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PDF LM2696 Data sheet ( Hoja de datos )

Número de pieza LM2696
Descripción Constant On Time Buck Regulator
Fabricantes National Semiconductor 
Logotipo National Semiconductor Logotipo



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October 2005
LM2696
3A, Constant On Time Buck Regulator
General Description
The LM2696 is a pulse width modulation (PWM) buck regu-
lator capable of delivering up to 3A into a load. The control
loop utilizes a constant on-time control scheme with input
voltage feed forward. This provides a topology that has
excellent transient response without the need for compen-
sation. The input voltage feed forward ensures that a con-
stant switching frequency is maintained across the entire VIN
range.
The LM2696 is capable of switching frequencies in the range
www.DataSohf e1e0t04kUH.czotmo 500 kHz. Combined with an integrated 130 m
high side NMOS switch the LM2696 can utilize small sized
external components and provide high efficiency. An internal
soft-start and power-good flag are also provided to allow for
simple sequencing between multiple regulators.
The LM2696 is available with an adjustable output in an
exposed pad TSSOP-16 package.
Features
n Input voltage range of 4.5V–24V
n Constant On-Time
n No compensation needed
n Maximum Load Current of 3A
n Switching frequency of 100 kHz–500 kHz
n Constant frequency across input range
n TTL compatible shutdown thresholds
n Low standby current of 12 µA
n 130 minternal MOSFET switch
Applications
n High efficiency step-down switching regulators
n LCD Monitors
n Set-Top Boxes
Typical Application Circuit
© 2005 National Semiconductor Corporation DS201534
20153401
www.national.com

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LM2696 pdf
Typical Performance Characteristics (Continued)
EXTVCC vs VIN
EXTVCC vs Load Current
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Feedback Threshold Voltage vs Temp
kON vs Temp
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Switch ON Time vs RON Pin Current
Min Off-Time vs Temp
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LM2696 arduino
Application Information (Continued)
While COUT and VOUT control the slew rate of the output
voltage, the total amount of time the LM2696 takes to startup
is dependent on two other terms. See the “Startup” section
for more information.
EXTVCC CAPACITOR
External VCC is a 3.65V rail generated by an internal sub-
regulator that powers the parts internal circuitry. This rail
should be bypassed with a 1 µF ceramic capacitor (X5R or
equivalent dielectric). Although EXTVCC is for internal use, it
can be used as an external rail for extremely light loads (<50
µA). If EXTVCC is accidentally shorted to GND the part is
protected by a 5 mA current limit. This rail also has an
under-voltage lockout that will prevent the part from switch-
ing if the EXTVCC voltage drops.
SHUTDOWN
The state of the shutdown pin enables the device or places
www.iDt iantaaSshleeeept4sUta.cteo.mThis pin has an internal pull-up and may be
left floating or connected to a high logic level. Connecting
this pin to GND will shutdown the part. Shutting down the
part will prevent the part from switching and reduce the
quiescent current drawn by the part. This pin must be by-
passed with a 1 nF ceramic capacitor (X5R or Y5V) to
ensure proper logic thresholds.
CBOOT CAPACITOR
The purpose of an external bootstrap capacitor is to turn the
FET on by using the SW node as a pedestal. This allows the
voltage on the CBOOT pin to be greater than VIN. Whenever
the catch diode is conducting and the SW node is at GND,
an internal diode will conduct that charges the CBOOT ca-
pacitor to approximately 4V. When the SW node rises, the
CBOOT pin will rise to approximately 4V above the SW
node. For optimal performance, a 0.1 µF ceramic capacitor
(X5R or equivalent dielectric) should be used.
PGOOD RESISTOR
The PGOOD resistor is used to pull the PGOOD pin high
whenever a steady state operating range is achieved. This
resistor needs to be sized to prevent excessive current from
flowing into the PGOOD pin whenever the open drain FET is
turned on. The recommendation is to use a 10 k–100 k
resistor. This range of values is a compromise between rise
time and power dissipation.
CATCH DIODE
The catch or freewheeling diode acts as the bottom switch in
a non-synchronous buck switcher. Because of this, the diode
has to handle the full output current whenever the FET is not
conducting. Therefore, it must be sized appropriately to
handle the current. The average current through the diode
can be calculated by the equation:
ID_AVG = IOUT(1–D)
Care should also be taken to ensure that the reverse voltage
rating of the diode is adequate. Whenever the FET is con-
ducting the voltage across the diode will be approximately
equal to VIN. It is recommended to have a reverse rating that
is equal to 120% of VIN to ensure adequate guard banding
against any ringing that could occur on the switch node.
Selection of the catch diode is critical to overall switcher
performance. To obtain the optimal performance, a Schottky
diode should be used due to their low forward voltage drop
and fast recovery.
BYPASS CAPACITOR
A bypass capacitor must be used on the AVIN line to help
decouple any noise that could interfere with the analog
circuitry. Typically, a small (1 µF) ceramic capacitor is placed
as close as possible to the AVIN pin.
EXTERNAL OPERATION STARTUP
The total startup time, from the initial VIN rise to the time
VOUT reaches its nominal value is determined by three sepa-
rate steps. Upon the rise of VIN, the first step to occur is that
the EXTVCC voltage has to reach its nominal output voltage
of 3.65V before the internal circuitry is active. This time is
dictated by the output capacitance (1 µF) and the current
limit of the regulator (5 mA typical), which will always be on
the order of 730 µs. Upon reaching its steady state value, an
internal delay of 200 µs will occur to ensure stable operation.
Upon completion the LM2696 will begin switching and the
output will rise. The rise time of the output will be governed
by the soft-start capacitor. To highlight these three steps a
timing diagram please refer to Figure 3.
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