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LM2937ES-2.5 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM2937ES-2.5 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
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6 / 10 page  Application Hints EXTERNAL CAPACITORS The output capacitor is critical to maintaining regulator stabil- ity, and must meet the required conditions for both ESR (Equivalent Series Resistance) and minimum amount of ca- pacitance. MINIMUM CAPACITANCE: The minimum output capacitance required to maintain stabil- ity is 10 µF (this value may be increased without limit). Larger values of output capacitance will give improved tran- sient response. ESR LIMITS: The ESR of the output capacitor will cause loop instability if it is too high or too low. The acceptable range of ESR plotted versus load current is shown in the graph below. It is essen- tial that the output capacitor meet these requirements, or oscillations can result. It is important to note that for most capacitors, ESR is speci- fied only at room temperature. However, the designer must ensure that the ESR will stay inside the limits shown over the entire operating temperature range for the design. For aluminum electrolytic capacitors, ESR will increase by about 30X as the temperature is reduced from 25˚C to −40˚C. This type of capacitor is not well-suited for low tem- perature operation. Solid tantalum capacitors have a more stable ESR over tem- perature, but are more expensive than aluminum electrolyt- ics. A cost-effective approach sometimes used is to parallel an aluminum electrolytic with a solid Tantalum, with the total capacitance split about 75/25% with the Aluminum being the larger value. If two capacitors are paralleled, the effective ESR is the par- allel of the two individual values. The “flatter” ESR of the Tan- talum will keep the effective ESR from rising as quickly at low temperatures. HEATSINKING A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of the application. Under all possible operating conditions, the junc- tion temperature must be within the range specified under Absolute Maximum Ratings. To determine if a heatsink is required, the power dissipated by the regulator, P D, must be calculated. The figure below shows the voltages and currents which are present in the circuit, as well as the formula for calculating the power dissipated in the regulator: The next parameter which must be calculated is the maxi- mum allowable temperature rise, T R (max). This is calcu- lated by using the formula: T R (max) = TJ(max) − TA (max) where: T J (max) is the maximum allowable junction tem- perature, which is 125˚C for commercial grade parts. T A (max) is the maximum ambient temperature which will be encountered in the applica- tion. Using the calculated values for T R(max) and PD, the maxi- mum allowable value for the junction-to-ambient thermal re- sistance, θ (J−A), can now be found: θ (J−A) = TR (max)/PD IMPORTANT: If the maximum allowable value for θ (J−A) is found to be ≥ 53˚C/W for the TO-220 package, ≥ 80˚C/W for the TO-263 package, or ≥174˚C/W for the SOT-223 pack- age, no heatsink is needed since the package alone will dis- sipate enough heat to satisfy these requirements. If the calculated value for θ (J−A)falls below these limits, a heatsink is required. HEATSINKING TO-220 PACKAGE PARTS The TO-220 can be attached to a typical heatsink, or se- cured to a copper plane on a PC board. If a copper plane is to be used, the values of θ (J−A) will be the same as shown in the next section for the TO-263. If a manufactured heatsink is to be selected, the value of heatsink-to-ambient thermal resistance, θ (H−A), must first be calculated: θ (H−A) = θ(J−A) − θ(C−H) − θ(J−C) Where: θ (J−C) is defined as the thermal resistance from the junction to the surface of the case. A value of 3˚C/W can be assumed for θ (J−C) for this calculation. θ (C−H) is defined as the thermal resistance be- tween the case and the surface of the heat- sink. The value of θ (C−H) will vary from about 1.5˚C/W to about 2.5˚C/W (depend- ing on method of attachment, insulator, etc.). If the exact value is unknown, 2˚C/W should be assumed for θ (C−H). When a value for θ (H−A) is found using the equation shown, a heatsink must be selected that has a value that is less than or equal to this number. Output Capacitor ESR DS100113-17 FIGURE 1. ESR Limits DS100113-19 IIN = IL ÷ IG PD = (VIN −VOUT)IL +(VIN)IG FIGURE 2. Power Dissipation Diagram www.national.com 6 |
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