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LT1761 Datasheet(PDF) 10 Page - Linear Technology |
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LT1761 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT3020/LT3020-1.2/ LT3020-1.5/LT3020-1.8 3020fc APPLICATIO S I FOR ATIO an output current change of 1mA to 100mA is typically 0.4mV at VADJ = 200mV. At VOUT = 1.5V, load regulation is: (1.5V/200mV) • (0.4mV) = 3mV Output Capacitance and Transient Response The LT3020’s design is stable with a wide range of output capacitors, but is optimized for low ESR ceramic capaci- tors. The output capacitor’s ESR affects stability, most notably with small value capacitors. Use a minimum output capacitor of 2.2 µF with an ESR of 0.3Ω or less to prevent oscillations. The LT3020 is a low voltage device, and output load transient response is a function of output capacitance. Larger values of output capacitance decrease the peak deviations and provide improved transient re- sponse for larger load current changes. For output capaci- tor values greater than 20 µF a small feedforward capacitor with a value of 300pF across the upper divider resistor (R2 in Figure 1) is required. Give extra consideration to the use of ceramic capacitors. Manufacturers make ceramic capacitors with a variety of dielectrics, each with a different behavior across tempera- ture and applied voltage. The most common dielectrics are Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics provide high C-V products in a small package at low cost, but exhibit strong voltage and temperature coefficients. The X5R and X7R dielectrics yield highly stable characterisitics and are more suitable for use as the output capacitor at fractionally increased cost. The X5R and X7R dielectrics both exhibit excellent voltage coefficient char- acteristics. The X7R type works over a larger temperature range and exhibits better temperature stability whereas X5R is less expensive and is available in higher values. Figures 2 and 3 show voltage coefficient and temperature coefficient comparisons between Y5V and X5R material. 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, simi- lar to the way a piezoelectric accelerometer or microphone works. For a ceramic capacitor, the stress can be induced by vibrations in the system or thermal transients. The re- sulting voltages produced can cause appreciable amounts of noise. A ceramic capacitor produced Figure 4’s trace in DC BIAS VOLTAGE (V) 3020 F02 20 0 –20 –40 –60 –80 –100 0 4 8 10 26 12 14 X5R Y5V 16 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10 µF TEMPERATURE ( °C) –50 40 20 0 –20 –40 –60 –80 –100 25 75 3020 F03 –25 0 50 100 125 Y5V X5R BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10 µF Figure 2. Ceramic Capacitor DC Bias Characteristics Figure 3. Ceramic Capacitor Temperature Characteristics 1ms/DIV 3020 F04 1mV/DIV VOUT = 1.3V COUT = 10µF ILOAD = 0 Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor |
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