FeaTures
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LTC1872
Constant Frequency
Current Mode Step-Up
DC/DC Controller in SOT-23
DescripTion
The
LTC
®
1872
is a constant frequency current mode step-
up DC/DC controller providing excellent AC and DC load
and line regulation. The device incorporates an accurate
undervoltage lockout feature that shuts down the LTC1872
when the input voltage falls below 2.0V.
The LTC1872 boasts a ±2.5% output voltage accuracy
and consumes only 270µA of quiescent current. For ap-
plications where efficiency is a prime consideration, the
LTC1872 is configured for Burst Mode operation, which
enhances efficiency at low output current.
In shutdown, the device draws a mere 8µA. The high
550kHz constant operating frequency allows the use of a
small external inductor.
The LTC1872 is available in a small footprint 6-lead
SOT-23.
L,
LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
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High Efficiency: Over 90%
High Output Currents Easily Achieved
Wide V
IN
Range: 2.5V to 9.8V
V
OUT
Limited Only by External Components
Constant Frequency 550kHz Operation
Burst Mode™ Operation at Light Load
Current Mode Operation for Excellent Line and Load
Transient Response
Low Quiescent Current: 270µA
Shutdown Mode Draws Only 8µA Supply Current
±2.5% Reference Accuracy
Tiny 6-Lead SOT-23 Package
applicaTions
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Lithium-Ion-Powered Applications
Cellular Telephones
Wireless Modems
Portable Computers
Scanners
Typical applicaTion
R1
0.03
147k
220pF
80.6k
1
2
3
I
TH
/RUN
LTC1872
GND
V
FB
SENSE
–
NGATE
422k
C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: MURATA GRM42-2X5R226K6.3
D1: IR10BQ015
L1: MURATA LQN6C4R7M04
M1: IRLMS2002
R1: DALE 0.25W
V
IN
5
4
6
M1
D1
L1
4.7µH
C1
10µF
10V
V
IN
3.3V
100
95
90
Efficiency vs Load Current
V
IN
= 3.3V
V
OUT
= 5V
+
C2
2× 22F
6.3V
V
OUT
5V
1A
EFFICIENCY (%)
85
80
75
70
65
1
10
100
LOAD CURRENT (mA)
1000
1872 TA01b
1872 TA01
Figure 1. LTC1872 High Output Current 3.3V to 5V Boost Converter
1872fa
For more information
www.linear.com/LTC1872
1
LTC1872
absoluTe MaxiMuM raTings
(Note 1)
pin conFiguraTion
TOP VIEW
I
TH
/RUN 1
GND 2
V
FB
3
6 NGATE
5 V
IN
4 SENSE
–
Input Supply Voltage (V
IN
) ......................... – 0.3V to 10V
SENSE
–
, NGATE Voltages ............. –0.3V to (V
IN
+ 0.3V)
V
FB
, I
TH
/RUN Voltages .............................. –0.3V to 2.4V
NGATE Peak Output Current (< 10µs) ........................ 1A
Storage Ambient Temperature Range ....– 65°C to 150°C
Operating Temperature Range (Note 2)....– 40°C to 85°C
Junction Temperature (Note 3) ............................ 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
S6 PACKAGE
6-LEAD PLASTIC SOT-23
T
JMAX
= 150°C,
θ
JA
= 230°C/W
orDer inForMaTion
LEAD FREE FINISH
LTC1872ES6#PBF
TAPE AND REEL
LTC1872ES6#TRPBF
PART MARKING
LTMK
PACKAGE DESCRIPTION
6-Lead Plastic SOT-23
TEMPERATURE RANGE
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on nonstandard lead based finish parts.
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/
The
l
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 specified. (Note 2)
PARAMETER
Input DC Supply Current
Normal Operation
Sleep Mode
Shutdown
UVLO
Undervoltage Lockout Threshold
Shutdown Threshold (at I
TH
/RUN)
Start-Up Current Source
Regulated Feedback Voltage
V
FB
Input Current
Oscillator Frequency
Gate Drive Rise Time
Gate Drive Fall Time
Peak Current Sense Voltage
V
ITH
/RUN = 0V
0°C to 70°C(Note 5)
– 40°C to 85°C(Note 5)
(Note 5)
V
FB
= 0.8V
C
LOAD
= 3000pF
C
LOAD
= 3000pF
(Note 6)
114
500
l
l
elecTrical characTerisTics
CONDITIONS
Typicals at V
IN
= 4.2V (Note 4)
2.4V ≤ V
IN
≤ 9.8V
2.4V ≤ V
IN
≤ 9.8V
2.4V ≤ V
IN
≤ 9.8V, V
ITH
/RUN = 0V
V
IN
< UVLO Threshold
V
IN
Falling
V
IN
Rising
l
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MIN
TYP
270
230
8
6
MAX
420
370
22
10
2.35
2.40
0.55
0.85
0.820
0.830
50
650
UNITS
µA
µA
µA
µA
V
V
V
µA
V
V
nA
kHz
ns
ns
mV
1.55
1.85
0.15
0.25
0.780
0.770
2.00
2.10
0.35
0.5
0.800
0.800
10
550
40
40
120
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1872E is guaranteed to meet performance 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:
T
J
= T
A
+ (P
D
•
θ
JA
°C/W)
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5:
The LTC1872 is tested in a feedback loop that servos V
FB
to the
output of the error amplifier.
Note 6:
Guaranteed by design at duty cycle = 30%. Peak current sense
voltage is V
REF
/6.67 at duty cycle <40%, and decreases as duty cycle
increases due to slope compensation as shown in Figure 2.
2
1872fa
For more information
www.linear.com/LTC1872
LTC1872
Typical perForMance characTerisTics
Reference Voltage
vs Temperature
825
820
815
V
FB
VOLTAGE (mV)
810
805
800
795
790
785
780
775
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1872 G01
Normalized Oscillator Frequency
vs Temperature
10
8
NORMALIZED FREQUENCY (%)
6
4
2
0
–2
–4
–6
–8
–10
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1872 G02
Undervoltage Lockout Trip
Voltage vs Temperature
2.24
2.20
2.16
UVLO TRIP VOLTAGE (V)
2.12
2.08
2.04
2.00
1.96
1.92
1.88
1.84
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1872 G03
V
IN
= 4.2V
V
IN
= 4.2V
V
IN
FALLING
Maximum Current Sense Trip
Voltage vs Duty Cycle
130
120
V
IN
– V
SENSE
– (mV)
110
100
90
80
70
60
50
20
30
40
50 60 70 80
DUTY CYCLE (%)
90
100
V
IN
= 4.2V
T
A
= 25°C
I
TH
/RUN VOLTAGE (mV)
600
560
520
480
440
400
360
320
280
240
Shutdown Threshold
vs Temperature
V
IN
= 4.2V
200
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1872 G05
1872 G04
pin FuncTions
I
TH
/RUN (Pin 1):
This pin performs two functions. It
serves as the error amplifier compensation point as well
as the run control input. Nominal voltage range for this
pin is 0.7V to 1.9V. Forcing this pin below 0.35V causes
the device to be shut down. In shutdown all functions are
disabled and the NGATE pin is held low.
GND (Pin 2):
Ground Pin.
V
FB
(Pin 3):
Receives the feedback voltage from an external
resistive divider across the output.
1872fa
SENSE
–
(Pin 4):
The Negative Input to the Current Com-
parator.
V
IN
(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
IN
.
For more information
www.linear.com/LTC1872
3
LTC1872
FuncTional DiagraM
V
IN
5
SENSE
–
4
+
ICMP
–
SLOPE
COMP
RS
R
S
Q
V
IN
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
NGATE
6
OSC
V
IN
0.5µA
+
V
IN
0.3V
–
VOLTAGE
REFERENCE
GND
2
UNDERVOLTAGE
LOCKOUT
V
REF
0.8V
operaTion
Main Control Loop
(Refer to Functional Diagram)
The LTC1872 is a constant frequency current mode
switching regulator. During normal operation, the external
N-channel power MOSFET is turned on each cycle by the
oscillator and turned off when the current comparator
(ICMP) resets the RS latch. The peak inductor current
at which ICMP resets the RS latch is controlled by the
voltage on the I
TH
/RUN pin, which is the output of the
error amplifier EAMP. An external resistive divider con-
nected between V
OUT
and ground allows the EAMP to
receive an output feedback voltage V
FB
. When the load
current increases, it causes a slight decrease in V
FB
4
For more information
www.linear.com/LTC1872
+
FREQ
FOLDBACK
–
0.3V
+
–
BURST
CMP
SLEEP
OVP
+
–
V
REF
+
60mV
V
REF
0.8V
V
FB
3
V
IN
0.15V
EAMP
+
–
1 I
TH
/RUN
0.35V
+
–
SHDN
CMP
SHDN
UV
1.2V
1872FD
relative to the 0.8V reference, which in turn causes the
I
TH
/RUN voltage to increase until the average inductor
current matches the new load current.
The main control loop is shut down by pulling the I
TH
/
RUN pin low. Releasing I
TH
/RUN allows an internal 0.5µA
current source to charge up the external compensation
network. When the I
TH
/RUN pin reaches 0.35V, the main
control loop is enabled with the I
TH
/RUN voltage then
pulled up to its zero current level of approximately 0.7V.
As the external compensation network continues to charge
up, the corresponding output current trip level follows,
allowing normal operation.
1872fa
LTC1872
operaTion
Comparator OVP guards against transient overshoots
> 7.5% by turning off the external N-channel power
MOSFET and keeping it off until the fault is removed.
Burst Mode Operation
The LTC1872 enters Burst Mode operation at low load
currents. In this mode, the peak current of the inductor is
set as if V
ITH
/RUN = 1V (at low duty cycles) even though
the voltage at the I
TH
/RUN pin is at a lower value. If the
inductor’s average current is greater than the load require-
ment, the voltage at the I
TH
/RUN pin will drop. When the
I
TH
/RUN voltage goes below 0.85V, the sleep signal goes
high, turning off the external MOSFET. The sleep signal
goes low when the I
TH
/RUN voltage goes above 0.925V
and the LTC1872 resumes normal operation. The next
oscillator cycle will turn the external MOSFET on and the
switching cycle repeats.
Undervoltage Lockout
To prevent operation of the N-channel MOSFET below safe
input voltage levels, an undervoltage lockout is incorpo-
rated into the LTC1872. When the input supply voltage
drops below approximately 2.0V, the N-channel MOSFET
and all circuitry is turned off except the undervoltage block,
which draws only several microamperes.
110
100
90
SF = I
OUT
/I
OUT(MAX)
(%)
80
70
60
50
40
30
20
10
0
I
RIPPLE
= 0.4I
PK
AT 5% DUTY CYCLE
I
RIPPLE
= 0.2I
PK
AT 5% DUTY CYCLE
V
IN
= 4.2V
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1872 F02
Overvoltage Protection
The overvoltage comparator in the LTC1872 will turn the
external MOSFET off when the feedback voltage has risen
7.5% above the reference voltage of 0.8V. This comparator
has a typical hysteresis of 20mV.
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
I
PK
=
V
ITH
−0.7
10
(
R
SENSE
)
when the LTC1872 is operating below 40% duty cycle.
However, once the duty cycle exceeds 40%, slope com-
pensation begins and effectively reduces the peak inductor
current. The amount of reduction is given by the curves
in Figure 2.
Short-Circuit Protection
Since the power switch in a boost converter is not in
series with the power path from input to load, turning off
the switch provides no protection from a short-circuit at
the output. External means such as a fuse in series with
the boost inductor must be employed to handle this fault
condition.
Figure 2. Maximum Output Current vs Duty Cycle
1872fa
For more information
www.linear.com/LTC1872
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