LTC1872B
Constant Frequency
Current Mode Step-Up
DC/DC Controller in ThinSOT
FEATURES
s
s
s
s
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DESCRIPTIO
s
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Burst Mode
TM
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
TM
Package
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 accu-
rate 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and ThinSOT are trademarks of Linear Technology Corporation.
APPLICATIO S
s
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Optical Communications
Lithium-Ion-Powered Applications
Cellular Telephones
Wireless Devices
Portable Computers
Scanners
TYPICAL APPLICATION
R1
0.03Ω
147k
220pF
80.6k
1
I
TH
/RUN
GND
V
FB
V
IN
SENSE
–
NGATE
422k
C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: MURATA GRM42-2X5R106K010AL
D1: IR10BQ015
L1: MURATA LQN6C4R7M04
M1: Si2302DS
R1: DALE 0.25W
5
L1
4.7µH
V
OUT
5V
1A
C1
10µF
10V
V
IN
3.3V
100
95
90
EFFICIENCY (%)
85
80
75
70
65
1
10
100
LOAD CURRENT (mA)
1000
1872B TA01b
LTC1872B
2
3
4
6
M1
D1
C2
4× 10µF
10V
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.
U
Typical Efficiency vs Load Current*
V
IN
= 3.3V
V
OUT
= 5V
U
U
1
LTC1872B
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
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
ORDER PART
NUMBER
LTC1872BES6
S6 PART MARKING
LTXY
S6 PACKAGE
6-LEAD PLASTIC SOT-23
T
JMAX
= 150°C,
θ
JA
= 230°C/ W
Consult LTC marketing for parts specified with wider operating temperature ranges.
The
q
denotes specifications that 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
q
q
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
q
q
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 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
U
W
U
U
W W
W
LTC1872B
TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage
vs Temperature
825
820
815
V
IN
= 4.2V
10
8
NORMALIZED FREQUENCY (%)
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
4
2
0
–2
–4
–6
–8
–10
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1872B G02
UVLO TRIP VOLTAGE (V)
Maximum Current Sense Trip
Voltage vs Duty Cycle
130
120
V
IN
– V
SENSE
– (mV)
I
TH
/RUN VOLTAGE (mV)
110
100
90
80
70
60
50
20
30
40
50 60 70 80
DUTY CYCLE (%)
90
100
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 exter-
nal resistive divider across the output.
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
.
U W
Normalized Oscillator Frequency
vs Temperature
V
IN
= 4.2V
2.24
2.20
2.16
2.12
2.08
2.04
2.00
1.96
1.92
1.88
Undervoltage Lockout Trip
Voltage vs Temperature
V
IN
FALLING
6
1.84
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1872B G03
Shutdown Threshold
vs Temperature
600
560
520
480
440
400
360
320
280
240
200
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1872B G05
V
IN
= 4.2V
T
A
= 25°C
V
IN
= 4.2V
187B2 G04
U
U
U
3
LTC1872B
FUNCTIONAL DIAGRA
V
IN
5
SENSE
–
4
15mV
OSC
FREQ
FOLDBACK
0.5µA
+
V
IN
–
VOLTAGE
REFERENCE
GND
2
UNDERVOLTAGE
LOCKOUT
V
REF
0.8V
OPERATIO
(Refer to Functional Diagram)
Main Control Loop
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 connected
between V
OUT
and ground allows the EAMP to receive an
output feedback voltage V
FB
. When the load current in-
creases, 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
–
+
W
+
ICMP
U
U
U
–
RS
SLOPE
COMP
R
Q
S
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
V
IN
NGATE
6
OVP
0.3V
+
–
V
REF
+
60mV
EAMP
+
–
V
REF
0.8V
V
FB
3
V
IN
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 genera-
tion over a wide load current range.
LTC1872B
OPERATIO
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 improve-
ment 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 compara-
tor has a typical hysteresis of 20mV.
SF = I
OUT
/I
OUT(MAX)
(%)
20mV AC/DIV
V
IN
= 3.3V
V
OUT
= 5V
I
OUT
= 50mA
(2a) V
OUT
Ripple for Figure 1 Circuit
Using LTC1872 Burst Mode Operation
U
(Refer to Functional Diagram)
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 induc-
tor current. The amount of reduction is given by the curves
in Figure 3.
110
100
90
80
70
60
50
40
30
20
10
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1872B F03
I
RIPPLE
= 0.4I
PK
AT 5% DUTY CYCLE
I
RIPPLE
= 0.2I
PK
AT 5% DUTY CYCLE
V
IN
= 4.2V
Figure 3. Maximum Output Current vs Duty Cycle
20mV AC/DIV
5µs/DIV
1872B F02a
V
IN
= 3.3V
V
OUT
= 5V
I
OUT
= 50mA
5µs/DIV
1872B F02b
(2b) V
OUT
Ripple for Figure 1 Circuit Using
LTC1872B Non-Burst Mode Operation
Figure 2. Output Ripple Waveforms for the Circuit of Figure 1
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