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LTC1779 데이터시트 PDF : 부품 기능 및 핀배열

부품번호 LTC1779
기능 250mA Current Mode Step-Down DC/DC Converter
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LTC1779 데이터시트 및 LTC1779 PDF

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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
4

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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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