Datasheet LTC1844 (Analog Devices) - 9

LTC1844 Series
U U W U APPLICATIONS INFORMATION
temperature and applied voltage. The most common di- Additionally, some ceramic capacitors have a piezoelectric electrics used are Z5U, Y5V, X5R and X7R. The Z5U and response. A piezoelectric device generates voltage across Y5V dielectrics are good for providing high capacitances its terminals due to mechanical stress, similar to the way in a small package, but exhibit strong voltage and tem- a piezoelectric accelerometer or microphone works. For a perature coefficients as shown in Figures 4 and 5. When ceramic capacitor the stress can be induced by vibrations used with a 5V regulator, a 10µF Y5V capacitor can exhibit in the system or thermal transients. The resulting voltages an effective value as low as 1µF to 2µF over the operating produced can cause appreciable amounts of noise, espe- temperature range. The X5R and X7R dielectrics result in cially when a ceramic capacitor is used for noise bypass- more stable characteristics and are more suitable for use ing. A ceramic capacitor produced Figure 6’s trace in as the output capacitor. The X7R type has better stability response to light tapping from a pencil. Similar vibration- across temperature, while the X5R is less expensive and induced behavior can masquerade as increased output is available in higher values. voltage noise. LTC1844-2.8 20 COUT = 10µF BOTH CAPACITORS ARE 16V, CBYP = 0.01µF 1210 CASE SIZE, 10µF ILOAD = 100mA 0 X5R –20 VOUT 500µV/DIV –40 –60 CHANGE IN VALUE (%) Y5V –80 100ms/DIV 1844 F06 –100 0 2 4 6 8 10 12 14 16 DC BIAS VOLTAGE (V)
Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor
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Figure 4. Ceramic Capacitor DC Bias Characteristics Dropout Recovery and Output Overshoot
40 If the input supply voltage drops too low for the LTC1844 20 to maintain regulation, the internal feedback loop goes into dropout and the internal pass transistor turns fully on. 0 X5R If the input supply then suddenly rises, the output may –20 briefly overshoot the intended output voltage while the –40 LTC1844 transitions back from dropout to normal opera- Y5V tion. This behavior occurs when the input supply slew rate –60 CHANGE IN VALUE (%) is greater than 1V/ms and the output bypass capacitor is –80 BOTH CAPACITORS ARE 16V, small. If the input is expected to slew rapidly, an output 1210 CASE SIZE, 10µF –100 bypass capacitor of 10µF or greater should be used to –50 –25 0 25 50 75 100 125 minimize output overshoot. Note that overshoot typically TEMPERATURE (°C) 1844 F05 does not occur at start-up since the feedback loop does not spend a significant amount of time in dropout.
Figure 5. Ceramic Capacitor Temperature Characteristics
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