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LTC1771EMS8 데이터시트 PDF




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부품번호 LTC1771EMS8 기능
기능 Low Quiescent Current High Efficiency Step-Down DC/DC Controller
제조업체 Linear Technology
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LTC1771EMS8 데이터시트, 핀배열, 회로
FEATURES
s Very Low Standby Current: 10µA
s Available in Space-Saving 8-Lead MSOP Package
s High Output Currents
s Wide VIN Range: 2.8V to 20V Operation
s VOUT Range: 1.23V to 18V
s High Efficiency: Over 93% Possible
s ±2% Output Accuracy
s Very Low Dropout Operation: 100% Duty Cycle
s Current Mode Operation for Excellent Line and
Load Transient Response
s Defeatable Burst ModeTM Operation
s Short-Circuit Protected
s Optional Programmable Soft-Start
s Micropower Shutdown: IQ = 2µA
U
APPLICATIO S
s Cellular Telephones and Wireless Modems
s 1- to 4-Cell Lithium-Ion-Powered Applications
s Portable Instruments
s Battery-Powered Equipment
s Battery Chargers
s Scanners
Final Electrical Specifications
LTC1771
Low Quiescent Current
High Efficiency Step-Down
DC/DC Controller
DESCRIPTIO
February 2000
The LTC®1771 is a high efficiency current mode step-
down DC/DC controller that draws as little as 10µA DC
supply current to regulate the output at no load while
maintaining high efficiency for loads up to several amps.
The LTC1771 drives an external P-channel power MOSFET
using a current mode, constant off-time architecture. An
external sense resistor is used to program the operating
current level. Current mode control provides short-circuit
protection, excellent transient response and controlled
start-up behavior. Burst Mode operation enables the
LTC1771 to maintain high efficiency down to extremely
low currents. Shutdown mode further reduces the supply
current to a mere 2µA. For low noise applications, Burst
Mode operation can be easily disabled with the MODE pin.
Wide input supply range of 2.8V to 18V (20V maximum)
and 100% duty cycle operation for low dropout make the
LTC1771 ideal for a wide variety of battery-powered appli-
cations where maximizing battery life is important.
The LTC1771’s availability in both 8-lead MSOP and SO
packages provides for a minimum area solution.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
CSS
0.01µF
VIN
4.5V TO 18V
RSENSE
0.05
10µF
25V
CER
RC
10k
CC
22OpF
VIN SENSE
RUN/SS PGATE
ITH LTC1771
VFB MODE
GND
M1
Si6447DQ
VIN
R2
1.64M
1%
R1
1M
1%
5pF
L1
15µH
UPS5817
VOUT
3.3V
+ COUT 2A
150µF
6.3V
1771 F01
Figure 1. High Efficiency Step-Down Converter
LTC1771 Efficiency
100
VIN = 5V
90 VIN = 10V
80 VIN = 15V
70
60
50
VOUT = 3.3V
40 RSENSE = 0.05
0.1 1
10
100 1000
LOAD CURRENT (mA)
10000
1771 F01b
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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LTC1771EMS8 pdf, 반도체, 판매, 대치품
LTC1771
W
FUNCTIONAL BLOCK DIAGRA
VIN
CSS RUN/SS
1
1µA
MODE
(BURST ENABLE)
8
READY
1.23V
VOUT
+
ON
EA
SOFT-START
10% CURRENT
*
ITH
2
RC
CC GND
4
2V
1V
B
1V +
SLEEP
READY
*OPTIONAL FOR FOLDBACK
CURRENT LIMITING
MODE
ON TRIGGER
1-SHOT
STRETCH
3.5µs
1.23V
REFERENCE
10% CURRENT
C
ON
VIN
6
22k
VIN
+
CIN
RSENSE
SENSE
7
250k
BLANKING
VIN
SW
5
L
VOUT
VFB +
3 COUT
1771 BD
4

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LTC1771EMS8 전자부품, 판매, 대치품
LTC1771
APPLICATIO S I FOR ATIO
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC1771. The main selection criteria for
the power MOSFET are the threshold voltage VGS(TH) and
the “on” resistance RDS(ON), reverse transfer capacitance
and total gate charge.
Since the LTC1771 can operate down to input voltages as
low as 2.8V, a sublogic level threshold MOSFET (RDS(ON)
guaranteed at VGS = 2.5V) is required for applications that
work close to this voltage. When these MOSFETs are used,
make sure that the input supply to the LTC1771 is less than
the absolute maximum VGS rating (typically 12V), as the
MOSFET gate will see the full supply voltage.
The required RDS(ON) of the MOSFET is governed by its
allowable power dissipation. For applications that may
operate the LTC1771 in dropout, i.e. 100% duty cycle, at
its worst case the required RDS(ON) is given by:
( ) ( )RDS(ON) =
PP
IOUT(MAX) 2 1+ δP
where PP is the allowable power dissipation and δP is the
temperature dependency of RDS(ON). (1 + δP) is generally
given for a MOSFET in the form of a normalized RDS(ON) vs
temperature curve, but = 0.005/°C can be used as an
approximation for low voltage MOSFETs.
In applications where the maximum duty cycle is less than
100% and the LTC1771 is in continuous mode, the RDS(ON)
is governed by:
RDS(ON) =
PP
2
( ) ( )DC IOUT 1+ δP
DC = VOUT + VD
VIN + VD
where DC is the maximum operating duty cycle of the
LTC1771.
Catch Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As VIN approaches VOUT
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short-circuited. Under this condition, the diode must
safely handle IPEAK at close to 100% duty cycle.
To maximize both low and high current efficiencies, a fast
switching diode with low forward drop and low reverse
leakage should be used. Low reverse leakage current is
critical to maximize low current efficiency since the leak-
age can potentially exceed the magnitude of the LTC1771
supply current. Low forward drop is critical for high
current efficiency since loss is proportional to forward
drop. The effect of reverse leakage and forward drop on
no- load supply current and efficiency for various Schottky
diodes is shown in Table 1. As can be seen, these are
conflicting parameters and the user must weigh the
importance of each spec in choosing the best diode for the
application.
Table 1. Effect of Catch Diode on Performance
DIODE
MBR0540
LEAKAGE
NO-LOAD EFFICIENCY
(VR = 3.3V) VF @ 1A SUPPLY CURRENT AT 10V/1A
0.25µA 0.50V
10.4µA
86.3%
UPS5817
2.8µA 0.41V
11.8µA
88.2%
MBR0520
3.7µA 0.36V
12.2µA
88.4%
MBRS120T3 4.4µA 0.43V
12.2µA
87.9%
MBRM120LT3 8.3µA 0.32V
14.0µA
89.4%
MBRS320
19.7µA 0.29V
20.0µA
89.8%
CIN and COUT Selection
At higher load currents, when the inductor current is
continuous, the source current of the P-channel MOSFET
is a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
capacitor current is given by:
[ ]( )CIN required IRMS
=
IMAX
VOUT
VIN VOUT
VIN
1/ 2
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
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부품번호상세설명 및 기능제조사
LTC1771EMS8

Low Quiescent Current High Efficiency Step-Down DC/DC Controller

Linear Technology
Linear Technology

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