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PDF LTC1779 Data sheet ( Hoja de datos )
| Número de pieza | LTC1779 | |
| Descripción | 250mA Current Mode Step-Down DC/DC Converter | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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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
SENSE– 4
C3
0.1µF L1
22µH
+
D1
R1
10Ω
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 = 10Ω
1 10 100
LOAD CURRENT (mA)
1000
1779 F01b
1
http://www.Datasheet4U.com
1 page  
U
OPERATIO (Refer to Functional Diagram)
LTC1779
up, the corre sponding output current trip level follows,
allowing normal operation.
Comparator OVP guards against transient overshoots
> 7.5% by turning off the internal P-channel power
MOSFET and keeping it off until the fault is removed.
Burst Mode Operation
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 100kHz. 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.
The LTC1779 enters Burst Mode operation at low load
currents. In this mode, the peak current of the inductor is
Overvoltage Protection
set as if VITH/RUN = 1V (at low duty cycles) even though As a further protection, the overvoltage comparator in the
the voltage at the I TH/RUN pin is at a lower value. If the LTC1779 will turn the internal MOSFET off when the
inductor’s average current is greater than the load require- feedback voltage has risen 7.5% above the reference
ment, the voltage at the ITH/RUN pin will drop. When the voltage of 0.8V. This comparator has a typical hysteresis
ITH/RUN voltage goes below 0.85V, the sleep signal goes of 30mV.
high, turning off the internal MOSFET. The sleep signal
goes low when the I TH/RUN voltage goes above 0.925V
and the LTC1779 resumes normal operation. The next
oscillator cycle will turn the internal MOSFET on and the
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
switching cycle repeats.
Dropout Operation
()IPK
=
M(VITH/RUN – 0.7
10 RSENSE +Ω2
)
When the input supply voltage decreases towards the
when the LTC1779 is operating below 40% duty cycle.
output voltage, the rate of change of inductor current
However, once the duty cycle exceeds 40%, slope com-
during the ON cycle decreases. This reduction means that pensation begins and effectively reduces the peak induc-
the internal P-channel MOSFET will remain on for more tor current. The amount of reduction is given by the curves
than one oscillator cycle since the inductor current has not in Figure 2.
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
110
100
output voltage will then be determined by the input voltage
90
minus the voltage drop across the MOSFET, the sense
80
resistor and the inductor.
70
60
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below safe
input voltage levels, an undervoltage lockout is incorpo-
rated into the LTC1779. When the input supply voltage
drops below approximately 2.0V, the P-channel MOSFET
and all circuitry is turned off except the undervoltage
50
IRIPPLE = 0.4IPK
40 AT 5% DUTY CYCLE
30
IRIPPLE = 0.2IPK
AT 5% DUTY CYCLE
20 VIN = 4.2V
10
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1779 F02
block, which draws only several microamperes.
Figure 2. Maximum Output Current vs Duty Cycle
5
5 Page 
PACKAGE DESCRIPTION
S6 Package
6-Lead Plastic SOT-23
(LTC DWG # 05-08-1634)
(LTC DWG # 05-08-1636)
SOT-23
(Original)
A
.90 – 1.45
(.035 – .057)
A1
.00 – 0.15
(.00 – .006)
A2
.90 – 1.30
(.035 – .051)
L
.35 – .55
(.014 – .021)
SOT-23
(ThinSOT)
1.00 MAX
(.039 MAX)
.01 – .10
(.0004 – .004)
.80 – .90
(.031 – .035)
.30 – .50 REF
(.012 – .019 REF)
2.60 – 3.00
(.102 – .118)
1.50 – 1.75
(.059 – .069)
(NOTE 3)
2.80 – 3.10
(.110 – .118)
(NOTE 3)
PIN ONE ID
.20
(.008)
DATUM ‘A’
.95
(.037)
REF
A A2
L
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
.09 – .20
(.004 – .008)
(NOTE 2)
2.
DIMENSIONS
ARE
IN
MILLIMETERS
(INCHES)
3. DRAWING NOT TO SCALE
4. DIMENSIONS ARE INCLUSIVE OF PLATING
5. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
6. MOLD FLASH SHALL NOT EXCEED .254mm
7. PACKAGE EIAJ REFERENCE IS:
SC-74A (EIAJ) FOR ORIGINAL
JEDEL MO-193 FOR THIN
1.90
(.074)
REF
LTC1779
.25 – .50
(.010 – .020)
(6PLCS, NOTE 2)
A1
S6 SOT-23 0401
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.
11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet LTC1779.PDF ] | |
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