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PDF LTC3801 Data sheet ( Hoja de datos )
| Número de pieza | LTC3801 | |
| Descripción | Micropower Constant Frequency Step-Down DC/DC Controllers | |
| Fabricantes | Linear Technology | |
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LTC3801/LTC3801B
Micropower
Constant Frequency Step-Down
DC/DC Controllers in ThinSOT
www.datasheet4u.com
FEATURES
DESCRIPTIO
s High Efficiency: Up to 94%
s Very Low No-Load Quiescent Current:
Only 16µA (LTC3801)
s High Output Currents Easily Achieved
s Internal Soft-Start
s Wide VIN Range: 2.4V to 9.8V
s Low Dropout: 100% Duty Cycle
s Constant Frequency 550kHz Operation
s Burst Mode® Operation for High Efficiency
at Light Loads (LTC3801)
s Burst Mode Operation Disabled for Lower Output
Ripple at Light Loads (LTC3801B)
s Output Voltage as Low as 0.8V
s ±1.5% Voltage Reference Accuracy
s Current Mode Operation for Excellent Line and Load
Transient Response
s Only 6µA Supply Current in Shutdown (LTC3801)
s Low Profile (1mm) SOT-23 Package
U
APPLICATIO S
s 1- or 2-Cell Li-Ion Battery-Powered Applications
s Wireless Devices
s Portable Computers
s Distributed Power Systems
The LTC®3801/LTC3801B are constant frequency cur-
rent mode step-down DC/DC controllers in a low profile
(1mm max) 6-lead SOT-23 (ThinSOTTM) package. The
parts provide excellent AC and DC load and line regula-
tion with ±1.5% output voltage accuracy. The LTC3801
consumes only 195µA of quiescent current in normal
operation, dropping to 16µA under no-load conditions.
The LTC3801/LTC3801B incorporate an undervoltage lock-
out feature that shuts down the device when the input
voltage falls below 2.2V. The LTC3801 automatically
switches into Burst Mode operation at light loads which
enhances efficiency at low output current. In the LTC3801B,
Burst Mode operation is disabled for lower output ripple at
light loads.
To further maximize the life of a battery source, the
external P-channel MOSFET is turned on continuously in
dropout (100% duty cycle). High switching frequency of
550kHz allows the use of a small inductor.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
550kHz Micropower Step-Down DC/DC Controller
220pF
10k
402k
ITH/RUN VIN
LTC3801/
LTC3801B
GND SENSE–
VFB PGATE
866k
0.025Ω
VIN
2.7V TO 9.8V
10µF
4.7µH
+
47µF
VOUT
2.5V
2A
3801 TA01
LTC3801 Efficiency vs Load Current*
100
VOUT = 2.5V
95
VIN = 4.2V
90
VIN = 3.3V
85 VIN = 6.6V
80
75
70 VIN = 9.8V VIN = 8.4V
65
60
55
50
0.1
1
10 100 1000 10000
LOAD CURRENT (mA)
3801 TA02
*SEE NO-LOAD IQ vs INPUT VOLTAGE ON THE LAST PAGE OF THIS DATA SHEET
3801f
1
1 page  
U
OPERATIO (Refer to the Functional Diagram)
Main Control Loop (Normal Operation)
The LTC3801/LTC3801B are constant frequency current
wwwm.daotadsehesette4pu.-cdoomwn switching regulator controllers. During
normal operation, an external P-channel MOSFET is turned
on each cycle when the oscillator sets the RS latch and
turned off when the current comparator resets the latch.
The peak inductor current at which the current comparator
trips is controlled by the voltage on the ITH/RUN pin, which
is the output of the error amplifier. The negative input to
the error amplifier is the output feedback voltage VFB
which is generated by an external resistor divider con-
nected between VOUT and ground. When the load current
increases, it causes a slight decrease in VFB 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 main control loop is shut down by pulling the ITH/RUN
pin to ground. Releasing the ITH/RUN pin allows an
internal 1µA current source (2µA on LTC3801B) to charge
up the external compensation network. When the ITH/
RUN pin voltage reaches approximately 0.6V, the main
control loop is enabled and the ITH/RUN voltage is pulled
up by a clamp to its zero current level of approximately
one diode voltage drop (0.7V). As the external compensa-
tion network continues to charge up, the corresponding
peak inductor current level follows, allowing normal op-
eration. The maximum peak inductor current attainable is
set by a clamp on the ITH/RUN pin at 1.2V above the zero
current level (approximately 1.9V).
Burst Mode Operation (LTC3801 Only)
The LTC3801 incorporates Burst Mode operation at low
load currents (<25% of IMAX). In this mode, an internal
clamp sets the peak current of the inductor at a level cor-
responding to an ITH/RUN voltage 0.3V above its zero
current level (approximately 1V), even though the actual
ITH/RUN voltage is lower. When the inductor’s average
current is greater than the load requirement, the voltage at
the ITH/RUN pin will drop. When the ITH/RUN voltage falls
to 0.15V above its zero current level (approximately 0.85V),
the sleep comparator will trip, turning off the external
MOSFET. In sleep, the input DC supply current to the IC is
reduced to 16µA from 195µA in normal operation. With the
switch held off, average inductor current will decay to zero
LTC3801/LTC3801B
and the load will eventually cause the error amplifier out-
put to start drifting higher. When the error amplifier output
rises to 0.225V above its zero current level (approximately
0.925V), the sleep comparator will untrip and normal op-
eration will resume. The next oscillator cycle will turn the
external MOSFET on and the switching cycle will repeat.
Low Load Current Operation (LTC3801B Only)
Under very light load current conditions, the ITH/RUN pin
voltage will be very close to the zero current level of 0.85V.
As the load current decreases further, an internal offset at
the current comparator input will ensure that the current
comparator remains tripped (even at zero load current)
and the regulator will start to skip cycles, as it must, in
order to maintain regulation. This behavior allows the
regulator to maintain constant frequency down to very
light loads, resulting in less low frequency noise genera-
tion over a wide load current range.
Figure 1 illustrates this result for the circuit on the front
page of this data sheet using both an LTC3801 (in Burst
Mode operation) and an LTC3801B (with Burst Mode
operation disabled). At an output current of 100mA, the
LTC3801 exhibits an output ripple of 81.6mVP-P, whereas
the LTC3801B has an output ripple of only 17.6mVP-P. At
lower output current levels, the improvement is even
greater. This comes at a tradeoff of lower efficiency for the
non Burst Mode part at light load currents (see Figure 2).
Also notice the constant frequency operation of the
LTC3801B, even at 5% of maximum output current.
Dropout Operation
When the input supply voltage decreases towards the
output voltage, the rate of change of inductor current
during the on cycle decreases. This reduction means that
at some input-output differential, the external P-channel
MOSFET will remain on for more than one oscillator cycle
(start dropping off-cycles) since the inductor current has
not ramped up to the threshold set by the error amplifier.
Further reduction in input supply voltage will eventually
cause the external P-channel MOSFET to be turned on
100%, i.e., DC. The output voltage will then be determined
by the input voltage minus the voltage drop across the
sense resistor, the MOSFET and the inductor.
3801f
5
5 Page 
APPLICATIO S I FOR ATIO
Foldback Current Limiting
As described in the Output Diode Selection, the worst-
wwwc.daastaeshdeiest4suip.caotmion occurs with a short-circuited output
when the diode conducts the current limit value almost
continuously. To prevent excessive heating in the diode,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding di-
odes DFB1 and DFB2 between the output and the ITH/RUN
pin as shown in Figure 5. In a hard short (VOUT = 0V), the
current will be reduced to approximately 50% of the
maximum output current.
LTC3801/LTC3801B
LTC3801/
LTC3801B
ITH/RUN VFB
R2 +
R1
VOUT
DFB1
DFB2
3801 F05
Figure 5. Foldback Current Limiting
PACKAGE DESCRIPTIO
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
0.62 0.95
MAX REF
1.22 REF
2.90 BSC
(NOTE 4)
3.85 MAX 2.62 REF
1.4 MIN
2.80 BSC
1.50 – 1.75
(NOTE 4)
PIN ONE ID
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.20 BSC
DATUM ‘A’
0.95 BSC
0.80 – 0.90
1.00 MAX
0.30 – 0.50 REF
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
0.09 – 0.20
(NOTE 3)
1.90 BSC
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
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.
0.30 – 0.45
6 PLCS (NOTE 3)
0.01 – 0.10
S6 TSOT-23 0302
3801f
11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet LTC3801.PDF ] | |
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