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 ef-
ficient operation.
The LM2670 is part of the SIMPLE SWITCHER family of
power converters. A complete design uses a minimum num-
ber 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 sup-
ply can be designed quickly. The software is provided free of
charge and can be downloaded from National Semiconduc-
tor’s Internet site located at http://www.national.com.
PIN 1 - Switch Output
This is the output of a power MOSFET switch connected di-
rectly 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 oscil-
lator. 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 volt-
age. The voltage on pin 1 switches between Vin (switch ON)
and below ground by the voltage drop of the external Schot-
tky diode (switch OFF).
PIN 2 - 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 guaranteed performance the input voltage must be in the
range of 8V to 40V. For best performance of the power sup-
ply the input pin should always be bypassed with an input ca-
pacitor located close to pin 2.
PIN 3 - C Boost
A capacitor must be connected from pin 3 to the switch out-
put, pin 1. This capacitor boosts the gate drive to the internal
MOSFET above Vin to fully turn it ON. This minimizes con-
duction losses in the power switch to maintain high effi-
ciency. The recommended value for C Boost is 0.01µF.
PIN 4 - Ground
This is the ground reference connection for all components
in the power supply. In fast-switching, high-current applica-
tions such as those implemented with the LM2670, it is rec-
ommended that a broad ground plane be used to minimize
signal coupling throughout the circuit
PIN 5 - Sync
This input allows control of the switching clock frequency. If
left open-circuited the regulator will be switched at the inter-
nal 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 regula-
tor. This capability provides for consistent filtering of the out-
put ripple from system to system as well as precise fre-
quency 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
1KΩ resistor to ground at pin 5 as shown inFigure
1.
PIN 6 - Feedback
This is the input to a two-stage high gain amplifier, which
drives the PWM controller. It is necessary to connect pin 6 to
the actual output of the power supply to set the dc output
voltage. For the fixed output devices (3.3V, 5V and 12V out-
puts), a direct wire connection to the output is all that is re-
quired as internal gain setting resistors are provided inside
the LM2670. For the adjustable output version two external
resistors are required to set the dc output voltage. For stable
operation of the power supply it is important to prevent cou-
pling of any inductor flux to the feedback input.
PIN 7 - ON/OFF
This input provides an electrical ON/OFF control of the
power supply. Connecting this pin to ground or to any volt-
age less than 0.8V will completely turn OFF the regulator.
The current drain from the input supply when OFF is only
50µA. Pin 7 has an internal pull-up current source of approxi-
mately 20µA and a protection clamp zener diode of 7V to
ground. When electrically driving the ON/OFF pin the high
voltage level for the ON condition should not exceed the 6V
absolute maximum limit. When ON/OFF control is not re-
quired pin 7 should be left open circuited.
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