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LT1763 Datasheet(PDF) 9 Page - Linear Technology |
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LT1763 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page LT1964 9 1964f Bypass Capacitance and Low Noise Performance The LT1964 may be used with the addition of a bypass capacitor from VOUT to the BYP pin to lower output voltage noise. A good quality low leakage capacitor is recommended. This capacitor will bypass the reference of the LT1964, providing a low frequency noise pole. The noise pole provided by this bypass capacitor will lower the output voltage noise to as low as 30 µVRMS with the addition of a 0.01 µF bypass capacitor. Using a bypass capacitor has the added benefit of improving transient response. With no bypass capacitor and a 10 µF output capacitor, a –10mA to –200mA load step will settle to within 1% of its final value in less than 100 µs. With the addition of a 0.01 µF bypass capacitor, the output will stay within 1% for the same –10mA to –200mA load step (see LT1964-5 Transient Response in the Typical Characteris- tics section). However, regulator start-up time is inversely proportional to the size of the bypass capacitor. Higher values of output voltage noise may be measured if care is not exercised with regard to circuit layout and testing. Crosstalk from nearby traces can induce unwanted noise onto the output of the LT1964-X. APPLICATIO S I FOR ATIO 1964 F01 GND ADJ IN OUT LT1964 VIN VOUT + R1 R2 VOUT = –1.22V(1 + ) – (IADJ)(R2) VADJ = –1.22V IADJ = 30nA AT 25°C OUTPUT RANGE = –1.22V TO –20V R2 R1 Output Capacitance and Transient Response The LT1964 is designed to be stable with a wide range of output capacitors. The ESR of the output capacitor affects stability, most notably with small capacitors. A minimum output capacitor of 1 µF with an ESR of 3Ω or less is recommended to prevent oscillations. The LT1964 is a micropower device and output transient response will be a function of output capacitance. Larger values of output capacitance decrease the peak deviations and provide improved transient response for larger load current changes. Bypass capacitors, used to decouple individual components powered by the LT1964, will increase the effective output capacitor value. Extra consideration must be given to the use of ceramic capacitors. Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior across temperature and applied voltage. The most common di- electrics used are Z5U, Y5V, X5R, and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances in a small package, but exhibit strong voltage and tem- perature coefficients as shown in Figures 2 and 3. When used with a –5V regulator, a 10 µF Y5V capacitor can exhibit an effective value as low as 1 µF to 2µF over the operating temperature range. The X5R and X7R dielectrics result in more stable characteristics and are more suitable for use as the output capacitor. The X7R type has better stability across temperature, while the X5R is less expen- sive and is available in higher values. Voltage and temperature coefficients are not the only sources of problems. Some ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals due to mechanical stress, similar to the way a piezoelectric accelerometer or micro- phone works. For a ceramic capacitor the stress can be induced by vibrations in the system or thermal transients. The resulting voltages produced can cause appreciable amounts of noise, especially when a ceramic capacitor is used for noise bypassing. A ceramic capacitor produced Figure 4’s trace in response to light tapping from a pencil. Similar vibration induced behavior can masquerade as increased output voltage noise. Figure 1. Adjustable Operation |
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