LTC3803-3
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
FEATURES
DESCRIPTION
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The LTC®3803-3 is a constant frequency current mode
flyback controller optimized for driving 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.
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VIN and VOUT Limited Only by External Components
Adjustable Slope Compensation
Internal Soft-Start
–55°C to 150°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
APPLICATIONS
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Telecom Power Supplies
42V and 12V Automotive Power Supplies
Auxiliary/Housekeeping Power Supplies
Power over Ethernet Powered Devices
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.
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 shunt regulator. An
internal undervoltage lockout shuts down the LTC3803-3
when the input voltage is too low to provide sufficient gate
drive to the external MOSFET.
The LTC3803-3 is available in a low profile (1mm) 6-lead
SOT-23 (ThinSOT™) package.
TYPICAL APPLICATION
5V Output Nonisolated Telecom Housekeeping Power Supply
100
UPS840
T1
1μF
10V
X5R
1μF
100V
X5R
470pF
300μF*
6.3V
X5R
•
VCC
ITH/RUN NGATE
82k
•
LTC3803-3
GND
68mΩ
VOUT = 5V
90
85
80
75
70
65
150pF
200V
SENSE
VFB
20k
FDC2512
4.7k
95
VOUT
5V
2A MAX
EFFICIENCY (%)
VIN
36V TO 72V
10k
Efficiency vs Load Current
55
220Ω
50
250
105k
38033 TA01
T1: COOPER CTX02-15242
*THREE 100μF UNITS IN PARALLEL
VIN = 36V
VIN = 48V
VIN = 60V
VIN = 72V
60
500
750 1000 1250 1500 1750 2000
LOAD CURRENT (mA)
38033 TA02
38033fd
1
LTC3803-3
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 ( VTURNON, VITH/RUN Falling
LTC3803E-3
LTC3803I-3, LTC3803H-3
LTC3803MP-3
IITHSTART
Start-Up Current Source
VITH/RUN = 0V
VFB
Regulated Feedback Voltage
(Note 5)
0°C ≤ TJ ≤ 85°C
LTC3803E-3: –40°C ≤ TJ ≤ 85°C
LTC3803I-3: –40°C ≤ TJ ≤ 125°C
LTC3803H-3: –40°C ≤ TJ ≤ 150°C
LTC3803MP-3: –55°C ≤ TJ ≤ 150°C
RSL = 0 (Note 6)
LTC3803E-3
LTC3803I-3, LTC3803H-3
LTC3803MP-3
VIMAX
Peak Current Sense Voltage
gm
Error Amplifier Transconductance
ITH/RUN Pin Load = ±5μA (Note 5)
ΔVO(LINE)
Output Voltage Line Regulation
(Note 5)
IFB
VFB Input Current
(Note 5)
fOSC
Oscillator Frequency
VITH/RUN = 1.3V
MIN
TYP
MAX
UNITS
l
l
l
8.4
8.4
8.1
9.5
9.5
9.5
10.5
10.7
10.7
V
V
V
l
l
0.05
0.03
0.6
0.6
V
V
240
40
350
90
μA
μA
0.15
0.10
0.09
0.28
0.28
0.28
0.45
0.45
0.45
V
V
V
0.2
0.3
0.4
μA
l
l
l
l
0.788
0.780
0.780
0.780
0.780
0.800
0.800
0.800
0.800
0.800
0.812
0.812
0.820
0.820
0.820
V
V
V
V
V
l
l
l
90
85
85
100
100
100
115
115
120
333
500
l
l
l
200
0.05
mV
mV
mV
μA/V
mV/V
10
50
nA
270
300
330
kHz
8
9.6
%
70
80
90
%
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)
5
μA
tSFST
Soft-Start Time
1.4
ms
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 LTC3803-3 is tested under pulsed load conditions such
that TJ ≈ TA. The LTC3803E-3 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-3 is guaranteed over the –40°C to 125°C operating junction
temperature range, the LTC3803H-3 is guaranteed over the –40°C to
150°C operating junction temperature range and the LTC3803MP-3 is
tested and guaranteed over the full –55°C to 150°C operating junction
temperature range. High junction temperatures degrade operating
lifetimes; operating lifetime is derated for junction temperatures greater
than 125°C. 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.
Note 3: Junction temperature TJ is calculated from the ambient
temperature TA and power dissipation PD according to the following
formula:
TJ = TA + (PD • 230°C/W).
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 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.
38033fd
3
LTC3803-3
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted.
Reference Voltage
vs Supply Voltage
Reference Voltage vs Temperature
820
801.0
VCC = 8V
804
VCC b VCLAMP1mA
800.8
815
Reference Voltage
vs VCC Shunt Regulator Current
803
800.6
805
800
795
790
802
800.4
VFB VOLTAGE (mV)
VFB VOLTAGE (mV)
VFB VOLTAGE (mV)
810
800.2
800.0
799.8
799.6
799.0
–30
0
30
60
90
TEMPERATURE (°C)
120
150
6
6.5
8
7.5
8.5
VCC SUPPLY VOLTAGE (V)
7
9
38033 G01
310
300
290
280
–30
0
30
60
90
TEMPERATURE (°C)
120
330
330
320
320
310
300
290
280
270
150
6
6.5
8.5
7
7.5
8
VCC SUPPLY VOLTAGE (V)
38033 G04
9.7
7.0
9.6
6.5
VTURNOFF
350
9.3
5.0
9.2
4.5
9.1
150
38033 G07
5
10
15
20
ICC (mA)
25
9.0
–60
30
35
VCC = 8V
VITH/RUN = 1.3V
325
ICC = 25mA
ICC = 1mA
9.4
120
0
ICC Supply Current
vs Temperature
9.5
5.5
0
30
60
90
TEMPERATURE (°C)
280
38033 G06
SUPPLY CURRENT (μA)
7.5
VCC (V)
VCC UNDERVOLTAGE LOCKOUT (V)
9.8
–30
290
9.9
8.0
4.0
–60
300
270
9
10.0
VTURNON
25
310
VCC Shunt Regulator Voltage
vs Temperature
9.0
6.0
20
15
ICC (mA)
38033 G05
VCC Undervoltage Lockout
Thresholds vs Temperature
8.5
10
Oscillator Frequency
vs VCC Shunt Regulator Current
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY (kHz)
320
5
0
38033 G03
Oscillator Frequency
vs Supply Voltage
VCC = 8V
270
–60
796
9.5
38033 F02
Oscillator Frequency
vs Temperature
330
799
797
799.2
780
–60
800
798
799.4
785
801
300
275
250
225
–30
0
30
60
90
TEMPERATURE (°C)
120
150
38033 G08
200
–60
–30
0
30
60
90
TEMPERATURE (°C)
120
150
38033 G09
38033fd
4
LTC3803-3
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted.
Start-Up ICC Supply Current
vs Temperature
800
70
60
50
40
30
20
ITH/RUN PIN CURRENT SOURCE (nA)
SHUTDOWN THRESHOLD (mV)
80
400
350
300
250
200
150
10
–30
0
30
60
90
TEMPERATURE (°C)
120
100
–60
150
–30
0
30
60
120
90
Peak Current Sense Voltage
vs Temperature
600
500
400
300
200
100
0
–60
150
–30
0
30
60
90
TEMPERATURE (°C)
120
150
38033 G12
Soft-Start Time vs Temperature
4.0
VCC = 8V
115
3.5
110
3.0
105
100
95
90
85
80
–60
VCC = VTURNON + 0.1V
VITH/RUN = 0V
38033 G11
38033 G10
120
700
TEMPERATURE (°C)
SOFT-START TIME (ms)
0
–60
ITH/RUN Start-Up Current Source
vs Temperature
450
VCC = VTURNON – 0.1V
SENSE PIN VOLTAGE (mV)
START-UP SUPPLY CURRENT (μA)
90
ITH/RUN Shutdown Threshold
vs Temperature
2.5
2.0
1.5
1.0
0.5
–30
0
30
60
90
120
150
TEMPERATURE (°C)
0
–60
–30
0
30
60
90
120
150
TEMPERATURE (°C)
38033 G13
38033 G14
38033fd
5
LTC3803-3
PIN FUNCTIONS
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 the shutdown
threshold (VITHSHDN) 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.
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
300kHz
OSCILLATOR
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
GATE
DRIVER NGATE
SLOPE
COMP
CURRENT
RAMP
SENSE
1
6
4
ITH/RUN
38033 BD
38033fd
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LTC3803-3
OPERATION
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.
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)
V
> VITHSHDN
VITH/RUN < VITHSHDN ITH/RUN
AND VCC > VTURNON
(NOMINALLY 0.28V)
(NOMINALLY 8.7V)
LTC3803-3
ENABLED
38033 F01
Figure 1. Start-Up/Shutdown State Diagram
38033fd
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LTC3803-3
OPERATION
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
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 the shutdown
threshold (VITHSHDN) 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 VITHSHDN, 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
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 LTC3803-3-based power
supplies to rise in a smooth and controlled manner on
start-up.
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 the wide UVLO hysteresis (1V min) and small VCC
supply current draw (
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