LTC1872B
Constant Frequency
Current Mode Step-Up
DC/DC Controller in ThinSOT
FeaTures
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DescripTion
The
LTC
®
1872B
is a constant frequency current mode
step-up DC/DC controller providing excellent AC and DC
load and line regulation. The device incorporates an ac-
curate undervoltage lockout feature that shuts down the
LTC1872B when the input voltage falls below 2.0V.
The LTC1872B provides a ± 2.5% output voltage accuracy
and consumes only 270µA of quiescent current. In shut-
down, the device draws a mere 8µA.
High constant operating frequency of 550kHz allows the
use of a small external inductor. The constant frequency
operation is maintained down to very light loads, resulting
in less low frequency noise generation over a wide load
current range.
The LTC1872B is available in a 6-lead low profile (1mm)
ThinSOT package. For a Burst Mode operation enabled
version of the LTC1872B, please refer to the LTC1872
data sheet.
L,
LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
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Burst Mode™ Operation Disabled for Lower Output
Ripple at Light Loads
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
Current Mode Operation for Excellent Line and Load
Transient Response
Shutdown Mode Draws Only 8µA Supply Current
Low Profile (1mm) ThinSOT™ Package
applicaTions
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Optical Communications
Lithium-Ion-Powered Applications
Cellular Telephones
Wireless Devices
Portable Computers
Scanners
Typical applicaTion
R1
0.03
147k
220pF
80.6k
1
2
3
I
TH
/RUN
GND
V
FB
V
IN
SENSE
–
C1
10µF
10V
V
IN
3.3V
Typical Efficiency vs Load Current*
100
95
90
V
IN
= 3.3V
V
OUT
= 5V
5
4
6
NGATE
422k
M1
D1
C2
4× 10µF
10V
V
OUT
5V
1A
EFFICIENCY (%)
LTC1872B
L1
4.7µH
85
80
75
70
65
1
10
100
LOAD CURRENT (mA)
1000
1872B TA01b
C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: MURATA GRM42-2X5R106K010AL
D1: IR10BQ015
L1: MURATA LQN6C4R7M04
M1: Si2302DS
R1: DALE 0.25W
1872B TA01
Figure 1. LTC1872B High Output Current 3.3V to 5V Boost Converter
*Output ripple waveforms for the circuit of Figure 1 appear in Figure 2.
1872bfa
For more information
www.linear.com/LTC1872B
1
LTC1872B
absoluTe MaxiMuM raTings
(Note 1)
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
pin conFiguraTion
TOP VIEW
I
TH
/RUN 1
GND 2
V
FB
3
6 NGATE
5 V
IN
4 SENSE
–
S6 PACKAGE
6-LEAD PLASTIC SOT-23
T
JMAX
= 150°C,
θ
JA
= 230°C/W
orDer inForMaTion
LEAD FREE FINISH
LTC1872BES6#PBF
TAPE AND REEL
LTC1872BES6#TRPBF
PART MARKING
LTXY
PACKAGE DESCRIPTION
16-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
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
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)
l
elecTrical characTerisTics
MIN
TYP
270
230
8
6
2.00
2.10
0.35
0.5
0.800
0.800
10
550
40
40
120
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
l
l
1.55
1.85
0.15
0.25
0.780
0.770
500
l
l
114
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1872BE 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 LTC1872B 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 3.
2
1872bfa
For more information
www.linear.com/LTC1872B
LTC1872B
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)
1872B G01
Normalized Oscillator Frequency
vs Temperature
10
8
NORMALIZED FREQUENCY (%)
6
UVLO TRIP VOLTAGE (V)
4
2
0
–2
–4
–6
–8
–10
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1872B G02
Undervoltage Lockout Trip
Voltage vs Temperature
2.24
2.20
2.16
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)
1872B 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)
1872B G05
187B2 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.
1872bfa
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/LTC1872B
3
LTC1872B
FuncTional DiagraM
V
IN
5
SENSE
–
4
15mV
+
ICMP
–
SLOPE
COMP
RS
R
S
Q
V
IN
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
NGATE
6
OSC
0.5µA
+
V
IN
–
VOLTAGE
REFERENCE
GND
2
UNDERVOLTAGE
LOCKOUT
V
REF
0.8V
operaTion
Main Control Loop
(Refer to Functional Diagram)
The LTC1872B 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
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
4
For more information
www.linear.com/LTC1872B
+
FREQ
FOLDBACK
–
OVP
0.3V
+
–
V
REF
+
60mV
V
REF
0.8V
V
FB
3
V
IN
EAMP
+
–
1 I
TH
/RUN
0.35V
+
–
SHDN
CMP
SHDN
UV
1.2V
1872B FD
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.
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.
Low Load Current Operation
Under very light load current conditions, the I
TH
/RUN pin
voltage will be very close to the zero current level of 0.85V.
As the load current decreases further, an internal offset at
the current comparator input will assure that the current
comparator remains tripped (even at zero load current) and
the regulator will start to skip cycles, as it must, in order
to maintain regulation. This behavior allows the regulator
to maintain constant frequency down to very light loads,
resulting in less low frequency noise generation over a
wide load current range.
1872bfa
LTC1872B
operaTion
(Refer to Functional Diagram)
Figure 2 illustrates this result for the circuit of Figure 1
using both an LTC1872 in Burst Mode operation and an
LTC1872B (non-Burst Mode operation). At an output
current of 50mA, the Burst Mode operation part exhibits
an output ripple of approximately 80mV
P-P
, whereas the
non-Burst Mode operation part has an output ripple of
≈45mV
P-P
. At lower output current levels, the improvement
is even greater. This comes at a trade off of slightly lower
efficiency for the non-Burst Mode operation part. Also
notice the constant frequency operation of the LTC1872B,
even at 5% of maximum output current.
Undervoltage Lockout
To prevent operation of the N-channel MOSFET below safe
input voltage levels, an undervoltage lockout is incorpo-
rated into the LTC1872B. 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.
Overvoltage Protection
The overvoltage comparator in the LTC1872B 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 LTC1872B 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 3.
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 (%)
1872B F03
Figure 3. Maximum Output Current vs Duty Cycle
20mV AC/DIV
20mV AC/DIV
V
IN
= 3.3V
V
OUT
= 5V
I
OUT
= 50mA
5µs/DIV
1872B F02a
V
IN
= 3.3V
V
OUT
= 5V
I
OUT
= 50mA
5µs/DIV
1872B F02b
(2a) V
OUT
Ripple for Figure 1 Circuit
Using LTC1872 Burst Mode Operation
(2b) V
OUT
Ripple for Figure 1 Circuit Using
LTC1872B Non-Burst Mode Operation
1872bfa
Figure 2. Output Ripple Waveforms for the Circuit of Figure 1
For more information
www.linear.com/LTC1872B
5
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