LTC1772
Constant Frequency
Current Mode Step-Down
DC/DC Controller in SOT-23
FEATURES
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DESCRIPTIO
High Efficiency: Up to 94%
High Output Currents Easily Achieved
Wide V
IN
Range: 2.5V to 9.8V
Constant Frequency 550kHz Operation
Burst Mode
®
Operation at Light Load
Low Dropout: 100% Duty Cycle
Tiny 6-Lead SOT-23 Package
0.8V Reference Allows Low Output Voltages
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
The LTC
®
1772 is a constant frequency current mode step-
down DC/DC controller providing excellent AC and DC load
and line regulation. The device incorporates an accurate
undervoltage lockout feature that shuts down the LTC1772
when the input voltage falls below 2.0V.
The LTC1772 provides a
±2.5%
output voltage accuracy
and consumes only 270µA of quiescent current. For
applications where efficiency is a prime consideration, the
LTC1772 is configured for Burst Mode operation, which
enhances efficiency at low output current.
To further maximize the life of a battery source, the
external P-channel MOSFET is turned on continuously in
dropout (100%dutycycle).In shutdown, the device draws
a mere 8µA. High constant operating frequency of 550kHz
allows the use of a small external inductor.
The LTC1772 is available in a small footprint 6-lead
SOT-23.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
APPLICATIO S
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One or Two Lithium-Ion-Powered Applications
Cellular Telephones
Wireless Modems
Portable Computers
Distributed 3.3V, 2.5V or 1.8V Power Systems
Scanners
TYPICAL APPLICATIO
R1
0.03Ω
1
10k
220pF
I
TH
/RUN PGATE
LTC1772
2
3
GND
V
FB
V
IN
SENSE
–
5
4
D1
6
L1
M1 4.7µH
C1
10µF
10V
V
IN
2.5V
TO 9.8V
+
EFFICIENCY (%)
C2A
47µF
6V
C2B
1µF
10V
V
OUT
2.5V
2A
174k
C1: TAIYO YUDEN LMK325BJ106K-T
C2A: SANYO 6TPA47M
C2B: AVX 0805ZC105KAT1A
D1: MOTOROLA MBRM120T3
L1: MURATA LQN6C-4R7
M1: FAIRCHILD FDC638P
R1: IRC LRC-LR1206-01-R030F
80.6k
1772 F01a
Figure 1. High Efficiency, High Output Current 2.5V/2A Regulator
1772fb
U
Efficiency vs Load Current
100
90
80
V
IN
= 6V
70
V
IN
= 9.8V
60
50
V
OUT
= 2.5V
40
1
10
100
1000
LOAD CURRENT (mA)
10000
1772 F01b
U
U
V
IN
= 3.3V
V
IN
= 4.2V
V
IN
= 8.4V
1
LTC1772
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
I
TH
/RUN 1
GND 2
V
FB
3
6 PGATE
5 V
IN
4 SENSE
–
Input Supply Voltage (V
IN
).........................– 0.3V to 10V
SENSE
–
, PGATE Voltages ............. – 0.3V to (V
IN
+ 0.3V)
V
FB
, I
TH
/RUN Voltages .............................– 0.3V to 2.4V
PGATE Peak Output Current (<10µs) ......................... 1A
Storage Ambient Temperature Range ... – 65°C to 150°C
Operating Temperature Range
LTC1772CS6 ........................................... 0°C to 70°C
LTC1772ES6 (Note 2) ........................ – 40°C to 85°C
LTC1772IS6 (Note 2) ......................... – 40°C to 85°C
LTC1772HS6 (Notes 2,3) ................. – 40°C to 140°C
Junction Temperature (Note 3) ............................. 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART NUMBER
LTC1772CS6
LTC1772ES6
LTC1772IS6
LTC1772HS6
S6 PART MARKING
LTIL
LTIM
LTB7
LTBRY
S6 PACKAGE
6-LEAD PLASTIC SOT-23
θ
JA
= 230°C/ W
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 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
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 (LTC1772C)
V
IN
Falling (LTC1772E, LTC1772I, LTC1772H)
V
IN
Rising
(LTC1772C)
(LTC1772E, LTC1772I, LTC1772H)
V
ITH
/RUN = 0V
(Note 5) (LTC1772C)
(Note 5) (LTC1772E, LTC1772I, LTC1772H)
2.4V
≤
V
IN
≤
9.8V (Note 5)
I
TH
/RUN Sinking 5µA (Note 5)
I
TH
/RUN Sourcing 5µA (Note 5)
(Note 5)
Measured at V
FB
V
FB
= 0.8V
V
FB
= 0V
C
LOAD
= 3000pF
C
LOAD
= 3000pF
(Note 6)
0.820
500
●
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●
●
●
ELECTRICAL CHARACTERISTICS
MIN
TYP
270
230
8
6
MAX
420
370
22
10
2.30
2.35
2.40
0.50
0.55
0.85
0.820
0.830
UNITS
µA
µA
µA
µA
V
V
V
V
V
µA
V
V
mV/V
mV/µA
mV/µA
1.60
1.55
1.85
0.20
0.15
0.25
0.780
0.770
2.00
2.00
2.10
0.35
0.35
0.5
0.800
0.800
0.05
2.5
2.5
10
0.860
20
550
120
40
40
120
Shutdown Threshold (at I
TH
/RUN)
Start-Up Current Source
Regulated Feedback Voltage
Output Voltage Line Regulation
Output Voltage Load Regulation
V
FB
Input Current
Overvoltage Protect Threshold
Overvoltage Protect Hysteresis
Oscillator Frequency
Gate Drive Rise Time
Gate Drive Fall Time
Peak Current Sense Voltage
50
0.895
650
2
U
W
U
U
W W
W
nA
V
mV
kHz
kHz
ns
ns
mV
1772fb
LTC1772
ELECTRICAL CHARACTERISTICS
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1772E is guaranteed to meet 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. The LTC1772I is guaranteed to meet specified
performance from –40°C to 85°C. The LTC1772H is guaranteed to meet
specified performance from –40°C to 140°C.
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)
Operation at high junction temperatures degrades operating lifetimes.
Operating lifetimes at junction temperatures greater than 125°C is derated
to 1000 hours.
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5:
The LTC1772 is tested in a feedback loop that servos V
FB
to the
output of the error amplifier.
Note 6:
Peak current sense voltage is reduced dependent on duty cycle to
a percentage of value as given in Figure 2.
TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage
vs Temperature
825
820
815
V
FB
VOLTAGE (mV)
V
IN
= 4.2V
NORMALIZED FREQUENCY (%)
TRIP VOLTAGE (V)
810
805
800
795
790
785
780
775
–55 –35 –15 5 25 45 65 85 105 125 145
TEMPERATURE (°C)
1772 G01
Maximum (V
IN
– SENSE
–
) Voltage
vs Duty Cycle
130
120
TRIP VOLTAGE (mV)
110
100
90
80
70
60
50
20
30
40
50 60 70 80
DUTY CYCLE (%)
90
100
TRIP VOLTAGE (V)
U W
Normalized Frequency
vs Temperature
10
8
6
4
2
0
–2
–4
–6
–8
–10
–55 –35 –15 5 25 45 65 85 105 125 145
TEMPERATURE (°C)
1772 G02
Undervoltage Lockout Trip
Voltage vs Temperature
2.14
2.10
2.06
2.02
1.98
1.94
1.90
1.86
1.82
1.78
1.74
–55 –35 –15 5 25 45 65 85 105 125 145
TEMPERATURE (°C)
1772 G03
V
IN
= 4.2V
V
IN
FALLING
Shutdown Threshold
vs Temperature
550
510
470
430
390
350
310
270
230
190
150
–55 –35 –15 5 25 45 65 85 105 125 145
TEMPERATURE (°C)
1772 G05
V
IN
= 4.2V
T
A
= 25°C
V
IN
= 4.2V
1772 G04
1772fb
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LTC1772
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. The current comparator threshold
increases with this control voltage. 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 PGATE pin is held high.
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.
PGATE (Pin 6):
Gate Drive for the External P-Channel
MOSFET. This pin swings from 0V to V
IN
.
FUNCTIONAL DIAGRA
V
IN
5
SENSE
–
4
+
ICMP
–
RS1
OSC
SLOPE
COMP
R
Q
S
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
V
IN
PGATE
6
FREQ
FOLDBACK
SHORT-CIRCUIT
DETECT
V
IN
0.5µA
+
V
IN
0.3V
–
VOLTAGE
REFERENCE
GND
2
UNDERVOLTAGE
LOCKOUT
V
REF
0.8V
4
–
+
W
U
U
U
U
U
0.3V
0.15V
+
–
BURST
CMP
OVP
+
–
V
REF
+
60mV
SLEEP
EAMP
+
–
V
REF
0.8V
V
FB
3
V
IN
1 I
TH
/RUN
0.35V
+
SHDN
CMP
SHDN
UV
–
1.2V
1772 FD
1772fb
LTC1772
OPERATIO
Main Control Loop
The LTC1772 is a constant frequency current mode switch-
ing regulator. During normal operation, the external
P-channel power MOSFET is turned on each cycle when
the oscillator sets the RS latch (RS1) and turned off when
the current comparator (ICMP) resets the 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 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
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 P-channel power
MOSFET and keeping it off until the fault is removed.
Burst Mode Operation
The LTC1772 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 LTC1772 resumes normal operation. The next
U
(Refer to Functional Diagram)
oscillator cycle will turn the external MOSFET on and the
switching cycle repeats.
Dropout Operation
When the input supply voltage decreases towards the
output voltage, the rate of change of inductor current
during the ON cycle decreases. This reduction means that
the external P-channel MOSFET will remain on for more
than one oscillator cycle since the inductor current has not
ramped up to the threshold set by EAMP. Further reduc-
tion in input supply voltage will eventually cause the
P-channel MOSFET to be turned on 100%, i.e., DC. The
output voltage will then be determined by the input voltage
minus the voltage drop across the MOSFET, the sense
resistor and the inductor.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below safe
input voltage levels, an undervoltage lockout is incorpo-
rated into the LTC1772. When the input supply voltage
drops below approximately 2.0V, the P-channel MOSFET
and all circuitry is turned off except the undervoltage block,
which draws only several microamperes.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 120kHz. This lower
frequency allows the inductor current to safely discharge,
thereby preventing current runaway. The oscillator’s fre-
quency will gradually increase to its designed rate when
the feedback voltage again approaches 0.8V.
Overvoltage Protection
As a further protection, the overvoltage comparator in the
LTC1772 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.
1772fb
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