Datasheet LT3012B (Analog Devices) - 8

LT3012B
U U W U APPLICATIO S I FOR ATIO
The LT3012B is a 250mA high voltage low dropout regu- to the ratio of the desired output voltage to 1.24V; (VOUT/ lator with micropower quiescent current. The device is 1.24V). For example, load regulation for an output current capable of supplying 250mA at a dropout voltage of change of 1mA to 250mA is –7mV typical at VOUT = 1.24V. 400mV. Operating quiescent current is only 40µA. In At VOUT = 12V, load regulation is: addition to the low quiescent current, the LT3012B incor- (12V/1.24V) • (–7mV) = –68mV porates several protection features which make it ideal for use in battery-powered systems. The device is protected
Output Capacitance and Transient Response
against both reverse input and reverse output voltages. In battery backup applications where the output can be held The LT3012B is designed to be stable with a wide range of up by a backup battery when the input is pulled to ground, output capacitors. The ESR of the output capacitor affects the LT3012B acts like it has a diode in series with its output stability, most notably with small capacitors. A minimum and prevents reverse current flow. output capacitor of 3.3µF with an ESR of 3Ω or less is recommended to prevent oscillations. The LT3012B is a
Adjustable Operation
micropower device and output transient response will be a function of output capacitance. Larger values of output The LT3012B has an output voltage range of 1.24V to capacitance decrease the peak deviations and provide 60V. The output voltage is set by the ratio of two external improved transient response for larger load current resistors as shown in Figure 1. The device servos the changes. Bypass capacitors, used to decouple individual output to maintain the voltage at the adjust pin at 1.24V components powered by the LT3012B, will increase the referenced to ground. The current in R1 is then equal to effective output capacitor value. 1.24V/R1 and the current in R2 is the current in R1 plus the ADJ pin bias current. The ADJ pin bias current, 30nA Extra consideration must be given to the use of ceramic at 25°C, flows through R2 into the ADJ pin. The output capacitors. Ceramic capacitors are manufactured with a voltage can be calculated using the formula in Figure 1. variety of dielectrics, each with different behavior across The value of R1 should be less than 250k to minimize temperature and applied voltage. The most common di- errors in the output voltage caused by the ADJ pin bias electrics used are specified with EIA temperature charac- current. teristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances in a The adjustable device is tested and specified with the ADJ small package, but they tend to have strong voltage and pin tied to the OUT pin and a 5µA DC load (unless otherwise temperature coefficients as shown in Figures 2 and 3. specified) for an output voltage of 1.24V. Specifications When used with a 5V regulator, a 16V 10µF Y5V capacitor for output voltages greater than 1.24V will be proportional IN OUT VOUT LT3012B R2 C1 + VIN ADJ GND R1 3012B F01 R2 V ( ) OUT = 1.24V 1 + + (IADJ)(R2) R1 VADJ = 1.24V IADJ = 30nA AT 25°C OUTPUT RANGE = 1.24V TO 60V
Figure 1. Adjustable Operation
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