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PDF LM1770 Data sheet ( Hoja de datos )

Número de pieza LM1770
Descripción SOT-23 Synchronous Buck Controller
Fabricantes National Semiconductor 
Logotipo National Semiconductor Logotipo



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November 2005
LM1770
Low-Voltage SOT23 Synchronous Buck Controller With
No External Compensation
General Description
The LM1770 is an efficient synchronous buck switching con-
troller in a tiny SOT23 package. The constant on-time control
scheme provides a simple design free of compensation com-
ponents, allowing minimal component count and board
space. It also incorporates a unique input feed-forward to
maintain a constant frequency independent of the input volt-
age. The LM1770 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 LM1770 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 No compensation required
n Constant frequency across input range
n Low quiescent current of 400µA
n Internal soft-start
n Short circuit protection
n Tiny SOT-23 package
Applications
n Simple To Design, High Efficiency Step Down Switching
Regulators
n Set-Top Boxes
n Cable Modems
n Printers
n Digital Video Recorders
n Servers
Typical Application Circuit
© 2005 National Semiconductor Corporation DS201662
20166201
www.national.com

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LM1770 pdf
Typical Performance Characteristics (Continued)
UVLO Threshold vs Temperature
TON vs VIN (LM1770S)
20166227
TON vs Temperature (LM1770T)
TON vs VIN (LM1770T)
20166230
20166228
TON vs Temperature (LM1770U)
TON vs VIN (LM1770U)
20166231
20166229
5
20166232
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LM1770 arduino
Design Guide (Continued)
20166219
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
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
of a CFF capacitor is recommended as it improves the regu-
lation and stability of the design. However, its benefit is
diminished as VOUT starts approaching VREF , therefore it is
not needed in this situation.
INPUT CAPACITOR
The dominating factor that usually sets an input capacitors’
size is the current handling ability. This is usually determined
by the package size and ESR of the capacitor. If these two
criteria are met then there usually should be enough capaci-
tance to prevent impedance interactions with the source. In
general it is recommended to use a ceramic capacitor for the
input as they provide a low impedance and small footprint.
One important note is to use a good dielectric for the ceramic
capacitor such as X5R or X7R. These provide better over
temperature performance and also minimize the DC voltage
derating that occurs on Y5V capacitors. To calculate the
input capacitor RMS current, the equation below can be
used:
MOSFET Selection
The two FETs used in the LM1770 requires attention to
selection of parameters to ensure optimal performance of
the power supply. The high side FET should be a PFET and
the low side an NFET. These can be integrated in one
package or as two separate packages. The criteria that
matter in selection are listed below:
VDS VOLTAGE RATING
The first selection criteria is to select FETs that have suffi-
cient VDS voltage ratings to handle the maximum voltage
seen at the input plus any transient spikes that can occur
from parasitic ringing. In general most FETs available for this
application will have ratings from 8V to 20V. If a larger
voltage rating is used then the performance will most likely
be degraded because of higher gate capacitance.
RDSON
The RDS(ON) specification is important as it determines sev-
eral attributes of the FET and the overall power supply. The
first is that it sets the maximum current of the FET for a given
package. A lower RDS(ON) will permit a higher allowable
current and reduce conduction losses, however, it will in-
crease the gate capacitance and the switching losses.
GATE DRIVE
The next step is to ensure that the FETs are capable of
switching at the low Vin supplies used by the LM1770. The
FET should have the Rdson specified at either 1.8V or 2.5V
to ensure that it can switch effectively as soon as the
LM1770 starts up.
which can be approximated by,
GATE CHARGE
Because the LM1770 utilizes a fixed dead-time scheme to
prevent cross conduction, the FET transitions must occur in
this time. The rise and fall time of the FETs gate can be
influenced by several factors including the gate capacitance.
11 www.national.com

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