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LM2595 Datasheet(PDF) 17 Page - National Semiconductor (TI) |
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LM2595 Datasheet(HTML) 17 Page - National Semiconductor (TI) |
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17 / 29 page  Block Diagram Application Information PIN FUNCTIONS +V 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 C 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. www.national.com 17 |
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