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LT1764 Datasheet(PDF) 11 Page - Linear Technology |
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LT1764 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1764 Series 1764fb 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. 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 Figure 2. Adjustable Operation 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 transi- ent response for larger load current changes. Bypass capacitors, used to decouple individual components pow- ered by the LT1764-X, 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 across temperature and applied voltage. The most common di- electrics used are specified with EIA temperature charac- teristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances in a small package, but they tend to have strong voltage and temperature coefficients as shown in Figures 3 and 4. When used with a 5V regulator, a 16V 10 µF Y5V capacitor can exhibit an effective value as low as 1 µF to 2µF for the DC bias voltage applied and over the operating tempera- ture 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. Care still must be exercised when using X5R and X7R capacitors; the X5R and X7R codes only specify operating temperature range and maxi- mum capacitance change over temperature. Capacitance change due to DC bias with X5R and X7R capacitors is better than Y5V and Z5U capacitors, but can still be significant enough to drop capacitor values below appro- priate levels. Capacitor DC bias characteristics tend to improve as component case size increases, but expected capacitance at operating voltage should be verified. 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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