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LTC1779 PDF 데이터시트 : 부품 기능 및 핀배열

부품번호 LTC1779
기능 250mA Current Mode Step-Down DC/DC Converter
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LTC1779 데이터시트, 핀배열, 회로
LTC1779
250mA Current Mode
Step-Down DC/DC Converter
in ThinSOT
FEATURES
s High Efficiency: Up to 94%
s 250mA Output Current
s Wide VIN Range: 2.5V to 9.8V
s 550kHz Constant Frequency Operation
s Burst ModeTM Operation at Light Load
s Low Dropout: 100% Duty Cycle
s 0.8V Reference Allows Low Output Voltages
s ±2.5% Reference Accuracy
s Current Mode Operation for Excellent Line and Load
Transient Response
s Low Quiescent Current: 135µA
s Shutdown Mode Draws Only 8µA Supply Current
s Low Profile (1mm) ThinSOTTM Package
U
APPLICATIO S
s 1- or 2-Cell Lithium-Ion-Powered Applications
s Cellular Telephones
s Wireless Modems
s Portable Computers
s Distributed 3.3V, 2.5V or 1.8V Power Systems
s Scanners
DESCRIPTIO
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 pro-
vides excellent AC and DC load and line regulation. The
device incorporates an accurate undervoltage lockout fea-
ture that shuts down the LTC1779 when the input voltage
falls below 2V.
The LTC1779 boasts a±2.5% output voltage accuracy and
consumes only 135µA of quiescent current. For applica-
tions 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and ThinSOT are trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
20k
100pF
16
ITH/RUN SW
LTC1779
25
GND VIN
3
VFB
SENSE4
C3
0.1µF L1
22µH
+
D1
R1
10
C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
D1: IR10BQ015
L1: COILTRONICS UP1B220
C1
10µF
16V
C2
47µF
6V
VIN
2.5V
TO 9.8V
169k
VOUT
2.5V
100mA
78.7k
1779 F01a
Figure 1. LTC1779 High Efficiency 2.5V/100mA Step-Down Converter
Efficiency vs Load Current
100
90
VIN = 3.3V
80
70
60
VIN = 6V
VIN = 9.8V
50
40
30
0.1
VOUT = 2.5V
RSENSE = 10
1 10 100
LOAD CURRENT (mA)
1000
1779 F01b
1
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LTC1779 pdf, 반도체, 판매, 대치품
LTC1779
W
FU CTIO AL DIAGRA
SENSE
4
VIN
5
+
ICMP
SLOPE
OSC COMP
OVERTEMP
DETECT
2
RS1 VIN
R
Q
SWITCHING
LOGIC AND
BLANKING
S CIRCUIT
1× 24×
SW
6
VIN
0.3V
GND
2
FREQ
FOLDBACK
SHORT-CIRCUIT
DETECT
0.3V
0.15V
BURST
+ CMP
SLEEP
0.5µA
+
VIN
1 ITH/RUN
0.325V +
VOLTAGE
REFERENCE
VREF
0.8V
SHDN
CMP
SHDN
UV
UNDERVOLTAGE
LOCKOUT
OVP
EAMP
+
VREF
+
60mV
+
VREF
0.8V
VFB
3
VIN
1.2V
1779FD
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
load current increases, it causes a slight decrease in VFB
The LTC1779 is a constant frequency current mode switch-
ing regulator. During normal operation, the internal
P-channel power MOSFET is turned on each cycle when
relative to the 0.8V reference, which in turn causes the
ITH/RUN voltage to increase until the average inductor
current matches the new load current.
the oscillator sets the RS latch (RS1) and turned off when The main control loop is shut down by pulling thTeHI/RUN
the current comparator (ICMP) resets the latch. The peak pin low. Releasing I TH/RUN allows an internal 0.5 µA
inductor current at which ICMP resets the RS latch is
current source to charge up the external compensation
controlled by the voltage on the TIH/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 voltageFVB. When the
network. When the TIH/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
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LTC1779 전자부품, 판매, 대치품
LTC1779
APPLICATIO S I FOR ATIO
A smaller value than L MIN could be used in the circuit;
however, the inductor current will not be continuous
understand how it is going to work over the entire input
voltage range.
during burst periods.
Inductor Core Selection
RSENSE Selection for Output Current
Once the value for L is known, the type of inductor must be
The selection of R SENSE determines the output current
limit, the maximum possible output current before the
selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
internal current limit threshold is reached. IOUT(MAX), the forcing the use of more expensive ferrite, molypermalloy
maximum specified output current in a design, must be or Kool Mu® cores. Actual core loss is independent of core
less than ICL. With the current comparator monitoring the size for a fixed inductor value, but it is very dependent on
voltage developed across R SENSE, the threshold of the inductance selected. As inductance increases, core losses
comparator determines the inductor’s peak current. The go down. Unfortunately, increased inductance requires
maximum output current, ICL, the LTC1779 can provide is more turns of wire and therefore copper losses will
given by:
increase. Ferrite designs have very low core losses and are
ICL
=
M
1S0F0

01. 2V
RSENSE +Ω22

IRIPPLE
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
where SF and M are as defined in the previous section,
Figures 2 and 3. Typically, RSENSE is chosen between 0
and 20. Current limit is at a minimum at minimum input
voltage and maximum at maximum input voltage. Both
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
conditions should be considered in a design where current Molypermalloy (from Magnetics, Inc.) is a very good, low
limit is important.
loss core material for toroids, but it is more expensive than
To calculate several current limit conditions and choose ferrite. A reasonable compromise from the same manu-
the best sense resistor for your design, first use minimum facturer is Kool Mu. Toroids are very space efficient,
input voltage. Calculate the duty cycle at minimum input especially when you can use several layers of wire.
voltage.
Because they generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount that do
DC = VOUT
VIN(M) IN
not increase the height significantly are available.
Output Diode Selection
Choose the slope factor, SF, from Figure 2 based on the
duty cycle. The ripple current calculated at minimum input The catch diode carries load current during the off-time.
voltage and the chosen L should be used in the current The average diode current is therefore dependent on the
limit equation (see Inductor Value Calculation). Figure 3 internal P-channel switch duty cycle. At high input volt-
provides several values of RSENSE and their corresponding
M values at different input voltages. Select the minimum
ages the diode conducts most of the time. As V IN ap-
proaches VOUT the diode conducts only a small fraction of
input voltage and calculate the resulting minimum current the time. The most stressful condition for the diode is
limit settings.
when the output is short-circuited. Under this condition
the diode must safely handle I PK at close to 100% duty
The process must be repeated for maximum current limit cycle. Therefore, it is important to adequately specify the
using duty cycle, slope factor, ripple current and mirror diode peak current and average power dissipation so as
ratio based onmaximum input voltage in order to choose not to exceed the diode ratings.
the best sense resistor for a particular design and to
Kool Mu is a registered trademark of Magnetics, Inc.
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