FeaTures
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LTC1779
250mA Current Mode
Step-Down DC/DC Converter
in ThinSOT
DescripTion
The
LTC
®
1779
is a constant frequency current mode step-
down DC/DC converter in a 6-lead ThinSOT package. The
part operates with a 2.5V to 9.8V input and can provide
up to 250mA of output current. Current mode control
provides excellent AC and DC load and line regulation.
The device incorporates an accurate undervoltage lockout
feature that shuts down the LTC1779 when the input volt-
age falls below 2V.
The LTC1779 boasts a ± 2.5% output voltage accuracy
and consumes only 135µA of quiescent current. For ap-
plications where efficiency is a prime consideration, the
LTC1779 is configured for Burst Mode operation, which
enhances efficiency at low output current.
To further maximize the life of a battery source, the
internal P-channel MOSFET is turned on continuously in
dropout (100% duty cycle). In shutdown, the device draws
a mere 8µA. High constant operating frequency of 550kHz
allows the use of a small external inductor.
The LTC1779 is available in a low profile (1mm) ThinSOT
package.
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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High Efficiency: Up to 94%
250mA Output Current
Wide V
IN
Range: 2.5V to 9.8V
550kHz Constant Frequency Operation
Burst Mode™ Operation at Light Load
Low Dropout: 100% Duty Cycle
0.8V Reference Allows Low Output Voltages
±2.5% Reference Accuracy
Current Mode Operation for Excellent Line and Load
Transient Response
Low Quiescent Current: 135µA
Shutdown Mode Draws Only 8µA Supply Current
Low Profile (1mm) ThinSOT™ Package
applicaTions
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1- or 2-Cell Lithium-Ion-Powered Applications
Cellular Telephones
Wireless Modems
Portable Computers
Distributed 3.3V, 2.5V or 1.8V Power Systems
Scanners
Typical applicaTion
C3
0.1µF
1
20k
100pF
3
2
I
TH
/RUN
LTC1779
GND
V
FB
V
IN
SENSE
–
SW
6
5
4
R1
10
D1
L1
22µH
C1
10µF
16V
V
IN
2.5V
TO 9.8V
V
OUT
2.5V
100mA
EFFICIENCY (%)
Efficiency vs Load Current
100
90
80
70
60
50
40
30
0.1
1
V
OUT
= 2.5V
R
SENSE
= 10
10
100
LOAD CURRENT (mA)
1000
1779 F01b
+
C2
47µF
6V
V
IN
= 3.3V
V
IN
= 6V
V
IN
= 9.8V
169k
78.7k
1779 F01a
C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
D1: IR10BQ015
L1: COILTRONICS UP1B220
Figure 1. LTC1779 High Efficiency 2.5V/100mA Step-Down Converter
1779fa
For more information
www.linear.com/LTC1779
1
LTC1779
absoluTe MaxiMuM raTings
Input Supply Voltage (V
IN
) ......................... – 0.3V to 10V
SENSE
–
, SW Voltages ................... –0.3V to (V
IN
+ 0.3V)
V
FB
, I
TH
/RUN Voltages .............................. –0.3V to 2.4V
SW Peak Output Current (< 10µs) ........................... 0.5A
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
(Note 1)
pin conFiguraTion
TOP VIEW
I
TH
/RUN 1
GND 2
V
FB
3
6 SW
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
LTC1779ES6#PBF
TAPE AND REEL
LTC1779ES6#TRPBF
PART MARKING
LTLP
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/
2
1779fa
For more information
www.linear.com/LTC1779
LTC1779
The
l
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
Shutdown
UVLO
Undervoltage Lockout Threshold
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
Overtemperature Protect Threshold
Overtemperature Protect Hysteresis
Oscillator Frequency
R
DS(ON)
of Internal P-Channel FET
Peak Current Sense Voltage
V
FB
= 0.8V
V
FB
= 0V
V
IN
= 4.2V, I
SW
= 100mA
(Note 6)
500
V
ITH
/RUN = 0V
(Note 5) 0°C to 70°C
(Note 5) – 40°C to 85°C
2.5V ≤ 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
0.820
l
l
elecTrical characTerisTics
CONDITIONS
Typicals at V
IN
= 4.2V (Note 4)
2.5V ≤ V
IN
≤ 9.8V
2.5V ≤ 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
135
8
1
MAX
240
22
13
2.5
0.5
0.85
0.820
0.830
3
UNITS
µA
µA
µA
V
V
V
µA
V
V
mV/V
mV/µA
mV/µA
1.60
0.15
0.25
0.780
0.770
–3
2.0
2.1
0.325
0.5
0.800
0.800
0
2.5
2.5
5
0.860
30
170
15
550
100
0.85
120
25
0.895
nA
V
mV
°C
°C
650
1.4
kHz
kHz
Ω
mV
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1779E 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
J
°C/W)
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5:
The LTC1779 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 upon duty cycle
to a percentage of value as given in Figure 2.
1779fa
For more information
www.linear.com/LTC1779
3
LTC1779
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)
1779 G01
Normalized Oscillator Frequency
vs Temperature
15
12
NORMALIZED FREQUENCY (%)
9
TRIP VOLTAGE (V)
6
3
0
–3
–6
–9
–12
–15
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G02
Undervoltage Lockout Trip
Voltage vs Temperature
2.20
2.16
2.12
2.08
2.04
2.00
1.96
1.92
1.88
1.84
1.80
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G03
V
IN
= 4.2V
V
IN
= 4.2V
V
IN
FALLING
Shutdown Threshold
vs Temperature
600
560
520
I
TH
/RUN VOLTAGE (mV)
480
440
400
360
320
280
240
200
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G04
R
DS(ON)
of Internal P-Channel FET
vs Input Voltage
1.85
1.70
1.55
1.40
R
DS(ON)
( )
R
DS(ON)
( )
1.25
1.10
0.95
0.80
0.65
0.50
0.35
2
3
4
5
7
8
6
INPUT VOLTAGE (V)
9
10
T
A
= 125°C
T
A
= 25°C
T
A
= –55°C
I
SW
= 100mA
SENSE
–
= V
IN
1.85
R
DS(ON)
of Internal P-Channel FET
vs Temperature
I
SW
= 100mA
1.70 SENSE
–
= V
IN
1.55
1.40
1.25
1.10
0.95
0.80
0.65
0.50
0.35
–55 –35 –15
V
IN
= 9.8V
V
IN
= 8.4V
V
IN
= 6V
V
IN
= 2.4V
V
IN
= 4.2V
V
IN
= 4.2V
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G06
1779 G05
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 0.325V causes
the device to be shut down. In shutdown all functions are
disabled and the internal P-channel MOSFET is turned off.
The SW pin will be high impedance.
GND (Pin 2):
Ground Pin.
V
FB
(Pin 3):
Receives the feedback voltage from an external
resistive divider across the output.
SENSE
–
(Pin 4):
The Negative Input to the Current Com-
parator. Can be connected to V
IN
for default minimum
peak current of 250mA. Connecting a resistor between
SENSE
–
and V
IN
specifies a lower peak current. (See Ap-
plications Information for specifying resistor value.)
V
IN
(Pin 5):
Supply Pin. Must be closely decoupled to
GND Pin 2.
SW (Pin 6):
Switching Node and Drain of Internal
P-Channel Power MOSFET. Connects to external inductor
and catch diode.
1779fa
4
For more information
www.linear.com/LTC1779
LTC1779
FuncTional DiagraM
SENSE
–
4
V
IN
5
+
ICMP
–
SLOPE
COMP
RS1
R
S
Q
OVERTEMP
DETECT
2
V
IN
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
1×
24×
SW
6
OSC
SHORT-CIRCUIT
DETECT
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 LTC1779 is a constant frequency current mode
switching regulator. During normal operation, the internal
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
For more information
www.linear.com/LTC1779
+
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
0.325V
+
–
SHDN
CMP
SHDN
UV
1.2V
1779FD
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 325mV, 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.
1779fa
5
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