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LT1764EQ-1.8 Datasheet(PDF) 10 Page - Linear Technology |
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LT1764EQ-1.8 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT1764 Series 1764fa The LT1764 series are 3A low dropout regulators opti- mized for fast transient response. The devices are capable of supplying 3A at a dropout voltage of 340mV. The low operating quiescent current (1mA) drops to less than 1 µA in shutdown. In addition to the low quiescent current, the LT1764 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 LT1764-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 LT1764 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. Figure 2. Adjustable Operation 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 3A 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 LT1764 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 LT1764-X, 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 ca- pacitors with an ESR below 50m Ω can cause oscillations. Please consult our Applications Engineering department for help with any issues concerning the use of ceramic output 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 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 expensive 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 IN 1764 F02 R2 OUT VIN VOUT ADJ GND LT1764 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 |
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