
60
0
90
TEMPERATURE (°C)
120
150
38035 G14
38035fd
5
LTC3803-5
PIN FUNCTIONS
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 the shutdown
threshold (VITHSHDN) causes the LTC3803-5 to shut down.
In shutdown mode, the NGATE pin is held low.
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.
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.
BLOCK DIAGRAM
5
VCC
0.3μA 0.28V
800mV
REFERENCE
VCC
SHUNT
REGULATOR
+
SHUTDOWN
COMPARATOR
VCC < VTURNON
–
SHUTDOWN
SOFTSTART
CLAMP
+
3
VFB
GND
2
UNDERVOLTAGE
LOCKOUT
–
ERROR
AMPLIFIER
CURRENT
COMPARATOR
VCC
R
+
Q
S
–
20mV
1.2V
200kHz
OSCILLATOR
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
GATE
DRIVER NGATE
SLOPE
COMP
CURRENT
RAMP
SENSE
1
6
4
ITH/RUN
38035 BD
38035fd
6
LTC3803-5
OPERATION
The LTC3803-5 is a constant frequency current mode
controller for flyback, SEPIC and DC/DC boost converter
applications in a tiny ThinSOT package. The LTC3803-5 is
designed so that none of its pins need to come in contact
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 LTC3803-5’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 LTC3803-5 turns on the external power
MOSFET when the internal free-running 200kHz oscillator
sets the RS latch. It turns off the MOSFET when the 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.
Start-Up/Shutdown
The LTC3803-5 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 LTC3803-5
transitions into and out of shutdown according to the state
diagram (Figure 1).
LTC3803-5
SHUT DOWN
VCC < VTURNOFF
(NOMINALLY 4V)
V
> VITHSHDN
VITH/RUN < VITHSHDN ITH/RUN
AND VCC > VTURNON
(NOMINALLY 0.28V)
(NOMINALLY 4.8V)
LTC3803-5
ENABLED
38035 F01
Figure 1. Start-Up/Shutdown State Diagram
38035fd
7
LTC3803-5
OPERATION
The undervoltage lockout (UVLO) mechanism prevents
the LTC3803-5 from trying to drive a MOSFET with insufficient VGS. The voltage at the VCC pin must exceed
VTURNON (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 lockout
disables the LTC3803-5.
The ITH/RUN pin can be driven below VITHSHDN (nominally
0.28V) to force the LTC3803-5 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
VITHSHDN, 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 represent current comparator thresholds from zero
to maximum.
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC3803-5 comes out of shutdown. Specifically, the ITH/
RUN voltage is clamped and is prevented from reaching
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.
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 8.1V 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.
The VCC pin must be bypassed to ground immediately adjacent to the IC pins with a ceramic or tantalum capacitor.
Proper supply bypassing is necessary to supply the high
transient currents required by the MOSFET gate driver.
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 Information section.
38035fd
8
LTC3803-5
APPLICATIONS INFORMATION
Many LTC3803-5 application circuits can be derived from
the topology shown in Figure 2.
The LTC3803-5 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.
VIN
D1
T1
CIN LPRI
LSEC
5
1
CC
2
VCC
ITH/RUN NGATE
LTC3803-5
GND
SENSE
6
4
VFB
R1
3
Q1
RSL
RSENSE
R2
38035 F02
Figure 2. Typical LTC3803-5 Application Circuit
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
The regulated output voltage is determined by the resistor
divider across VOUT (R1 and R2 in Figure 2). The ratio
of R2 to R1 needed to produce a desired VOUT can be
calculated:
R2 =
VOUT – 0.8V
• R1
0.8V
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 frequency power transformer design, the following should
prove useful.
Turns Ratios
COUT
•
CVCC
TRANSFORMER DESIGN CONSIDERATIONS
VOUT
•
RVCC
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.
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-PAC™
series in applications with high input to output voltage
ratios. For example, if a 6-winding VERSA-PAC is used
with three windings in series on the primary and three
windings in parallel on the secondary, a 3:1 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.
38035fd
9
LTC3803-5
APPLICATIONS INFORMATION
Leakage Inductance
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to occur after the
output switch (Q1) turn-off. This is increasingly 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.
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.
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 LTC3803-5 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.
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%, 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
38035fd
10
LTC3803-5
APPLICATIONS INFORMATION
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 – 6.5%
• 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.
Designs not needing slope compensation may replace
RSL with a short circuit.
GND to drop enough voltage across RVCC to regulate
VCC to around 8.1V. 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 LTC3803-5.
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
resistor RB brings VCC above the VCC turn-on threshold,
enabling the LTC3803-5.
8V TO
75 VIN
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
RB
100k
Q1
MMBTA42
LTC3803-5
VCC
D1
6.8V
CVCC
0.1μF
GND
38035 F04
VIN
RVCC
LTC3803-5
Figure 4. Powering the LTC3803-5
with an External Preregulator
VCC
CVCC
GND
38035 F03
Figure 3. Powering the LTC3803-5
Via the Internal Shunt Regulator
38035fd
11
LTC3803-5
TYPICAL APPLICATIONS
2W Isolated Housekeeping Telecom Converter
BAS516
PRIMARY SIDE
10V, 100mA
OUTPUT
T1
220k
•
MMBTA42
2.2μF
PDZ6.8B
130Ω
1μF
VIN
36V TO 75V
BAS516
•
9.09k
2.2μF
BAS516
1k
•
SECONDARY SIDE
10V, 100mA
OUTPUT
SECONDARY
SIDE GROUND
1nF
22k
787Ω
1
LTC3803-5
6
ITH/RUN NGATE
2
5
3
GND
VFB
VCC
SENSE
4
FDC2512
T1: PULSE ENGINEERING PA0648
OR TYCO TTI8698
5.6k
1μF
PRIMARY GROUND
0.1Ω
38035 TA03
38035fd
12
LTC3803-5
TYPICAL APPLICATIONS
4:1 Input Range 3.3V Output Isolated Flyback DC/DC Converter
T1
PA1277NL
VIN+
18 V TO 72V
•
2.2μF
220k
MMBTA42
GND
BAS516
68Ω
PDZ6.8B
100μF
6.3V
×3
PDS1040
•
150pF
130Ω
VCC
10Ω
22Ω BAS516
680Ω
•
1
2
3
ITH/RUN
GATE
6
0.1μF
FDC2512
LTC3803-5
5
VCC
GND
VFB
SENSE
4
VOUT+
4.7k
0.1μF
BAT760
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
38035 TA05
Efficiency vs Load Current
84
82
80
EFFICIENCY (%)
VIN–
VOUT+
3.3V
3A
78
76
74
72
70
VIN = 48V
VIN = 24V
0
1
2
IOUT (A)
3
4
38035 TA05a
38035fd
13
LTC3803-5
PACKAGE DESCRIPTION
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 REV B
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
38035fd
14
LTC3803-5
REVISION HISTORY
(Revision history begins at Rev D)
REV
DATE
DESCRIPTION
PAGE NUMBER
D
6/10
MP-grade part added. Reflected throughout the data sheet.
1 to 16
38035fd
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3803-5
TYPICAL APPLICATION
Synchronous Flyback Converter
VIN
36V TO 72V
220k MMBTA42
CIN
•
Q2
CO
PDZ6.8B
•
130Ω
1n
VOUT*
3.3V
1.5A
T1
D1
33k 1
8.06k
ITH/RUN
6
GATE
Q1
LTC3803-5
2
5
VCC
GND
560
3
5k
VFB
SENSE
25.5k*
RFB
VOUT
4
1μF
10V
T1: PULSE ENGINEERING PA1006
Q1: FAIRCHILD FDC2512
Q2: VISHAY Si9803
RCS
•
0.1μF
38035 TA04
D1: PHILIPS BAS516
RCS: VISHAY OR IRC, 80mΩ
CIN: TDK 1μF, 100V, X5R *FOR 5V OUTPUT CHANGE
CO: TDK 100μF, 6.3V, X5R RFB TO 42.2k
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3573
Isolated Flyback Switching Regulator with 60V
Integrated Switch
3V ≤ VIN ≤ 40V, No Opto-Isolator or Third Winding Required, Up to 7W
Output Power, MSOP-16E
LTC3805/
LTC3805-5
Adjustable Constant Frequency Flyback, Boost, SEPIC
DC/DC Controller
VIN and VOUT Limited Only by External Components, 3mm × 3mm DFN-10,
MSOP-10E Packages
LTC3873/
LTC3873-5
No RSENSE™ Constant Frequency Flyback, Boost, SEPIC
Controller
VIN and VOUT Limited Only by External Components, 8-pin ThinSOT or
2mm × 3mm DFN-8 Packages
LT3757
Boost, Flyback, SEPIC and Inverting Controller
2.9V ≤ VIN ≤ 40V, 100kHz to 1MHz Programmable Operating Frequency,
3mm × 3mm DFN-10 and MSOP-10E Package
LT3758
Boost, Flyback, SEPIC and Inverting Controller
5.5V ≤ VIN ≤ 100V, 100kHz to 1MHz Programmable Operating Frequency,
3mm × 3mm DFN-10 and MSOP-10E
LTC1871/LTC1871-1/ Wide Input Range, No RSENSE Low Quiescent Current
LTC1871-7
Flyback, Boost and SEPIC Controller
Programmable Operating Frequency, 2.5V ≤ VIN ≤ 36V, Burst Mode®
Operation at Light Load, MSOP-10
38035fd
16 Linear Technology Corporation
LT 0610 REV D • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
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