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LM2991WG-QML датащи(PDF) 8 Page - National Semiconductor (TI) |
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LM2991WG-QML датащи(HTML) 8 Page - National Semiconductor (TI) |
8 / 13 page Typical Performance Characteristics (Continued) Adjust Pin Current Low Voltage Behavior 20158725 20158726 Application Hints EXTERNAL CAPACITORS Like any low-dropout regulator, external capacitors are re- qired to stabilize the control loop. These capacitors must be correctly selected for proper performance. INPUT CAPACITOR An input capacitor is required if the regulator is located more than 6" from the input power supply filter capacitor (or if no other input capacitor is present). A solid Tantalum or ceramic capacitor whose value is at least 1 µF is recommended, but an aluminum electrolytic ( ≥ 10 µF) may be used. However, aluminum electrolytics should not be used in applications where the ambient temperature can drop below 0˚C because their internal impedance increases significantly at cold temperatures. OUTPUT CAPACITOR The output capacitor must meet the ESR limits shown in the graph, which means it must have an ESR between about 25 m Ω and 10Ω. A solid Tantalum (value ≥ 1 µF) is the best choice for the output capacitor. An aluminum electrolytic ( ≥ 10 µF) may be used if the ESR is in the stable range. It should be noted that the ESR of a typical aluminum electrolytic will increase by as much as 50X as the tempera- ture is reduced from 25˚C down to −40˚C, while a Tantalum will exhibit an ESR increase of about 2X over the same range. For this and other reasons, aluminum electrolytics should not be used in applications where low operating temperatures occur. The lower stable ESR limit of 25 m Ω means that ceramic capacitors can not be used directly on the output of an LDO. A ceramic ( ≥ 2.2 µF) can be used on the output if some external resistance is placed in series with it (1 Ω recom- mended). Dielectric types X7R or X5R must be used if the temperature range of the application varies more than ± 25˚C from ambient to assure the amount of capacitance is sufficient. CERAMIC BYPASS CAPACITORS Many designers place distributed ceramic capacitors whose value is in the range of 1000 pF to 0.1 µF at the power input pins of the IC’s across a circuit board. These can cause reduced phase margin or oscillations in LDO regulators. The advent of multi-layer boards with dedicated power and ground planes has removed the trace inductance that (pre- viously) provided the necessary "decoupling" to shield the output of the LDO from the effects of bypass capacitors. These capacitors should be avoided if possible, and kept as far away from the LDO output as is practical. Output Capacitor ESR Range 20158705 MINIMUM LOAD A minimum load current of 500 µA is required for proper operation. The external resistor divider can provide the mini- mum load, with the resistor from the adjust pin to ground set to 2.4 k Ω. SETTING THE OUTPUT VOLTAGE The output voltage of the LM2991 is set externally by a resistor divider using the following equation: V OUT =VREF x(1+R2/R1)−(IADJ xR2) where V REF = −1.21V. The output voltage can be pro- grammed within the range of −3V to −24V, typically an even greater range of −2V to −25V. The adjust pin current is about 60 nA, causing a slight error in the output voltage. However, www.national.com 8 |
Аналогичный номер детали - LM2991WG-QML |
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Аналогичное описание - LM2991WG-QML |
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