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LT1963EST Datasheet(PDF) 10 Page - Linear Integrated Systems |
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LT1963EST Datasheet(HTML) 10 Page - Linear Integrated Systems |
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10 / 16 page  10 LT1963 Series 1963fa Figure 2. Adjustable Operation IN 1963 F02 R2 OUT VIN VOUT ADJ GND LT1963 R1 + VV R R IR VV IA OUT ADJ ADJ ADJ =+ + ()( ) = =° 121 1 2 1 2 121 3 . . µ AT 25 C OUTPUT RANGE = 1.21V TO 20V APPLICATIO S I FOR ATIO The LT1963 series are 1.5A low dropout regulators opti- mized for fast transient response. The devices are capable of supplying 1.5A at a dropout voltage of 350mV. The low operating quiescent current (1mA) drops to less than 1 µA in shutdown. In addition to the low quiescent current, the LT1963 regulators incorporate several protection features which make them ideal for use in battery-powered sys- tems. The devices are protected against both reverse input and reverse output voltages. In battery backup applica- tions where the output can be held up by a backup battery when the input is pulled to ground, the LT1963-X acts like it has a diode in series with its output and prevents reverse current flow. Additionally, in dual supply applications where the regulator load is returned to a negative supply, the output can be pulled below ground by as much as 20V and still allow the device to start and operate. Adjustable Operation The adjustable version of the LT1963 has an output voltage range of 1.21V to 20V. The output voltage is set by the ratio of two external resistors as shown in Figure 2. The device servos the output to maintain the voltage at the ADJ pin at 1.21V referenced to ground. The current in R1 is then equal to 1.21V/R1 and the current in R2 is the current in R1 plus the ADJ pin bias current. The ADJ pin bias current, 3 µA at 25°C, flows through R2 into the ADJ pin. The output voltage can be calculated using the formula in Figure 2. The value of R1 should be less than 4.17k to minimize errors in the output voltage caused by the ADJ pin bias current. Note that in shutdown the output is turned off and the divider current will be zero. The adjustable device is tested and specified with the ADJ pin tied to the OUT pin for an output voltage of 1.21V. Specifications for output voltages greater than 1.21V will be proportional to the ratio of the desired output voltage to 1.21V: VOUT/1.21V. For example, load regulation for an output current change of 1mA to 1.5A is – 3mV typical at VOUT = 1.21V. At VOUT = 5V, load regulation is: (5V/1.21V)(–3mV) = – 12.4mV Output Capacitance and Transient Response The LT1963 regulators are designed to be stable with a wide range of output capacitors. The ESR of the output capacitor affects stability, most notably with small capaci- tors. A minimum output capacitor of 10 µF with an ESR in the range of 50m Ω to 3Ω is recommended to prevent oscillations. Larger values of output capacitance can de- crease the peak deviations and provide improved transient response for larger load current changes. Bypass capaci- tors, used to decouple individual components powered by the LT1963, will increase the effective output capacitor value. Extra consideration must be given to the use of ceramic capacitors. In some applications, the use of ceramic capacitors with an ESR below 50m Ω can cause oscilla- tions. Please consult our Applications Engineering depart- ment for help with any issues concerning the use of ceramic output capacitors. Ceramic capacitors are manu- factured with a variety of dielectrics, each with different behavior over temperature and applied voltage. The most common dielectrics 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 temperature coefficients as shown in Figures 3 and 4. 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 |
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