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LT1963A-1.8 Datasheet(PDF) 11 Page - Linear Technology |
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LT1963A-1.8 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1963A Series 1963af Figure 2. Adjustable Operation IN 1963 F02 R2 OUT VIN VOUT ADJ GND LT1963A R1 + VV R R IR VV IA OUT ADJ ADJ ADJ =+ Ê ËÁ ˆ ¯˜ + ()( ) = = ∞ 121 1 2 1 2 121 3 . . m AT 25 C OUTPUT RANGE = 1.21V TO 20V Figure 3. Ceramic Capacitor DC Bias Characteristics APPLICATIO S I FOR ATIO DC BIAS VOLTAGE (V) 1963 F03 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 The LT1963A series are 1.5A low dropout regulators op- timized for fast transient response. The devices are ca- pable of supplying 1.5A at a dropout voltage of 350mV. The low operating quiescent current (1mA) drops to less than 1 mA in shutdown. In addition to the low quiescent current, the LT1963A regulators incorporate several pro- tection features which make them ideal for use in battery- powered systems. The devices are protected against both reverse input and reverse output voltages. In battery backup applications where the output can be held up by a backup battery when the input is pulled to ground, the LT1963A-X acts like it has a diode in series with its output and prevents reverse current flow. Additionally, in dual supply applica- tions 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 LT1963A 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 mA 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 LT1963A 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 mF with an ESR of 3 Worlessisrecommendedtopreventoscillations.Larger values of output capacitance can decrease the peak devia- tions and provide improved transient response for larger load current changes. Bypass capacitors, used to decouple individual components powered by the LT1963A, 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 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 tem- perature coefficients as shown in Figures 3 and 4. When used with a 5V regulator, a 10 mF Y5V capacitor can exhibit |
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