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PDF LM1771 Data sheet ( Hoja de datos )
| Número de pieza | LM1771 | |
| Descripción | Low Voltage synchronous buck controller | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LM1771 (archivo pdf) en la parte inferior de esta página. Total 18 Páginas | ||
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www.DataSheet4U.com
June 2006
LM1771
Low-Voltage Synchronous Buck Controller with
Precision Enable and No External Compensation
General Description
The LM1771 is an efficient synchronous buck switching con-
troller with a precision enable requiring no external compen-
sation. The constant on-time control scheme provides a
simple design free of compensation components, allowing
minimal component count and board space. The precision
enable pin allows flexibility in sequencing multiple rails and
setting UVLO. The LM1771 also incorporates a unique input
feed-forward to maintain a constant frequency independent
of the input voltage. The LM1771 is optimized for a low
voltage input range of 2.8V to 5.5V and can provide an
adjustable output as low as 0.8V. Driving an external high
side PFET and low side NFET it can provide efficiencies as
high as 95%.
Three versions of the LM1771 are available depending on
the switching frequency desired for the application. Nominal
switching frequencies are in the range of 100kHz to
1000kHz.
Features
n Input voltage range of 2.8V to 5.5V
n 0.8V reference voltage
n Precision enable
n No compensation required
n Constant frequency across input range
n Low quiescent current of 400 µA
n Internal soft-start
n Short circuit protection
n Tiny LLP-6 package and MSOP-8 package
Applications
n Simple To Design, High Efficiency Step Down Switching
Regulators
n FPGAs, DSPs, and ASIC Power Supplies
n Set-Top Boxes
n Cable Modems
n Printers
n Digital Video Recorders
n Servers
n Graphic Cards
Typical Application Circuit
© 2006 National Semiconductor Corporation DS201890
20189001
www.national.com
1 page  
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Typical Performance Characteristics All curves taken at VIN = 3.3V with configuration in typical
application circuit shown in Application Information section of this datasheet. TJ = 25˚C, unless otherwise
specified. (Continued)
TOFF vs Temperature (LM1771S)
TOFF vs Temperature (LM1771T)
20189013
TOFF vs Temperature (LM1771U)
20189041
Feedback Voltage vs Temperature
20189042
VEN Threshold vs Temperature
20189017
Short Circuit Threshold vs Temperature
20189014
5
20189018
www.national.com
5 Page 
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Design Guide (Continued)
This circuit uses an additional resistor in series with the
inductor to add ripple at the output. It is placed in this location
and used in combination with the feed-forward capacitor
(CFF) to provide ripple to the feedback pin, without adding
ripple or a DC offset to the output. The benefit of using a
ceramic capacitor is still obtained with this technique. Be-
cause the addition of the resistor results in power loss, this
circuit implementation is only recommended for low currents
(2A and below). The power loss and rating of the resistor
should be taken into account when selecting this compo-
nent.
20189029
This circuit implementation utilizing the feed-forward capaci-
tor begins to experience limitations when the output voltage
is small. Previously the circuit relied on the CFF for all the
ripple at the feedback node by assuming that the resistor
divider was negligible. As VOUT decreases this can not be
assumed. The resistor divider contributes a larger amount of
ripple which is problematic as it is also out of phase. There-
fore the resistor location should be changed to be in series
with the output capacitor. This can be viewed as adding an
effective ESR to the output capacitor.
FEED-FORWARD CAPACITOR
The feed-forward capacitor is used across the top feedback
resistor to provide a lower impedance path for the high
frequency ripple without degrading the DC accuracy. Typi-
cally the value for this capacitor should be small enough to
prevent load transient errors because of the discharging
20189030
time, but large enough to prevent attenuation of the ripple
voltage. In general a small ceramic capacitor in the range of
1nF to 10nF is sufficient.
If CFF is used then it can be assumed that the ripple voltage
seen at the feedback pin is the same as the ripple voltage at
the output. The attenuation factor H no longer needs to be
used. However, in these conditions, it is recommended to
have a minimum of 20mV ripple at the feedback pin. The use
11 www.national.com
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet LM1771.PDF ] | |
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