LTC3872
No R
SENSE
Current Mode Boost
DC/DC Controller
FEATURES
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DESCRIPTIO
No Current Sense Resistor Required
V
OUT
up to 60V
Constant Frequency 550kHz Operation
Internal Soft-Start and Optional External Soft-Start
Adjustable Current Limit
Pulse Skipping at Light Load
V
IN
Range: 2.75V to 9.8V
±1.5% Voltage Reference Accuracy
Current Mode Operation for Excellent Line and Load
Transient Response
Low Profile (1mm) SOT-23 and 3mm
×
2mm DFN
Packages
The LTC
®
3872 is a constant frequency current mode
boost DC/DC controller that drives an N-channel power
MOSFET and requires very few external components. The
No R
SENSE
TM
architecture eliminates the need for a sense
resistor, improves efficiency and saves board space.
The LTC3872 provides excellent AC and DC load and line
regulation with ±1.5% output voltage accuracy. It incor-
porates an undervoltage lockout feature that shuts down
the device when the input voltage falls below 2.3V.
High switching frequency of 550kHz allows the use of a
small inductor. The LTC3872 is available in an 8-lead low
profile (1mm) ThinSOT
TM
package and 8-pin 3mm
×
2mm
DFN package.
, 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.
Protected by U.S. Patents including 6498466, 6611131, 5731694.
APPLICATIO S
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Telecom Power Supplies
42V Automotive Systems
24V Industrial Controls
IP Phone Power Supplies
TYPICAL APPLICATIO
1.8nF
High Efficiency 3.3V Input, 5V Output Boost Converter
17.4k
I
TH
47pF
V
IN
IPRG
LTC3872
GND
11k
1%
34.8k
1%
SW
M1
V
OUT
5V
2A
V
IN
10μF
EFFICIENCY (%)
1μH
D1
V
IN
3.3V
Efficiency and Power Loss vs Load Current
100
90
80
70
60
50
40
30
20
10
0.01
0.1
1
POWER LOSS (W)
10
V
FB
RUN/SS
NGATE
1nF
100μF
×2
3872 TA01
0
1
10
100
1000
LOAD CURRENT (mA)
U
0.001
10000
3872 TA01b
U
U
3872fa
1
LTC3872
ABSOLUTE
(Note 1)
AXI U RATI GS
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
Input Supply Voltage (V
IN
), RUN/SS .......... –0.3V to 10V
IPRG Voltage..................................–0.3V to (V
IN
+ 0.3V)
V
FB
, I
TH
Voltages ....................................... –0.3V to 2.4V
SW Voltage ................................................ –0.3V to 60V
PACKAGE/ORDER I FOR ATIO
TOP VIEW
IPRG 1
I
TH
2
V
FB
3
GND 4
8 SW
7 RUN/SS
6 V
IN
5 NGATE
TS8 PACKAGE
8-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 230°C/W
ORDER PART NUMBER
LTC3872ETS8
TS8 PART MARKING
LCGB
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.
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 4.2V unless otherwise noted. (Note 2)
PARAMETER
Input Voltage Range
Input DC Supply Current
Normal Operation
Shutdown
UVLO
Undervoltage Lockout Threshold
Shutdown Threshold (at RUN/SS)
Regulated Feedback Voltage
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
V
FB
Input Current
RUN/SS Pull Up Current
Typicals at V
IN
= 4.2V (Note 4)
2.75V ≤ V
IN
≤ 9.8V, V
ITH
= 1.3V
V
RUN/SS
= 0V
V
IN
< UVLO Threshold, V
RUN
= 0V
V
IN
Rising
V
IN
Falling
V
RUN/SS
Falling
V
RUN/SS
Rising
(Note 5)
2.75V < V
IN
< 9V (Note 5)
V
ITH
= 1.6V (Note 5)
V
ITH
= 1V (Note 5)
(Note 5)
V
RUN/SS
= 0
0.35
●
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
●
2
U
U
W
W W
U
W
TOP VIEW
GND 1
V
FB
2
I
TH
3
IPRG 4
9
8
7
6
5
NGATE
V
IN
RUN/SS
SW
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 PART NUMBER
LTC3872EDDB
DDB PART MARKING
LCHT
MIN
2.75
TYP
MAX
9.8
UNITS
V
μA
μA
μA
V
V
V
V
mV/V
%
%
250
8
20
2.3
2.05
0.6
0.7
1.182
2.45
2.3
0.85
0.95
1.2
0.14
0.05
–0.05
25
0.7
400
20
35
2.75
2.55
1.05
1.15
1.218
50
1.25
nA
μA
3872fa
LTC3872
ELECTRICAL CHARACTERISTICS
PARAMETER
Oscillator Frequency
Normal Operation
Gate Drive Rise Time
Gate Drive Fall Time
Peak Current Sense Voltage
V
FB
= 1.2V
C
LOAD
= 3000pF
C
LOAD
= 3000pF
IPRG = GND (Note 6)
IPRG = Float
IPRG = V
IN
●
●
●
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 4.2V unless otherwise noted. (Note 2)
CONDITIONS
MIN
500
TYP
550
40
40
80
145
240
100
170
270
1
120
195
290
MAX
650
UNITS
kHz
ns
ns
mV
mV
mV
ms
Default Internal 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 LTC3872E 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
• 110°C/W)
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 LTC3872 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.
TYPICAL PERFOR A CE CHARACTERISTICS
FB 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
FB VOLTAGE (V)
I
TH
VOLTAGE (V)
FB VOLTAGE (V)
40 60
TEMPERATURE (˚C)
0
20
U W
80
T
A
= 25°C unless otherwise noted.
I
TH
Voltage vs RUN/SS Voltage
2.5
FB Voltage Line Regulation
2.0
1.2015
1.2010
1.2005
1.2000
0.5
1.1995
1.1990
100
0
1
2
3
4
5
V
IN
(V)
3872 G01
3872 G02
1.5
1.0
0
6
7
8
9
10
V
IN
= 2.5V
V
IN
= 3.3V
V
IN
= 5V
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
RUN VOLTAGE (V)
3872 G03
3872fa
3
LTC3872
TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown I
Q
vs V
IN
14
12
SHUTDOWN MODE IQ (μA)
SHUTDOWN IQ (μA)
FREQUENCY (kHz)
10
8
6
4
2
0
2
3
4
5
6
V
IN
(V)
3872 G04
7
Gate Drive Rise and Fall Time
vs C
LOAD
100
90
80
RUN THRESHOLDS (V)
70
TIME (ns)
60
50
40
30
20
10
0
0
2000
6000
4000
C
LOAD
(pF)
8000
10000
3872 G07
RISE TIME
FALL TIME
0.94
0.92
0.90
0.88
0.86
0.84
0
2
4
6
V
IN
(V)
8
10
12
3872 G08
RUN THRESHOLDS (V)
Frequency vs Temperature
600
MAXIMUM SENSE THRESHOLD (mV)
300
575
FREQUENCY (kHz)
550
525
500
–50
4
U W
8
9
–5
0
T
A
= 25°C unless otherwise noted.
Frequency vs Duty Cycle
600
500
Shutdown I
Q
vs Temperature
20
15
400
300
200
100
0
–50 –25
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
3872 G05
10
5
10
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
3278 G06
RUN/SS Threshold vs V
IN
1.00
0.98
RISING
0.96
0.9
0.8
1.0
RUN/SS Threshold vs
Temperature
RISING
FALLING
0.7
FALLING
0.6
0.5
–50 –25
0
25 50
75 100 125 150
TEMPERATURE (°C)
3872 G09
Maximum Sense Threshold
vs Temperature
IPRG = V
IN
250
200
IPRG = FLOAT
150
100
50
0
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
3872 G11
IPRG = GND
25 50 75 100 125 150
TEMPERATURE (°C)
3872 G10
3872fa
LTC3872
PI FU CTIO S
IPRG (Pin 1/Pin 4):
Current Sense Limit Select Pin.
I
TH
(Pin 2/Pin 3):
It serves as the error amplifier com-
pensation point. Nominal voltage range for this pin is
0.7V to 1.9V.
V
FB
(Pin 3/Pin 2):
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
IN
(Pin 6/Pin 7):
Supply Pin. This pin must be closely
decoupled to GND (Pin 4).
FUNCTIONAL DIAGRAM
V
IN
GND
SW
UNDERVOLTAGE
LOCKOUT
SHUTDOWN
COMPARATOR
0.7μA
RUN/SS
I
TH
3872 FD
+
INTERNAL
SOFT-START
RAMP
–
U
U
U
(TS8/DD8)
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.
UV
VOLTAGE
REFERENCE
1.2V
SLOPE
COMPENSATION
–
+
CURRENT
COMPARATOR
IPRG
I
LIM
+
SHDN
I
TH
BUFFER
–
RS
LATCH
R
Q
S
550kHz
OSCILLATOR
CURRENT LIMIT
CLAMP
SWITCHING
LOGIC CIRCUIT
ERROR
AMPLIFIER
V
IN
NGATE
V
FB
1.2V
3872fa
5
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