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PDF LTC3538 Data sheet ( Hoja de datos )
| Número de pieza | LTC3538 | |
| Descripción | 800mA Synchronous Buck-Boost DC/DC Converter | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ Regulated Output with Input Voltages Above,
Below, or Equal to the Output
■ 800mA Continuous Output Current from a Single
Lithium-Ion/Polymer Cell
■ Single Inductor
■ 1MHz Fixed Frequency Operation
■ Output Disconnect in Shutdown
■ 2.4V to 5.5V Input
■ 35μA Quiesecent Current in Burst Mode Operation
■ <5μA Shutdown Current
■ Internal Soft-Start
■ Small, Thermally Enhanced 8-Lead (2mm x 3mm)
DFN package
APPLICATIONS
■ Miniature Hard Disk Drives
■ MP3 Players
■ Digital Cameras
■ Cellular Handsets
■ PDAs, Handheld PC
■ GPS Receivers
LTC3538
800mA Synchronous
Buck-Boost
DC/DC Converter
DESCRIPTION
The LTC®3538 is a highly efficient, low noise, buck-boost
DC/DC converter that operates from input voltages above,
below, and equal to the output voltage. The topology
incorporated in the IC provides a continuous transfer
function through all operating modes, making the product
ideal for single Lithium Ion or multicell Alkaline or NiMH
applications where the output voltage is within the battery
voltage range.
The LTC3538 is suited for use in Micro Hard Disk Drive
(μHDD) applications with its 800mA current capability. Burst
Mode® operation provides high efficiency at light loads.
The LTC3538 includes two 0.17Ω N-channel and two
0.2Ω P-channel MOSFET switches. Operating frequency
is internally set to 1MHz to minimize solution footprint
while maximizing efficiency.
Other features include <5μA shutdown current, internal
soft-start, short circuit protection and thermal shutdown.
The LTC3538 is available in a low profile (0.75mm), ther-
mally enhanced 8-lead (2mm × 3mm) DFN package.
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners. Protected by
U.S. Patents including 5481178, 6304066, 6580258, 6166527, 6404251.
TYPICAL APPLICATION
VIN
2.9V TO 4.2V
Li-Ion/Polymer to 3.3V at 800mA
L1
3.3μH
LTC3538
SW1 SW2
R1
464k
VOUT
3.3V
800mA
10k
CIN
10μF
PWM
VIN VOUT
BURST
BURST
GND
FB
VC
ON OFF SD
33pF
COUT
22μF
15k
330pF
R2
200k
3538 TA01
Efficiency vs VIN
100
VOUT = 3.3V
ILOAD = 200mA
95
90
85
80
2.4 2.9 3.4 3.9 4.4 4.9 5.4
VIN (V)
3538 TA01b
3538f
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LTC3538
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted
VC On/Off Threshold vs
Temperature
0.80
0.75
0.70
0.65 VC ON THRESHOLD
0.60
0.55
VC OFF THRESHOLD
0.50
0.45
0.40
–50
0 50
TEMPERATURE (°C)
100
3538 G10
Current Limit vs Temperature
4.0
VIN = VOUT = 3.6V
3.5 PEAK CURRENT LIMIT
Load Transient in Fixed
Frequency Mode
VOUT
100mV/DIV
3.0
2.5
2.0
LINEAR CURRENT LIMIT
1.5
–50 –25
0
25 50
TEMPERATURE (°C)
ILOAD
200mA/DIV
75 100
3538 G12
100μs/DIV
VIN = 3.3V
VOUT = 3.3V
ILOAD = 0mA TO 500mA
COUT = 22μF X5R CERAMIC
3538 G13
Burst Mode Operation
Transition From Burst Mode
Operation to Fixed Frequency
VOUT
50mV/DIV
IL
500mA/DIV
10μs/DIV
VIN = 3.3V
VOUT = 3.3V
ILOAD = 10mA
COUT = 22μF X5R CERAMIC
3538 G14
BURST
2V/DIV
VOUT
100mV/DIV
50μs/DIV
VIN = 3.3V
VOUT = 3.3V
ILOAD = 30mA
COUT = 22μF X5R CERAMIC
3538 G15
3538f
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LTC3538
OPERATION
Since the output current is discontinuous in boost mode,
the ripple in this mode will generally be much larger than
the magnitude of the ripple in buck mode.
Minimizing solution size is usually a priority. Please be
aware that ceramic capacitors can exhibit a significant
reduction in effective capacitance when a bias is applied.
The capacitors exhibiting the highest reduction are those
packaged in the smallest case size.
Input Capacitor Selection
Since VIN is the supply voltage for the IC it is recommended
to place at least a 4.7μF, low ESR ceramic bypass capaci-
tor close to VIN and GND. It is also important to minimize
any stray resistance from the converter to the battery or
other power source.
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and
D are not required, but do provide a lower drop during the
break-before-make time (typically 15ns), thus improving
efficiency. Use a surface mount Schottky diode such as an
MBRM120T3 or equivalent. Do not use ordinary rectifier
diodes since their slow recovery times will compromise
efficiency.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE
FAX
WEB SITE
AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com
Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com
Taiyo
Yuden
(408) 573-4150 (408) 573-4159 www.t-yuden.com
TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com
Shutdown MOSFET Selection
A discrete external N-channel MOSFET, open-drain pull-
down device or other suitable means can be used to put
the part in shutdown by pulling VC below 0.25V. Since
the error amplifier sources 13μA typically when active
and 1.5μA in shutdown, a relatively high resistance pull-
down device can be used to pull VC below 0.25V. More
importantly, leakage and parasitic capacitance need to
be minimized. During start-up, 1.5μA is typically sourced
from VC. The leakage of an external pull-down device and
compensation components tied to VC, must therefore be
minimized to ensure proper start-up. Capacitance from
the pull-down device should also be minimized as it can
affect converter stability. An N-channel MOSFET such as
the FDV301N or similar is recommended if an external
discrete N-channel MOSFET is needed.
PCB Layout Considerations
The LTC3538 switches large currents at high frequencies.
Special care should be given to the PCB layout to ensure
stable, noise-free operation. Figure 3 depicts the recom-
mended PCB layout to be utilized for the LTC3538. A few
key guidelines follow:
1. All circulating current paths should be kept as short as
possible. This can be accomplished by keeping the routes
to all components (except the FB divider network) in
Figure 3 as short and as wide as possible. Capacitor ground
connections should via down to the ground plane in the
shortest route possible. The bypass capacitor on VIN should
be placed as close to the IC as possible and should have
the shortest possible paths to ground.
2. The small signal ground pad (GND) should have a single
point connection to the power ground. A convenient way
to achieve this is to short this pin directly to the Exposed
Pad as shown in Figure 3.
3. The components in bold and their connections should
all be placed over a complete ground plane.
4. To prevent large circulating currents from disrupting
the output voltage sensing, the ground for the resistor
divider should be returned directly to the small signal
ground (GND) as shown.
5. Use of vias in the attach pad will enhance the thermal
environment of the converter especially if the vias extend
to a ground plane region on the exposed bottom surface
of the PCB.
3538f
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3538.PDF ] | |
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