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LP2985IM5X-1.8 Datasheet(PDF) 11 Page - Texas Instruments |
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LP2985IM5X-1.8 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 22 page LP2985LV-N www.ti.com SNOS510P – NOVEMBER 1999 – REVISED APRIL 2013 Noise Bypass Capacitor Connecting a 10 nF capacitor to the Bypass pin significantly reduces noise on the regulator output. It should be noted that the capacitor is connected directly to a high-impedance circuit in the bandgap reference. Because this circuit has only a few microamperes flowing in it, any significant loading on this node will cause a change in the regulated output voltage. For this reason, DC leakage current through the noise bypass capacitor must never exceed 100 nA, and should be kept as low as possible for best output voltage accuracy. The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High-quality ceramic capacitors with either NPO or COG dielectric typically have very low leakage. 10 nF polypropolene and polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low leakage current. CAPACITOR CHARACTERISTICS The LP2985LV-N was designed to work with ceramic capacitors on the output to take advantage of the benefits they offer: for capacitance values in the 2.2 µF to 4.7 µF range, ceramics are the least expensive and also have the lowest ESR values (which makes them best for eliminating high-frequency noise). The ESR of a typical 2.2 µF ceramic capacitor is in the range of 10 m Ω to 20 mΩ, which easily meets the ESR limits required for stability by the LP2985LV-N. One disadvantage of ceramic capacitors is that their capacitance can vary with temperature. Most large value ceramic capacitors ( ≥ 2.2 µF) are manufactured with the Z5U or Y5V temperature characteristic, which results in the capacitance dropping by more than 50% as the temperature goes from 25°C to 85°C. This could cause problems if a 2.2 µF capacitor were used on the output since it will drop down to approximately 1 µF at high ambient temperatures (which could cause the LP2985LV-N to oscillate). If Z5U or Y5V capacitors are used on the output, a minimum capacitance value of 4.7 µF must be observed. A better choice for temperature coefficient in ceramic capacitors is X7R, which holds the capacitance within ±15%. Unfortunately, the larger values of capacitance are not offered by all manufacturers in the X7R dielectric. Tantalum Tantalum capacitors are less desirable than ceramics for use as output capacitors because they are more expensive when comparing equivalent capacitance and voltage ratings in the 1 µF to 4.7 µF range. Another important consideration is that Tantalum capacitors have higher ESR values than equivalent size ceramics. This means that while it may be possible to find a Tantalum capacitor with an ESR value within the stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic capacitor with the same ESR value. It should also be noted that the ESR of a typical Tantalum will increase about 2:1 as the temperature goes from 25°C down to −40°C, so some guard band must be allowed. On/Off Input Operation The LP2985LV-N is shut off by driving the ON/OFF input low, and turned on by pulling it high. If this feature is not to be used, the ON/OFF input should be tied to VIN to keep the regulator output on at all times. To assure 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 listed in the ELECTRICAL CHARACTERISTICS(1) section under VON/OFF. To prevent mis-operation, the turn-on (and turn-off) voltage signals applied to the ON/OFF input must have a slew rate which is ≥ 40 mV/µs. CAUTION The regulator output voltage cannot be ensured if a slow-moving AC (or DC) signal is applied that is in the range between the specified turn-on and turn-off voltages listed under the electrical specification VON/OFF (see Electrical Characteristics). (1) Exposing the DSBGA device to direct sunlight will cause misoperation. See APPLICATION HINTS for additional information. Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LP2985LV-N |
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