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LT1764AEFE-3.3 Datasheet(PDF) 11 Page - Linear Technology |
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LT1764AEFE-3.3 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1764A Series 1764afb The LT1764A 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 LT1764A regulators incorporate several protection fea- tures 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 LT1764A-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 LT1764A 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 3A is – 3mV typical at VOUT = 1.21V. At VOUT = 5V, load regulation is: (5V/1.21V)(–3mV) = – 12.4mV APPLICATIO S I FOR ATIO Output Capacitors and Stability The LT1764A regulator is a feedback circuit. Like any feedback circuit, frequency compensation is needed to make it stable. For the LT1764A, the frequency compensa- tion is both internal and external—the output capacitor. The size of the output capacitor, the type of the output capacitor, and the ESR of the particular output capacitor all affect the stability. In addition to stability, the output capacitor also affects the high frequency transient response. The regulator loop has a finite band width. For high frequency transient loads, recovery from a transient is a combination of the output capacitor and the bandwidth of the regulator. The LT1764A was designed to be easy to use and accept a wide variety of output capacitors. However, the frequency compensation is affected by the output capacitor and optimum frequency stability may require some ESR, espe- cially with ceramic capacitors. For ease of use, low ESR polytantalum capacitors (POSCAP) are a good choice for both the transient response and stability of the regulator. These capacitors have intrinsic ESR that improves the stability. Ceramic capacitors have extremely low ESR, and while they are a good choice in many cases, placing a small series resistance element will sometimes achieve optimum stability and minimize ring- ing. In all cases, a minimum of 10 µF is required while the maximum ESR allowable is 3 Ω. The place where ESR is most helpful with ceramics is low output voltage. At low output voltages, below 2.5V, some ESR helps the stability when ceramic output capacitors are used. Also, some ESR allows a smaller capacitor value to be used. When small signal ringing occurs with ceramics due to insufficient ESR, adding ESR or increas- Figure 2. Adjustable Operation IN 1764 F02 R2 OUT VIN VOUT ADJ GND LT1764A 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 |
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