Datasheet LT3011 (Analog Devices) - 9

LT3011
APPLICATIONS INFORMATION
The LT3011 is a 50mA high voltage/low dropout regulator fi cations for output voltages greater than 1.24V will be with micropower quiescent current and shutdown. The proportional to the ratio of the desired output voltage to device is capable of supplying 50mA at a dropout voltage of 1.24V; (VOUT/1.24V). For example, load regulation for an 300mV. The low operating quiescent current (46μA) drops output current change of 1mA to 50mA is –6mV (typical) to 1μA in shutdown. In addition to low quiescent current, at VOUT = 1.24V. At VOUT = 12V, load regulation is: the LT3011 incorporates several protection features which 12V make it ideal for use in battery-powered systems. The • – 6mV = – 58mV 1.24V device is 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
Output Capacitance and Transient Response
is pulled to ground, the LT3011 acts like it has a diode in The LT3011 is designed to be stable with a wide range series with its output and prevents reverse current fl ow. of output capacitors. The ESR of the output capacitor affects stability, most notably with small capacitors. A
Adjustable Operation
minimum output capacitor of 1μF with an ESR of 3Ω or The LT3011 has an output voltage range of 1.24V to 60V. The less is recommended to prevent oscillations. The LT3011 output voltage is set by the ratio of two external resistors as is a micropower device and output transient response shown in Figure 1. The device servos the output to maintain will be a function of output capacitance. Larger values the voltage at the adjust pin at 1.24V referenced to ground. of output capacitance decrease the peak deviations and The current in R1 is then equal to 1.24V/R1 and the current provide improved transient response for larger load current in R2 is the current in R1 plus the ADJ pin bias current. changes. Bypass capacitors, used to decouple individual The ADJ pin bias current, 30nA at 25°C, fl ows through components powered by the LT3011, will increase the R2 into the ADJ pin. The output voltage can be calculated effective output capacitor value. using the formula in Figure 1. The value of R1 should be Extra consideration must be given to the use of ceramic less than 250k to minimize errors in the output voltage capacitors. Ceramic capacitors are manufactured with a caused by the ADJ pin bias current. Note that in shutdown variety of dielectrics, each with different behavior across the output is turned off and the divider current will be zero. temperature and applied voltage. The most common The adjustable device is tested and specifi ed with the dielectrics used are specifi ed with EIA temperature char- ADJ pin tied to the OUT pin and a 5μA DC load (unless acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and otherwise specifi ed) for an output voltage of 1.24V. Speci- Y5V dielectrics are good for providing high capacitances IN OUT VOUT R2 VOUT = VADJ 1 + + (IADJ)(R2) + R1 LT3011 R2 VIN VADJ = 1.24V IADJ = 30nA AT 25oC ADJ OUTPUT RANGE = 1.24V TO 60V R1 GND 3011 F01
Figure 1. Adjustable Operation
3011f 9