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PDF LTC3251-1.2 Data sheet ( Hoja de datos )
| Número de pieza | LTC3251-1.2 | |
| Descripción | Step-Down DC/DC Converter | |
| Fabricantes | Linear | |
| Logotipo |  |
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LTC3251/
LTC3251-1.2/LTC3251-1.5
500mA High Efficiency,
Low Noise, Inductorless
Step-Down DC/DC Converter
FEATURES
■ Up to 500mA Output Current
■ No Inductors
■ 2.7V to 5.5V Input Voltage Range
■ 2x Efficiency Improvement Over LDOs
■ 2-Phase, Spread Spectrum Operation
for Low Input and Output Noise
■ Shutdown Disconnects Load from VIN
■ Adjustable Output Voltage Range: 0.9V to 1.6V
Fixed Output Voltages: 1.2V, 1.5V
■ Super Burst, Burst and Burst Defeat Operating Modes
■ Low Operating Current: IIN = 35µA (Burst Mode®
Operation)
■ Super Burst Operating Current: IIN = 10µA
■ Low Shutdown Current: IIN = 0.01µA Typ
■ Soft-Start Limits Inrush Current at Turn-On
■ Short-Circuit and Overtemperature Protected
■ Available in a Thermally Enhanced
10-Pin MSOP Package
U
APPLICATIO S
■ Handheld Devices
■ Cellular Phones
■ Portable Electronic Equipment
■ DSP Power Supplies
DESCRIPTIO
The LTC®3251/LTC3251-1.2/LTC3251-1.5 are 2-phase
charge pump step-down DC/DC converters that produce a
regulated output from a 2.7V to 5.5V input. The parts use
switched capacitor fractional conversion to achieve twice
the typical efficiency of a linear regulator. No inductors are
required. VOUT is resistor programmable from 0.9V to 1.6V
or fixed at 1.2V or 1.5V, with up to 500mA of load current
available.
A unique 2-phase spread spectrum architecture provides
a very low noise regulated output as well as low noise at the
input.* The parts have four operating modes: Continuous
Spread Spectrum, Spread Spectrum with Burst Mode
operation, Super BurstTM mode operation and shutdown.
Low operating current (35µA in Burst Mode operation,
10µA in Super Burst mode operation) and low external
parts count make the LTC3251/LTC3251-1.2/LTC3251-1.5
ideally suited for space-constrained battery-powered
applications. The parts are short-circuit and overtempera-
ture protected, and are available in a thermally enhanced
10-pin MSOP package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
Super Burst is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by US Patents including 6411531.
TYPICAL APPLICATIO
1-CELL Li-Ion
OR
3-CELL NiMH
Spread Spectrum Step-Down Converter
OFF ON
19
MD0 MD1
LTC3251-1.5
2
VIN
VOUT 7
1µF 3 C1+
1µF 4 C1–
C2+ 8
C2– 6
5, 11
10
GND MODE
1µF
VOUT = 1.5V
500mA
10µF
3251 TA01
1.5V Efficiency vs Input Voltage
(Burst Mode Operation)
100
IOUT = 200mA
90
LTC3251-1.5
80
70
60
50
LDO
40
30
20
10
0
3 3.5 4 4.5 5 5.5
INPUT VOLTAGE (V)
3251 TA02
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1
1 page  
LTC3251/
LTC3251-1.2/LTC3251-1.5
TYPICAL PERFOR A CE CHARACTERISTICS
MD0/MD1 Input Threshold Voltage
vs Supply Voltage
1.2
1.1
1.0
–40°C
0.9
25°C
0.8
85°C
0.7
0.6
0.5
0.4
2.7 3.2 3.7 4.2 4.7 5.2
VIN (V)
3251 G10
Output Transient Response
(Continuous Mode)
450mA
IOUT
50mA
Max/Min Oscillator Frequency
vs Supply Voltage
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
2.7
–40°C MAX 25°C MAX
85°C MAX
25°C MIN
–40°C MIN
85°C MIN
3.2 3.7 4.2 4.7
VIN (V)
5.2
3251 G11
Output Transient Response
(Burst Mode Operation)
450mA
IOUT
50mA
VIN
VOUT
20mV/DIV
(AC)
VOUT
20mV/DIV
(AC)
VOUT
20mV/DIV
(AC)
TA = 25°C
10µs/DIV
COUT = 10µF X5R 6.3V
VOUT = 1.5V
3251 G13
TA = 25°C
10µs/DIV
COUT = 10µF X5R 6.3V
VOUT = 1.5V
3251 G14
Supply Transient Response
(Continuous Mode)
4.5V
VIN
3.5V
VOUT
20mV/DIV (AC)
TA = 25°C
20µs/DIV
COUT = 10µF X5R 6.3V
IOUT = 250mA
VOUT = 1.5V
3251 G15
LTC3251-1.5 Output Voltage
Ripple
SPREAD
SPECT
ENABLED
10mV/DIV (AC)
SPREAD
SPECT
DISABLED
10mV/DIV (AC)
TA = 25°C
200ns/DIV
COUT = 10µF X5R 6.3V
IOUT = 500mA
VOUT = 1.5V
3251 G16
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5
5 Page 
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OPERATIO (Refer to Block Diagram)
Further output noise reduction can be achieved by filtering
the LTC3251 output through a very small series inductor
as shown in Figure 4. A 10nH inductor will reject the fast
output transients caused by the blanking period. The 10nH
inductor can be fabricated on the PC board with about 1cm
(0.4") of 1mm wide PC board trace.
10nH
(TRACE INDUCTANCE)
VOUT
LTC3251
10µF
1µF
VOUT
GND
3251 F04
Figure 4. 10nH Inductor Used for
Additional Output Noise Reduction
VIN Capacitor Selection
The dual phase architecture used by the LTC3251 family
makes input noise filtering much less demanding than
conventional charge pump regulators. The input current
should be continuous at about IOUT/2. The blanking period
described in the VOUT section also effects the input. For
this reason it is recommended that a low ESR, 1µF (0.4µF
min) or greater ceramic capacitor be used for CIN (see
Ceramic Capacitor Selection Guidelines section).
In cases where the supply impedance is high, heavy output
transients can cause significant input transients. These
input transients feed back to the output which slows the
output transient recovery and increases overshoot and
output impedance. This effect can generally be avoided by
using low impedance supplies and short supply connec-
tions. If this is not possible, a ≥4.7µF capacitor is recom-
mended for the input capacitor. Aluminum and tantalum
capacitors are not recommended because of their high
ESR.
Further input noise reduction can be achieved by filtering
the input through a very small series inductor as shown in
Figure 5. A 10nH inductor will reject the fast input tran-
sients caused by the blanking period, thereby presenting
a nearly constant load to the input supply. For economy,
the 10nH inductor can be fabricated on the PC board with
about 1cm (0.4") of 1mm wide PC board trace.
LTC3251/
LTC3251-1.2/LTC3251-1.5
10nH
(TRACE INDUCTANCE)
VIN
SUPPLY
1µF
VIN
LTC3251
GND
3251 F05
Figure 5. 10nH Inductor Used for
Additional Input Noise Reduction
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or alumi-
num should never be used for the flying capacitors since
their voltages can reverse upon start-up of the LTC3251.
Ceramic capacitors should always be used for the flying
capacitors.
The flying capacitors control the strength of the charge
pump. In order to achieve the rated output current, it is
necessary for the flying capacitor to have at least 0.4µF of
capacitance over operating temperature with a 2V bias
(see Ceramic Capacitor Selection Guidelines). If only
200mA or less of output current is required for the
application, the flying capacitor minimum can be reduced
to 0.15µF.
Ceramic Capacitor Selection Guidelines
Capacitors of different materials lose their capacitance
with higher temperature and voltage at different rates. For
example, a ceramic capacitor made of X5R or X7R material
will retain most of its capacitance from – 40°C to 85°C,
whereas a Z5U or Y5V style capacitor will lose consider-
able capacitance over that range (60% to 80% loss typ).
Z5U and Y5V capacitors may also have a very strong
voltage coefficient, causing them to lose an additional
60% or more of their capacitance when the rated voltage
is applied. Therefore, when comparing different capaci-
tors, it is often more appropriate to compare the amount
of achievable capacitance for a given case size rather than
discussing the specified capacitance value. For example,
over rated voltage and temperature conditions, a 4.7µF,
10V, Y5V ceramic capacitor in an 0805 case may not
provide any more capacitance than a 1µF, 10V, X5R or X7R
available in the same 0805 case. In fact, over bias and
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11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3251-1.2.PDF ] | |
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