LM2595 datasheet(17/29 Pages) NSC

17 / 29 page

Block Diagram

Application Information

PIN FUNCTIONS

IN — This is the positive input supply for the IC switching

regulator. A suitable input bypass capacitor must be present

at this pin to minimize voltage transients and to supply the

switching currents needed by the regulator.

Ground — Circuit ground.

Output — Internal switch. The voltage at this pin switches

between (+V

IN −VSAT) and approximately −0.5V, with a duty

cycle of approximately V

OUT/VIN. To minimize coupling to

sensitive circuitry, the PC board copper area connected to

this pin should be kept to a minimum.

Feedback — Senses the regulated output voltage to com-

plete the feedback loop.

ON/OFF — Allows the switching regulator circuit to be shut

down using logic level signals thus dropping the total input

supply current to approximately 85 µA. Pulling this pin below

a threshold voltage of approximately 1.3V turns the regulator

on, and pulling this pin above 1.3V (up to a maximum of 25V)

shuts the regulator down. If this shutdown feature is not

needed, the ON/OFF pin can be wired to the ground pin or it

can be left open, in either case the regulator will be in the ON

condition.

EXTERNAL COMPONENTS

INPUT CAPACITOR

IN — A low ESR aluminum or tantalum bypass capacitor is

needed between the input pin and ground pin. It must be lo-

cated near the regulator using short leads. This capacitor

prevents large voltage transients from appearing at the in-

put, and provides the instantaneous current needed each

time the switch turns on.

The important parameters for the Input capacitor are the

voltage rating and the RMS current rating. Because of the

relatively high RMS currents flowing in a buck regulator’s in-

put capacitor, this capacitor should be chosen for its RMS

current rating rather than its capacitance or voltage ratings,

although the capacitance value and voltage rating are di-

rectly related to the RMS current rating.

The RMS current rating of a capacitor could be viewed as a

capacitor’s power rating. The RMS current flowing through

the capacitors internal ESR produces power which causes

the internal temperature of the capacitor to rise. The RMS

current rating of a capacitor is determined by the amount of

current required to raise the internal temperature approxi-

mately 10˚C above an ambient temperature of 105˚C. The

ability of the capacitor to dissipate this heat to the surround-

ing air will determine the amount of current the capacitor can

safely sustain. Capacitors that are physically large and have

a large surface area will typically have higher RMS current

ratings. For a given capacitor value, a higher voltage electro-

lytic capacitor will be physically larger than a lower voltage

capacitor, and thus be able to dissipate more heat to the sur-

rounding air, and therefore will have a higher RMS current

rating.

The consequences of operating an electrolytic capacitor

above the RMS current rating is a shortened operating life.

The higher temperature speeds up the evaporation of the ca-

pacitor’s electrolyte, resulting in eventual failure.

Selecting an input capacitor requires consulting the manu-

facturers data sheet for maximum allowable RMS ripple cur-

rent. For a maximum ambient temperature of 40˚C, a gen-

eral guideline would be to select a capacitor with a ripple

current rating of approximately 50% of the DC load current.

For ambient temperatures up to 70˚C, a current rating of

75% of the DC load current would be a good choice for a

conservative design. The capacitor voltage rating must be at

least 1.25 times greater than the maximum input voltage,

and often a much higher voltage capacitor is needed to sat-

isfy the RMS current requirements.

A graph shown in

Figure 13 shows the relationship between

an electrolytic capacitor value, its voltage rating, and the

RMS current it is rated for. These curves were obtained from

the Nichicon “PL” series of low ESR, high reliability electro-

DS012565-21

FIGURE 12.

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