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LM2670T-5.0 Fiches technique(PDF) 9 Page - Texas Instruments

No de pièce LM2670T-5.0
Description  LM2670 SIMPLE SWITCHER High Efficiency 3A Step-Down Voltage Regulator with Sync
Download  36 Pages
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Fabricant  TI [Texas Instruments]
Site Internet  http://www.ti.com
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LM2670T-5.0 Fiches technique(HTML) 9 Page - Texas Instruments

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LM2670
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SNVS036J – APRIL 2000 – REVISED APRIL 2013
APPLICATION HINTS
The LM2670 provides all of the active functions required for a step-down (buck) switching regulator. The internal
power switch is a DMOS power MOSFET to provide power supply designs with high current capability, up to 3A,
and highly efficient operation.
The LM2670 is part of the SIMPLE SWITCHER family of power converters. A complete design uses a minimum
number of external components, which have been pre-determined from a variety of manufacturers. Using either
this data sheet or a design software program called LM267X Made Simple (version 2.0) a complete switching
power supply can be designed quickly. The software is provided free of charge and can be downloaded from
Texas Instrument's Internet site located at www.ti.com.
SWITCH OUTPUT
This is the output of a power MOSFET switch connected directly to the input voltage. The switch provides energy
to an inductor, an output capacitor and the load circuitry under control of an internal pulse-width-modulator
(PWM). The PWM controller is internally clocked by a fixed 260KHz oscillator. In a standard step-down
application the duty cycle (Time ON/Time OFF) of the power switch is proportional to the ratio of the power
supply output voltage to the input voltage. The voltage on pin 1 switches between Vin (switch ON) and below
ground by the voltage drop of the external Schottky diode (switch OFF).
INPUT
The input voltage for the power supply is connected to pin 2. In addition to providing energy to the load the input
voltage also provides bias for the internal circuitry of the LM2670. For ensured performance the input voltage
must be in the range of 8V to 40V. For best performance of the power supply the input pin should always be
bypassed with an input capacitor located close to pin 2.
C BOOST
A capacitor must be connected from pin 3 to the switch output, pin 1. This capacitor boosts the gate drive to the
internal MOSFET above Vin to fully turn it ON. This minimizes conduction losses in the power switch to maintain
high efficiency. The recommended value for C Boost is 0.01
μF.
GROUND
This is the ground reference connection for all components in the power supply. In fast-switching, high-current
applications such as those implemented with the LM2670, it is recommended that a broad ground plane be used
to minimize signal coupling throughout the circuit
SYNC
This input allows control of the switching clock frequency. If left open-circuited the regulator will be switched at
the internal oscillator frequency, between 225KHz and 280KHz. An external clock can be used to force the
switching frequency and thereby control the output ripple frequency of the regulator. This capability provides for
consistent filtering of the output ripple from system to system as well as precise frequency spectrum positioning
of the ripple frequency which is often desired in communications and radio applications. This external frequency
must be greater than the LM2670 internal oscillator frequency, which could be as high as 280KHz, to prevent an
erroneous reset of the internal ramp oscillator and PWM control of the power switch. The ramp oscillator is reset
on the positive going edge of the sync input signal. It is recommended that the external TTL or CMOS compatible
clock (between 0V and a level greater than 3V) be ac coupled to the sync input through a 100pf capacitor and a
1K
Ω resistor to ground at pin 5 as shown in Figure 19.
When the SYNC function is used, current limit frequency foldback is not active. Therefore, the device my not be
fully protected against extreme output short circuit conditions. See ADDITIONAL APPLICATION INFORMATION.
Copyright © 2000–2013, Texas Instruments Incorporated
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