Final Electrical Specifications
LTC3803
Constant Frequency
Current Mode Flyback
DC/DC Controller in ThinSOT
August 2003
FEATURES
s
s
s
s
s
s
DESCRIPTION
The LTC
®
3803 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.
The LTC3803 provides
±1.5%
output voltage accuracy
and consumes only 240µA of quiescent current. Ground-
referenced current sensing allows LTC3803-based con-
verters to accept input supplies beyond the LTC3803’s
absolute maximum V
CC
. A micropower hysteretic start-up
feature allows efficient operation at high input voltages.
For simplicity, the LTC3803 can also be powered from a
high V
IN
through a resistor, due to its internal 9.4V shunt
regulator. An internal undervoltage lockout shuts down
the LTC3803 when the input voltage falls below 4.6V,
guaranteeing at least 4.6V of gate drive to the external
MOSFET.
The LTC3803 is available in a low profile (1mm) 6-lead
SOT-23 (ThinSOT
TM
) package.
s
s
s
V
IN
and V
OUT
Limited Only by External Components
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
APPLICATIO S
s
s
s
s
Telecom Power Supplies
42V and 12V Automotive Power Supplies
Auxiliary/Housekeeping Power Supplies
Power Over Ethernet
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
V
IN
36V TO 72V
5V Output Nonisolated Telecom Housekeeping Power Supply
UPS840
T1
10k
10µF
10V
X5R
56k
0.0022µF
4.7µF
100V
X5R
90
V
OUT
5V
2A MAX
80
70
EFFICIENCY (%)
•
•
FDC2512
300µF*
6.3V
X5R
60
50
40
30
20
V
IN
= 72V
V
CC
I
TH
/RUN NGATE
LTC3803
GND
V
FB
20k
105k
3803 TA01
SENSE
68mΩ
10
0
0.1
1
I
OUT
(A)
3803 TA02
T1: COOPER CTX02-15242
*THREE 100µF UNITS IN PARALLEL
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-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
U
U
Efficiency vs Load Current
V
IN
= 36V
V
IN
= 48V
V
IN
= 60V
10
3803i
1
LTC3803
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
I
TH
/RUN 1
GND 2
V
FB
3
6 NGATE
5 V
CC
4 SENSE
V
CC
to GND
Low Impedance Source .......................... – 0.3V to 8V
Current Fed ...................................... 25mA into V
CC
*
NGATE Voltage ......................................... – 0.3V to V
CC
V
FB
, I
TH
/RUN Voltages ..............................– 0.3V to 3.5V
SENSE Voltage ........................................... – 0.3V to 1V
NGATE Peak Output Current (<10µs) ........................ 1A
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Note 3) ............................ 150°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
*LTC3803 internal clamp circuit self regulates V
CC
voltage to 9.5V.
ORDER PART
NUMBER
LTC3803ES6
S6 PART
MARKING
LTACV
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
T
JMAX
= 150°C,
θ
JA
= 230°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
V
TURNON
V
TURNOFF
V
HYST
V
CLAMP1mA
V
CLAMP25mA
V
MARGIN
I
CC
PARAMETER
V
CC
Turn On Voltage
V
CC
Turn Off Voltage
V
CC
Hysteresis
V
CC
Shunt Regulator Voltage
V
CC
Shunt Regulator Voltage
V
CLAMP1mA
– V
TURNON
Margin
Input DC Supply Current
Normal Operation
Start-Up
Shutdown Threshold (at I
TH
/RUN)
Start-Up Current Source
Regulated Feedback Voltage
Error Amplifier Transconductance
Output Voltage Line Regulation
Output Voltage Load Regulation
V
FB
Input Current
Oscillator Frequency
Minimum Switch On Duty Cycle
Maximum Switch On Duty Cycle
Gate Drive Rise Time
Gate Drive Fall Time
Peak Current Sense Voltage
Peak Slope Compensation Output Current
Soft-Start Time
The
q
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 8V, unless otherwise noted. (Note 2)
CONDITIONS
q
q
V
TURNON
– V
TURNOFF
I
CC
= 1mA, V
ITH/RUN
= 0V
I
CC
= 25mA, V
ITH/RUN
= 0V
(Note 4)
V
ITH/RUN
= 1.3V
V
CC
= V
TURNON
– 100mV
V
CC
= V
TURNON
+ 100mV
V
ITH/RUN
= 0V
0°C
≤
T
A
≤
85°C (Note 5)
–40°C
≤
T
A
≤
85°CC (Note 5)
I
TH/RUN
Pin Load =
±5µA
(Note 5)
V
TURNOFF
< V
CC
< V
CLAMP
(Note 5)
I
TH
/RUN Sinking 5µA (Note 5)
I
TH
/RUN Sourcing 5µA (Note 5)
(Note 5)
V
ITH/RUN
= 1.3V
V
ITH/RUN
= 1.3V, V
FB
= 0.8V
V
ITH/RUN
= 1.3V, V
FB
= 0.8V
C
LOAD
= 3000pF
C
LOAD
= 3000pF
R
SL
= 0 (Note 6)
(Note 7)
q
q
q
q
MIN
7.8
4.6
1.5
8.3
8.4
0.05
TYP
8.7
5.7
3.0
9.4
9.5
0.6
240
40
0.28
0.3
0.800
0.800
333
0.05
3
3
10
200
6
80
40
40
100
5
1.4
MAX
9.2
6.8
10.3
10.5
UNITS
V
V
V
V
V
V
µA
µA
V
µA
V
V
µA/V
mV/V
mV/µA
mV/µA
nA
kHz
%
%
ns
ns
mV
µA
ms
q
q
V
ITHSHDN
I
ITHSTART
V
FB
g
m
∆V
O(LINE)
∆V
O(LOAD)
I
FB
f
OSC
DC
ON(MIN)
DC
ON(MAX)
t
RISE
t
FALL
V
IMAX
I
SLMAX
t
SFST
q
0.15
0.2
0.788
0.780
200
350
90
0.45
0.4
0.812
0.812
500
180
70
50
240
8
90
q
90
115
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3803E is guaranteed to meet specifications from 0°C to
70°C. Specifications over the – 40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls.
Note 3:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
3803i
2
U
W
U
U
W W
W
LTC3803
ELECTRICAL CHARACTERISTICS
T
J
= T
A
+ (P
D
• 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 is tested in a feedback loop that servos V
FB
to the
output of the error amplifier while maintaing I
TH
/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 (R
SL
). For
details, refer to the programmable slope compensation feature in the
Applications Information section.
Note 7:
Guaranteed by design.
TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage vs Temperature
800.5
800.0
V
FB
VOLTAGE (mV)
V
CC
= 8V
V
FB
VOLTAGE (mV)
V
FB
VOLTAGE (mV)
799.5
799.0
798.5
798.0
797.5
797.0
–50 –30 –10
10
30
50
TEMPERATURE (°C)
3803 G01
Oscillator Frequency
vs Temperature
210
V
CC
= 8V
OSCILLATOR FREQUENCY (kHz)
210
208
OSCILLATOR FREQUENCY (kHz)
206
204
202
200
198
196
194
192
OSCILLATOR FREQUENCY (kHz)
205
200
195
190
185
180
–50 –30 –10 10 30 50 70
TEMPERATURE (°C)
U W
70
90
Reference Voltage
vs Supply Voltage
801.0
800.8
800.6
800.4
800.2
800.0
799.8
799.6
799.4
799.2
799.0
802
801
800
799
798
797
6
6.5
8
7.5
8.5
V
CC
SUPPLY VOLTAGE (V)
7
9
9.5
3803 F02
Reference Voltage
vs V
CC
Shunt Regulator Current
804
803
T
A
= 25°C
T
A
= 25°C
V
CC
≤
V
CLAMP1mA
110
796
0
5
10
15
I
CC
(mA)
20
25
3803 G03
Oscillator Frequency
vs Supply Voltage
T
A
= 25°C
210
208
206
204
202
200
198
196
194
192
190
6
6.5
8.5
7.5
8
7
V
CC
SUPPLY VOLTAGE (V)
9
3803 G05
Oscillator Frequency
vs V
CC
Shunt Regulator Current
T
A
= 25°C
90
110
190
0
5
15
10
I
CC
(mA)
20
25
3803 G06
3803 G04
3803i
3
LTC3803
TYPICAL PERFOR A CE CHARACTERISTICS
V
CC
Undervoltage Lockout
Thresholds vs Temperature
10.0
V
CC
UNDERVOLTAGE LOCKOUT (V)
9.5
9.0
8.5
V
CC
(V)
V
TURNON
9.7
9.6
9.5
9.4
9.3
V
TURNOFF
SUPPLY CURRENT (µA)
8.0
7.5
7.0
6.5
6.0
5.5
5.0
–50 –30 –10 10 30 50 80
TEMPERATURE (°C)
90
110
Start-Up I
CC
Supply Current
vs Temperature
60
V
CC
= V
TURNON
– 0.1V
SHUTDOWN THRESHOLD (mV)
START-UP SUPPLY CURRENT (µA)
50
40
30
20
10
0
–50 –30 –10 10 30 50 70
TEMPERATURE (°C)
400
350
300
250
200
150
100
–50 –30 –10
I
TH
/RUN PIN CURRENT SOURCE (nA)
Peak Current Sense Voltage
vs Temperature
120
115
V
CC
= 8V
4.0
3.5
SENSE PIN VOLTAGE (mV)
105
100
95
90
85
80
–50 –30 –10 10 30 50 70
TEMPERATURE (°C)
90
110
SOFT-START TIME (ms)
110
4
U W
3803 G07
V
CC
Shunt Regulator Voltage
vs Temperature
10.0
9.9
9.8
265
260
255
250
245
240
235
230
225
220
I
CC
Supply Current
vs Temperature
V
CC
= 8V
V
ITH/RUN
= 1.3V
I
CC
= 25mA
I
CC
= 1mA
9.2
9.1
9.0
–50 –30 –10 10 30 50 70
TEMPERATURE (°C)
90
110
215
–50 –30 –10 10 30 50 70
TEMPERATURE (°C)
90
110
3803 G08
3803 G08
I
TH
/RUN Shutdown Threshold
vs Temperature
450
I
TH
/RUN Start-Up Current Source
vs Temperature
600
500
400
300
200
100
0
–50 –30 –10 10 30 50 70
TEMPERATURE (°C)
V
CC
= V
TURNON
+ 0.1V
V
ITH/RUN
= 0V
90
110
10
30
50
70
90
110
90
110
TEMPERATURE (°C)
3803 G10
3803 G11
3803 G12
Soft-Start Time vs Temperature
3.0
2.5
2.0
1.5
1.0
0.5
0
–50 –30 –10 10 30 50 70
TEMPERATURE (°C)
90
110
3803 G13
3803 G14
3803i
LTC3803
PI FU CTIO S
I
TH
/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 to shut down. In shutdown mode, the NGATE pin
is held low.
GND (Pin 2):
Ground Pin.
V
FB
(Pin 3):
Receives the feedback voltage from an exter-
nal resistive divider across the output.
SENSE (Pin 4):
This pin performs two functions. It moni-
tors switch current by reading the voltage across an
external current sense resistor to ground. It also injects a
current ramp that develops slope compensation voltage
across an optional external programming resistor.
V
CC
(Pin 5):
Supply Pin. Must be closely decoupled to GND
(Pin 2).
NGATE (Pin 6):
Gate Drive for the External N-Channel
MOSFET. This pin swings from 0V to V
CC
.
BLOCK DIAGRA
800mV
REFERENCE
V
CC
SHUNT
REGULATOR
SOFT-
START
CLAMP
CURRENT
COMPARATOR
R
Q
S
+
3
V
FB
–
GND
2
20mV
1.2V
1
I
TH
/RUN
3803 BD
+
–
W
U
U
U
5
V
CC
0.3µA 0.28V
+
–
SHUTDOWN
COMPARATOR
V
CC
< V
TURNON
UNDERVOLTAGE
LOCKOUT
SHUTDOWN
ERROR
AMPLIFIER
V
CC
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
GATE
DRIVER NGATE
6
200kHz
OSCILLATOR
SLOPE
COMP
CURRENT
RAMP
SENSE
4
3803i
5
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