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LP2980IM5X-1.8 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LP2980IM5X-1.8 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 14 page  Application Hints (Continued) Another factor to consider is that Tantalum manufacturers are presently designing their products toward the goal of getting the lowest possible ESR, in an attempt to compete with the new high-value ceramic capacitors. This means that the typical values will probably continue to decline in the future. SELECTING THE OUTPUT CAPACITOR This section contains guidelines for selecting an output ca- pacitor which will maintain good regulator stability over the entire operating temperature range (refer to ESR CURVES). 2.2 µF OUTPUT CAPACITOR The smallest output capacitor which can be used with the LP2980-1.8 is 2.2 µF. However, care must be exercised if this value is used because of the ESR requirement. At load currents ≤ 25mA, the stable ESR range is approxi- mately 0.5 Ω to 6Ω. This range is probably sufficiently wide that most 2.2 µF Tantalum capacitors would fall within it. At higher values of load currents (using a 2.2 µF output capacitor), the stable ESR window gets very narrow. It is likely that a Tantalum capacitor would not be a good choice for a design that must be robust enough for mass production. Instead, a 2.2 µF capacitor with very low ESR (either ce- ramic or film) should be used with a 2 Ω external resistor placed in series to provide the ESR. 4.7 µF OUTPUT CAPACITOR If a 4.7 µF capacitor is used, the stable range of ESR values for 50 mA operation is approximately 0.6 Ω to 6Ω. Because of the reduced ESR values of the new Tantalum, it is possible to find 4.7 µF Tantalum capacitors with ESR values at or below 0.6 Ω. To ensure a stable design, it is recommended that an external resistor (value about 0.5 Ω) be added in series with the 4.7 µF Tantalum to provide adequate mini- mum ESR. At values of load current ≤ 20 mA, the ESR range is wide enough that Tantalum can be used without external resis- tance for added ESR. Another acceptable configuration for 50 mA operation is to use a ceramic or film 4.7 µF capacitor (which has very low ESR) with an external 2 Ω resistor in series. 10 µF OUTPUT CAPACITOR 50 mA operation using a 10 µF output capacitor requires an ESR in the range of approximately 0.4 Ω to 7Ω. As stated previously, it is possible that solid Tantalum capacitors can be found with ESR values near to or below 0.4 Ω. An external resistor in series with the Tantalum (value of about 0.5 Ω)is recommended to assure unconditional stability. At values of load current ≤ 20 mA, the ESR range is wide enough that Tantalum can be used without external resis- tance for added ESR. Another acceptable configuration for 50 mA operation is to use a ceramic or film 10 µF capacitor (which has very low ESR) with an external 2 Ω resistor in series. 15 µF OUTPUT CAPACITOR The stable ESR range (for 50 mA load current) using a 15 µF output capacitor is approximately 0.1 Ω to 10Ω.Itisex- tremely unlikely that a 15 µF Tantalum capacitor would be found with an ESR below 0.1 Ω, so no external resistance is required with a Tantalum. As before, another acceptable configuration for 50 mA op- eration is to use a ceramic or film 15 µF capacitor with an external 2 Ω resistor in series. ON/OFF INPUT OPERATION The LP2980 is shut off by pulling the ON/OFF input low, and turned on by pulling it high. If this feature is not to be used, this pin should be tied to V IN to keep the regulator on at all times. To ensure proper operation, the signal source used to drive the ON/OFF input must be able to swing above and below the specified turn-on/turn-off voltage thresholds (see Electri- cal Characteristics). It is also important that the turn-on (and turn-off) voltage signals applied to the ON/OFF input have a slew rate which is not less than 40 mV/µs. CAUTION: The regulator output state can not be guaranteed if a slow-moving AC (or DC) signal is applied that it is in the range between the turn-on/turn-off voltage thresholds speci- fied in the Electrical Characteristics section. REVERSE INPUT/OUTPUT VOLTAGE The PNP power transistor used as the pass element in the LP2980 has an inherent diode connected between the regu- lator output and input. During normal operation (where the input voltage is higher than the output) this diode is reverse- biased. However, if the output is pulled above the input, this diode will turn ON and current will flow into the regulator output. In such cases, a parasitic SCR can latch which will allow a high current to flow into V IN (and out the ground pin), which can damage the part. In any application where the output may be pulled above the input, an external Schottky diode must be connected from V IN to VOUT (cathode on VIN, anode on VOUT), to limit the reverse voltage across the LP2980 to 0.3V (see Basic Ap- plication Circuit). www.national.com 11 |
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