Datasheet LT3014 (Analog Devices) - 8

LT3014
APPLICATIONS INFORMATION
The LT3014 is a 20mA high voltage low dropout regulator is –13mV typical at VOUT = 1.22V. At VOUT = 12V, load with micropower quiescent current and shutdown. The regulation is: device is capable of supplying 20mA at a dropout voltage (12V/1.22V) • (–13mV) = –128mV of 350mV. The low operating quiescent current (7μA) drops to 1μA in shutdown. In addition to the low quiescent cur-
Output Capacitance and Transient Response
rent, the LT3014 incorporates several protection features which make it ideal for use in battery-powered systems. The LT3014 is designed to be stable with a wide range of The device is protected against both reverse input and output capacitors. The ESR of the output capacitor affects reverse output voltages. In battery backup applications stability, most notably with small capacitors. A minimum where the output can be held up by a backup battery output capacitor of 0.47μF with an ESR of 3Ω or less is when the input is pulled to ground, the LT3014 acts like it recommended to prevent oscillations. The LT3014 is a has a diode in series with its output and prevents reverse micropower device and output transient response will be current fl ow. a function of output capacitance. Larger values of output capacitance decrease the peak deviations and provide
Adjustable Operation
improved transient response for larger load current changes. Bypass capacitors, used to decouple individual The LT3014 has an output voltage range of 1.22V to 60V. components powered by the LT3014, will increase the The output voltage is set by the ratio of two external effective output capacitor value. resistors as shown in Figure 1. The device servos the output to maintain the voltage at the adjust pin at 1.22V Extra consideration must be given to the use of ceramic referenced to ground. The current in R1 is then equal to capacitors. Ceramic capacitors are manufactured with a 1.22V/R1 and the current in R2 is the current in R1 plus variety of dielectrics, each with different behavior across the ADJ pin bias current. The ADJ pin bias current, 4nA temperature and applied voltage. The most common at 25°C, fl ows through R2 into the ADJ pin. The output dielectrics used are specifi ed with EIA temperature char- voltage can be calculated using the formula in Figure 1. acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and The value of R1 should be less than 1.62M to minimize Y5V dielectrics are good for providing high capacitances errors in the output voltage caused by the ADJ pin bias in a small package, but they tend to have strong voltage current. Note that in shutdown the output is turned off and temperature coeffi cients as shown in Figures 2 and 3. and the divider current will be zero. The device is tested When used with a 5V regulator, a 16V 10μF Y5V capacitor and specifi ed with the ADJ pin tied to the OUT pin and a can exhibit an effective value as low as 1μF to 2μF for the 5μA DC load (unless otherwise specifi ed) for an output DC bias voltage applied and over the operating tempera- voltage of 1.22V. Specifi cations for output voltages greater ture range. The X5R and X7R dielectrics result in more than 1.22V will be proportional to the ratio of the desired stable characteristics and are more suitable for use as the output voltage to 1.22V (V output capacitor. The X7R type has better stability across OUT/1.22V). For example, load regulation for an output current change of 1mA to 20mA temperature, while the X5R is less expensive and is avail- able in higher values. Care still must be exercised when IN OUT VOUT using X5R and X7R capacitors; the X5R and X7R codes LT3014 R2 + VIN only specify operating temperature range and maximum ADJ capacitance change over temperature. Capacitance change GND R1 due to DC bias with X5R and X7R capacitors is better than 3014 F01 Y5V and Z5U capacitors, but can still be signifi cant enough R2 VOUT = 1.22V • 1 + + (IADJ)(R2) to drop capacitor values below appropriate levels. Capaci- VADJ = 1.22V tor DC bias characteristics tend to improve as component IADJ = 4nA AT 25oC OUTPUT RANGE = 1.22V TO 60V case size increases, but expected capacitance at operating voltage should be verifi ed.
Figure 1. Adjustable Operation
3014fd 8