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LP2981IM5X-2.7 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LP2981IM5X-2.7 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page Typical Performance Characteristics Unless otherwise specified: T A = 25˚C, VIN =VO(NOM) + 1V, COUT = 4.7 µF, CIN = 1 µF all voltage options, ON/OFF pin tied to VIN. (Continued) Application Hints EXTERNAL CAPACITORS Like any low-dropout regulator, the external capacitors used with the LP2981 must be carefully selected to assure regula- tor loop stability. INPUT CAPACITOR: An input capacitor whose value is ≥ 1 µF is required with the LP2981 (amount of capacitance can be increased without limit). This capacitor must be located a distance of not more than 0.5" from the input pin of the LP2981 and returned to a clean analog ground. Any good quality ceramic or tantalum can be used for this capacitor. OUTPUT CAPACITOR: The output capacitor must meet both the requirement for minimum amount of capacitance and E.S.R. (equivalent series resistance) value. Curves are provided which show the allowable ESR range as a function of load current for various output voltages and capacitor val- ues (refer to Figures 1, 2, 3, 4). IMPORTANT: The output capacitor must maintain its ESR in the stable region over the full operating temperature range to assure stability. Also, capacitor tolerance and variation with temperature must be considered to assure the minimum amount of capacitance is provided at all times. This capacitor should be located not more than 0.5" from the output pin of the LP2981 and returned to a clean analog ground. CAPACITOR CHARACTERISTICS TANTALUM: Tantalum capacitors are the best choice for use with the LP2981. Most good quality tantalums can be used with the LP2981, but check the manufacturer’s data sheet to be sure the ESR is in range. It is important to remember that ESR increases at lower tem- peratures and a capacitor that is near the upper limit for sta- bility at room temperature can cause instability when it gets cold. In applications which must operate at very low temperatures, it may be necessary to parallel the output tantalum capacitor with a ceramic capacitor to prevent the ESR from going up too high (see next section for important information on ce- ramic capacitors). CERAMIC: Ceramic capacitors are not recommended for use at the output of the LP2981. This is because the ESR of a ceramic can be low enough to go below the minimum stable value for the LP2981. A 2.2 µF ceramic was measured and found to have an ESR of about 15 m Ω, which is low enough to cause oscillations. If a ceramic capacitor is used on the output, a 1 Ω resistor should be placed in series with the capacitor. ALUMINUM: Because of large physical size, aluminum elec- trolytics are not typically used with the LP2981. They must meet the same ESR requirements over the operating tem- perature range, more difficult because of their steep increase at cold temperature. An aluminum electrolytic can exhibit an ESR increase of as much as 50X when going from 20˚C to −40˚C. Also, some aluminum electrolytics are not operational below −25˚C be- cause the electrolyte can freeze. ON/OFF Pin Current vs V ON/OFF DS012506-36 ON/OFF Threshold vs Temperature DS012506-37 DS012506-38 FIGURE 1. 5V/3.3 µF ESR Curves www.national.com 11 |
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