LM2677

SNVS077I – MAY 2004 – REVISED JUNE 2012

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APPLICATION HINTS

The LM2677 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 5A,

and highly efficient operation.

The LM2677 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 Switchers Made Simple a complete switching power supply

can be designed quickly. The software is provided free of charge and can be downloaded from Texas

Instrument's website.

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 LM2677. 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 LM2677, 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 LM2677 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.

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