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PDF LM2757 Data sheet ( Hoja de datos )
| Número de pieza | LM2757 | |
| Descripción | Switched Capacitor Boost Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LM2757 (archivo pdf) en la parte inferior de esta página. Total 14 Páginas | ||
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October 2007
LM2757
Switched Capacitor Boost Regulator with High Impedance
Outputwww.datasheet4u.com in Shutdown
General Description
The LM2757 is a constant frequency pre-regulated switched-
capacitor charge pump that operates at 1.25 MHz to produce
a low-noise regulated output voltage. The device can be con-
figured to provide up to 100 mA at 4.1V, 110 mA at 4.5V, or
180 mA at 5V. Excellent efficiency is achieved without the use
of an inductor by operating the charge pump in a gain of either
3/2 or 2 according to the input voltage and output voltage op-
tion selection.
The LM2757 presents a high impedance at the VOUT pin when
shut down. This allows for use in applications that require the
regulated output bus to be driven by another supply while the
LM2757 is shut down.
A perfect fit for space-constrained, battery-operated applica-
tions, the LM2757 requires only 4 small, inexpensive ceramic
capacitors. LM2757 is a tiny 1.2 mm X 1.6 mm 12–bump micro
SMD device. Built in soft-start, over-current protection, and
thermal shutdown features are also included in this device.
Features
■ Inductorless solution uses only 4 small ceramic
capacitors.
■ True input-output and output-input disconnect.
■ Up to 180 mA output current capability (5V).
■ Selectable 4.1V, 4.5V or 5.0V output.
■ Pre-regulation minimizes input current ripple.
■ 1.24 MHz switching frequency for a low-noise, low-ripple
output voltage.
■ Efficient dual gain converter (2X, 3/2X).
■ Integrated Over Current Protection.
■ Integrated Thermal Shutdown Protection.
■ Tiny 1.2 mm X 1.6 mm X 0.4 mm pitch, 12–bump micro
SMD package.
Applications
■ Keypad LED Drive
■ USB/USB-OTG Power
■ Cellular Phone SIM cards
■ Audio amplifier power supplies
■ Low-current Camera Flash
■ General Purpose Li-Ion-to-5V Conversion
■ Supercapacitor Charger
Typical Application Circuit
30033701
30033723
© 2007 National Semiconductor Corporation 300337
www.national.com
1 page  
Block Diagram
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5 Page 
Capacitors with a Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2757. These
types of capacitors typically have wide capacitance tolerance
(+80%, -20%) and vary significantly over temperature (Y5V:
+22%, -82% over -30ºC to +85ºC range; Z5U: +22%, -56%
over +10ºC to +85ºC range). Under some conditions, a 1µF-
rated Y5V or Z5U capacitor could have a capacitance as low
as 0.1µF. Such detrimental deviation is likely to cause Y5V
and Z5U capacitors to fail to meet the minimum capacitance
wwrewq.udiaretamsheenetst4ouf.cthoemLM2757.
Net capacitance of a ceramic capacitor decreases with in-
creased DC bias. This degradation can result in lower capac-
itance than expected on the input and/or output, resulting in
higher ripple voltages and currents. Using capacitors at DC
bias voltages significantly below the capacitor voltage rating
will usually minimize DC bias effects. Consult capacitor man-
ufacturers for information on capacitor DC bias characteris-
tics.
Capacitance characteristics can vary quite dramatically with
different application conditions, capacitor types, and capaci-
tor manufacturers. It is strongly recommended that the
LM2757 circuit be thoroughly evaluated early in the design-in
process with the mass-production capacitors of choice. This
will help ensure that any such variability in capacitance does
not negatively impact circuit performance.
The voltage rating of the output capacitor should be 10V or
more. For example, a 10V 0603 1.0µF is acceptable for use
with the LM2757, as long as the capacitance does not fall
below a minimum of 0.5µF in the intended application. All
other capacitors should have a voltage rating at or above the
maximum input voltage of the application. The capacitors
should be selected such that the capacitance on the input
does not fall below 0.7µF, and the capacitance of the flying
capacitors does not fall below 0.2µF.
The table below lists some leading ceramic capacitor manu-
facturers.
Manufacturer
Contact Information
AVX
www.avx.com
Murata
www.murata.com
Taiyo-Yuden
www.t-yuden.com
TDK
www.component.tdk.com
Vishay-Vitramon
www.vishay.com
OUTPUT CAPACITOR AND OUTPUT VOLTAGE RIPPLE
The output capacitor in the LM2757 circuit (COUT) directly im-
pacts the magnitude of output voltage ripple. Other prominent
factors also affecting output voltage ripple include input volt-
age, output current and flying capacitance. Due to the com-
plexity of the regulation topology, providing equations or
models to approximate the magnitude of the ripple can not be
easily accomplished. But one important generalization can be
made: increasing (decreasing) the output capacitance will re-
sult in a proportional decrease (increase) in output voltage
ripple.
In typical high-current applications, a 1.0µF low-ESR ceramic
output capacitor is recommended. Different output capaci-
tance values can be used to reduce ripple, shrink the solution
size, and/or cut the cost of the solution. But changing the out-
put capacitor may also require changing the flying capacitor
and/or input capacitor to maintain good overall circuit perfor-
mance. Performance of the LM2757 with different capacitor
setups in discussed in the section Recommended Capacitor
Configurations.
High ESR in the output capacitor increases output voltage
ripple. If a ceramic capacitor is used at the output, this is usu-
ally not a concern because the ESR of a ceramic capacitor is
typically very low and has only a minimal impact on ripple
magnitudes. If a different capacitor type with higher ESR is
used (tantalum, for example), the ESR could result in high
ripple. To eliminate this effect, the net output ESR can be sig-
nificantly reduced by placing a low-ESR ceramic capacitor in
parallel with the primary output capacitor. The low ESR of the
ceramic capacitor will be in parallel with the higher ESR, re-
sulting in a low net ESR based on the principles of parallel
resistance reduction.
INPUT CAPACITOR AND INPUT VOLTAGE RIPPLE
The input capacitor (CIN) is a reservoir of charge that aids a
quick transfer of charge from the supply to the flying capaci-
tors during the charge phase of operation. The input capacitor
helps to keep the input voltage from drooping at the start of
the charge phase when the flying capacitors are connected
to the input. It also filters noise on the input pin, keeping this
noise out of sensitive internal analog circuitry that is biased
off the input line.
Much like the relationship between the output capacitance
and output voltage ripple, input capacitance has a dominant
and first-order effect on input ripple magnitude. Increasing
(decreasing) the input capacitance will result in a proportional
decrease (increase) in input voltage ripple. Input voltage, out-
put current, and flying capacitance also will affect input ripple
levels to some degree.
In typical high-current applications, a 1.0µF low-ESR ceramic
capacitor is recommended on the input. Different input ca-
pacitance values can be used to reduce ripple, shrink the
solution size, and/or cut the cost of the solution. But changing
the input capacitor may also require changing the flying ca-
pacitor and/or output capacitor to maintain good overall circuit
performance. Performance of the LM2757 with different ca-
pacitor setups is discussed below in Recommended Capac-
itor Configurations.
FLYING CAPACITORS
The flying capacitors (C1, C2) transfer charge from the input
to the output. Flying capacitance can impact both output cur-
rent capability and ripple magnitudes. If flying capacitance is
too small, the LM2757 may not be able to regulate the output
voltage when load currents are high. On the other hand, if the
flying capacitance is too large, the flying capacitor might over-
whelm the input and output capacitors, resulting in increased
input and output ripple.
In typical high-current applications, 0.47µF low-ESR ceramic
capacitors are recommended for the flying capacitors. Polar-
ized capacitors (tantalum, aluminum electrolytic, etc.) must
not be used for the flying capacitor, as they could become
reverse-biased during LM2757 operation.
RECOMMENDED CAPACITANCE
The data in Table 1 can be used to assist in the selection of
capacitance for each node that best balances solution size
and cost with the electrical requirements of the application.
As previously discussed, input and output ripple voltages will
vary with output current and input voltage. The numbers pro-
vided show expected ripple voltage with VIN = 3.6V and a load
current of 200mA at 5V output, 100mA at 4.5V output, and
100mA at 4.1V output. The table offers a first look at approx-
imate ripple levels and provides a comparison of different
capacitance configurations, but is not intended to be a guar-
antee of performance. With any capacitance configuration
11 www.national.com
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| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet LM2757.PDF ] | |
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