LTC3873
No R
SENSE
TM
Constant
Frequency Current Mode
Boost/Flyback/SEPIC
DC/DC Controller
FEATURES
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DESCRIPTION
The LTC
®
3873 is a constant frequency current mode,
boost, flyback or SEPIC DC/DC controller that drives an
N-channel power MOSFET in high input and output volt-
age converter applications. Soft-start can be programmed
using an external capacitor.
The LTC3873 provides ±1.5% output voltage accuracy and
consumes only 300μA quiescent current during normal
operation and only 55μA during micropower start-up.
Using a 9.3V internal shunt regulator, the LTC3873 can
be powered from a high input voltage through a resistor
or it can be powered directly from a low impedance DC
voltage of 9V or less.
The LTC3873 is available in 8-lead ThinSOT and 2mm
×
3mm DFN packages.
PARAMETER
V
CC
UV
+
V
CC
UV
–
LTC3873
8.4V
4V
LTC3873-5
3.9V
2.9V
V
IN
and V
OUT
Limited Only by External Components
Internal or Programmable External Soft-Start
Constant Frequency 200kHz Operation
Adjustable Current Limit
Current Sense Resistor Optional
Maximum 60V on SW Node with R
DS(ON)
Sensing
±1.5% Voltage Reference Accuracy
Current Mode Operation for Excellent Line and Load
Transient Response
Low Quiescent Current: 300μA
Low Profile (1mm) ThinSOT
TM
and (0.75mm)
2mm
×
3mm DFN Package
APPLICATIONS
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Telecom Power Supplies
42V and 12V Automotive Power Supplies
Portable Electronic Equipment
L,
LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
No R
SENSE
and ThinSOT are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
5V Output Nonisolated Telecom Power Supply
V
IN
36V TO 72V
4.7μF
100V
X5R
10μF
10V
X5R
7.5k
2.2nF
0.1μF
D2
V
OUT
5V
2A MAX
EFFICIENCY (%)
Efficiency and Power Loss vs Load Current
100
90
80
70
60
50
40
30
20
V
IN
= 72V
V
IN
= 60V
V
IN
= 48V
V
IN
= 36V
10
3873 TA01a
3000
EFFICIENCY
2500
POWER LOSS (mW)
2000
1500
POWER LOSS
1000
500
•
221k
D1
T1
100μF
6.3V
X5R
3
•
V
CC
I
TH
NGATE
LTC3873
RUN/SS
GND
SW
IPRG
V
FB
12.1k
38.3k
M1
68mΩ
10
0
LOAD CURRENT (mA)
10
1000
3873 TA01b
3873fa
1
LTC3873
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V
CC
to GND
Low Impedance Source ........................... –0.3V to 9V
Current Fed ..........................................25mA Into V
CC
RUN/SS........................................................ –0.3V to 9V
IPRG Voltage.................................–0.3V to (V
CC
+ 0.3V)
V
FB
, I
TH
Voltages ....................................... –0.3V to 2.4V
SW Voltage ................................................ –0.3V to 60V
Operating Temperature Range (Note 2).... –40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range................... –65°C to 125°C
Lead Temperature (Soldering, 10 sec)
TS8 Package ..................................................... 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
IPRG 1
I
TH
2
V
FB
3
GND 4
8 SW
7 RUN/SS
6 V
CC
5 NGATE
GND 1
V
FB
2
I
TH
3
IPRG 4
9
8
7
6
5
NGATE
V
CC
RUN/SS
SW
TS8 PACKAGE
8-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 230°C/W
DDB PACKAGE
8-LEAD (3mm 2mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 76°C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3873ETS8#PBF
LTC3873EDDB#PBF
LEAD BASED FINISH
LTC3873ETS8
LTC3873EDDB
TAPE AND REEL
LTC3873ETS8\#TRPBF
LTC3873EDDB#TRPBF
TAPE AND REEL
LTC3873ETS8#TR
LTC3873EDDB#TR
PART MARKING
LTCSN
LCSK
PART MARKING
LTCSN
LCSK
PACKAGE DESCRIPTION
8-Lead Plastic TSOT-23
8-Lead (3mm
×
2mm) Plastic DFN
PACKAGE DESCRIPTION
8-Lead Plastic TSOT-23
8-Lead (3mm
×
2mm) Plastic DFN
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
3873fa
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LTC3873
ELECTRICAL CHARACTERISTICS
PARAMETER
Input DC Supply Current
Normal Operation
Shutdown
UVLO
Undervoltage Lockout Threshold
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 9V unless otherwise noted. (Note 2)
CONDITIONS
Typicals
V
ITH
= 1.9V
V
RUN/SS
= 0V
V
CC
= UVLO Threshold – 100mV, V
RUN/SS
= V
CC
V
CC
Rising
V
CC
Falling
V
CC
Hysteresis
V
RUN/SS
Falling
V
RUN/SS
Rising
(Note 5)
5.6V < V
CC
< 9V (Note 5)
V
ITH
= 1.6V (Note 5)
V
ITH
= 1V (Note 5)
(Note 5)
70
V
RUN/SS
= 0V
V
RUN/SS
= 1.3V
1.5
5
160
C
LOAD
= 3000pF (Note 6)
C
LOAD
= 3000pF (Note 6)
IPRG = GND
IPRG = Float
IPRG = V
IN
I
IN
= 1mA, I
IN
= 25mA, V
RUN/SS
= 0V
l
l
l
l
l
l
l
l
l
MIN
TYP
300
55
45
MAX
400
100
60
8.8
4.4
4.8
0.9
1.0
1.218
UNITS
μA
μA
μA
V
V
V
V
V
V
mV/V
%
%
7.9
3.5
4.0
0.5
0.6
1.182
8.4
4.0
4.4
0.7
0.8
1.2
0.12
0.05
–0.05
25
78
3
15
20
200
40
40
Shutdown Threshold (at RUN/SS)
Regulated Feedback Voltage
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
V
FB
Input Current
Maximum Duty Cycle
RUN/SS Pull-Up Current
I
SLMAX
, Peak Slope Compensation Current
Oscillator Frequency
Gate Drive Rise Time
Gate Drive Fall Time
Peak Current Sense Voltage
50
84
4.5
25
240
nA
%
μA
μA
μA
kHz
ns
ns
95
165
265
9
110
185
295
9.3
3.3
125
210
325
9.6
mV
mV
mV
V
ms
V
IN
Shunt Regulator Voltage
Default Internal Soft-Start
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 LTC3873E is guaranteed to meet performance specifications
from 0°C to 85°C junction temperature. Specifications over the –40°C
to 85°C operating junction 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:
T
J
= T
A
+ (P
D
•
θ
JA
)
Note 4:
The dynamic input supply current is higher due to power MOSFET
gate charging (Q
G
• f
OSC
). See Applications Information.
Note 5:
The LTC3873 is tested in a feedback loop which servos V
FB
to
the reference voltage with the I
TH
pin forced to the midpoint of its voltage
range (0.7V ≤ V
ITH
≤ 1.9V, midpoint = 1.3V).
Note 6:
Rise and fall times are measured at 10% and 90% levels.
V
CC
= 5.6V.
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LTC3873
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage vs Temperature
1.25
1.24
1.23
1.22
1.21
1.20
1.19
1.18
–60 –40 –20
1.2025
1.2020
2.0
V
FB
VOLTAGE (V)
1.2015
V
FB
VOLTAGE (V)
I
TH
VOLTAGE (V)
1.2010
1.2005
1.2000
0.5
1.1995
1.1990
5
6
7
V
IN
(V)
3873 G02
Feedback Voltage Line Regulation
2.5
I
TH
Voltage vs RUN/SS Voltage
V
IN
= 5V
1.5
1.0
40 60 80 100 120
TEMPERATURE (°C)
3873 G01
0
20
8
9
10
0
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
RUN/SS VOLTAGE (V)
3873 G03
Shutdown Mode I
Q
vs V
IN
70
65
SHUTDOWN MODE I
Q
(μA)
SHUTDOWN MODE I
Q
(μA)
60
55
50
45
40
35
30
25
20
3
4
5
7
6
V
IN
(V)
8
9
10
3873 G04
Shutdown I
Q
vs Temperature
80
70
60
TIME (ns)
50
40
30
20
10
0
–60 –40 –20
0 20 40 60 80 100 120
TEMPERATURE (°C)
3873 G05
Gate Drive Rise and Fall Time
vs C
LOAD
100
90
80
70
60
50
40
30
20
10
0
0
2000
6000
4000
C
LOAD
(pF)
8000
10000
3873 G06
RISE TIME
FALL TIME
RUN Threshold vs Temperature
1.0
10.2
10.1
REGULATION VOLTAGE (V)
0.9
RUN THRESHOLDS (V)
0.8
RISING
10.0
9.9
9.8
9.7
9.6
9.5
9.4
9.3
0.5
–60 –40 –20
0 20 40 60 80 100 120
TEMPERATURE (°C)
3873 G07
Shunt Regulation Voltage
vs I
SHUNT
0.7
FALLING
0.6
9.2
0
5
10
15
20 25 30
I
SHUNT
(mA)
35
40
45
3873 G08
3873fa
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LTC3873
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency vs Temperature
250
MAXIMUM SENSE THRESHOLD (mV)
300
IPRG = V
IN
250
200
150
IPRG = GND
100
50
0
–60 –40 –20
IPRG = FLOAT
Maximum Sense Threshold
vs Temperature
230
FREQUECY (kHz)
210
190
170
150
–60 –40 –20
0 20 40 60 80 100 120
TEMPERATURE (°C)
3873 G09
0 20 40 60 80 100 120
TEMPERATURE (°C)
3873 G10
PIN FUNCTIONS
(TS8/DDB)
IPRG (Pin 1/Pin 4):
Current Sense Limit Select Pin.
I
TH
(Pin 2/Pin 3):
This pin serves as the error amplifier
compensation point. Nominal voltage range for this pin
is 0.7V to 1.9V.
V
FB
(Pin 3/Pin 2):
This pin receives the feedback voltage
from an external resistor divider across the output.
GND (Pin 4/Pin 1):
Ground Pin.
NGATE (Pin 5/Pin 8):
Gate Drive for the External N-Channel
MOSFET. This pin swings from 0V to V
IN
.
V
CC
(Pin 6/Pin 7):
Supply Pin. This pin must be closely
decoupled to GND (Pin 4).
RUN/SS (Pin 7/Pin 6):
Shutdown and External Soft-Start
Pin. In shutdown, all functions are disabled and the NGATE
pin is held low.
SW (Pin 8/Pin 5):
Switch node connection to inductor and
current sense input pin through external slope compensa-
tion resistor. Normally, the external N-channel MOSFET’s
drain is connected to this pin.
Exposed Pad (NA/Pin 9):
Ground. Must be soldered to PCB
for electrical contact and rated thermal performance.
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