Datasheet LT3014B (Analog Devices) - 8

LT3014B
APPLICATIONS INFORMATION
The LT3014B is a 20mA high voltage, low dropout regu-
Output Capacitance and Transient Response
lator with micropower quiescent current. The device is The LT3014B is designed to be stable with a wide range of capable of supplying 20mA at a dropout voltage of 350mV. output capacitors. The ESR of the output capacitor affects Operating quiescent current is only 7µA. In addition to stability, most notably with small capacitors. A minimum the low quiescent current, the LT3014B incorporates output capacitor of 0.47µF with an ESR of 3Ω or less is several protection features which make it ideal for use in recommended to prevent oscillations. The LT3014B is a battery-powered systems. The device is protected against micropower device and output transient response will be both reverse input and reverse output voltages. In battery a function of output capacitance. Larger values of output backup applications where the output can be held up by capacitance decrease the peak deviations and provide a backup battery when the input is pulled to ground, the improved transient response for larger load current LT3014B acts like it has a diode in series with its output changes. Bypass capacitors, used to decouple individual and prevents reverse current flow. components powered by the LT3014B, will increase the
Adjustable Operation
effective output capacitor value. The LT3014B has an output voltage range of 1.22V to Extra consideration must be given to the use of ceramic 60V. The output voltage is set by the ratio of two external capacitors. Ceramic capacitors are manufactured with a resistors as shown in Figure 1. The device servos the variety of dielectrics, each with different behavior across output to maintain the voltage at the adjust pin at 1.22V temperature and applied voltage. The most common referenced to ground. The current in R1 is then equal to dielectrics used are specified with EIA temperature char- 1.22V/R1 and the current in R2 is the current in R1 plus acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and the ADJ pin bias current. The ADJ pin bias current, 4nA Y5V dielectrics are good for providing high capacitances at 25°C, flows through R2 into the ADJ pin. The output in a small package, but they tend to have strong voltage voltage can be calculated using the formula in Figure 1. and temperature coefficients as shown in Figures 2 and 3. The value of R1 should be less than 1.62M to minimize When used with a 5V regulator, a 16V 10µF Y5V capacitor errors in the output voltage caused by the ADJ pin bias can exhibit an effective value as low as 1µF to 2µF for the current. The device is tested and specified with the ADJ pin DC bias voltage applied and over the operating tempera- tied to the OUT pin and a 5µA DC load (unless otherwise ture range. The X5R and X7R dielectrics result in more specified) for an output voltage of 1.22V. Specifications stable characteristics and are more suitable for use as the for output voltages greater than 1.22V will be propor- output capacitor. The X7R type has better stability across tional to the ratio of the desired output voltage to 1.22V temperature, while the X5R is less expensive and is avail- (VOUT/1.22V). For example, load regulation for an output current change of 1mA to 20mA is –13mV typical at VOUT 20 BOTH CAPACITORS ARE 16V, = 1.22V. At VOUT = 12V, load regulation is: 1210 CASE SIZE, 10µF 0 (12V/1.22V) • (–13mV) = –128mV X5R –20 ALUE (%) IN OUT VOUT –40 LT3014B R2 + VIN ADJ –60 GND R1 CHANGE IN V Y5V 3014B F01 –80 R2 V (• ) OUT = 1.22V 1 + + (I –100 R1 ADJ)(R2) 0 2 4 6 8 10 12 14 16 VADJ = 1.22V DC BIAS VOLTAGE (V) IADJ = 4nA AT 25°C OUTPUT RANGE = 1.22V TO 60V 3014B F02
Figure 1. Adjustable Operation Figure 2. Ceramic Capacitor DC Bias Characteristics
3014bfb 8 For more information www.linear.com/LT3014B Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Applications Information