LT3011

9

3011f

APPLICATIONS INFORMATION

The LT3011 is a 50mA high voltage/low dropout regulator

with micropower quiescent current and shutdown. The

device is capable of supplying 50mA at a dropout voltage of

300mV. The low operating quiescent current (46μA) drops

to 1μA in shutdown. In addition to low quiescent current,

the LT3011 incorporates several protection features which

make it ideal for use in battery-powered systems. The

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

is pulled to ground, the LT3011 acts like it has a diode in

series with its output and prevents reverse current flow.

Adjustable Operation

The LT3011 has an output voltage range of 1.24V to 60V. The

output voltage is set by the ratio of two external resistors as

shown in Figure 1. The device servos the output to maintain

the voltage at the adjust pin at 1.24V referenced to ground.

The current in R1 is then equal to 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 at 25°C, flows through

R2 into the ADJ pin. The output voltage can be calculated

using the formula in Figure 1. The value of R1 should be

less than 250k 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 and a 5μA DC load (unless

otherwise specified) for an output voltage of 1.24V. Speci-

fications for output voltages greater than 1.24V will be

proportional to the ratio of the desired output voltage to

output current change of 1mA to 50mA is –6mV (typical)

12

124

658

V

V

mV

mV

.

•–

–

=

Output Capacitance and Transient Response

The LT3011 is designed to be stable with a wide range

of output capacitors. The ESR of the output capacitor

affects stability, most notably with small capacitors. A

minimum output capacitor of 1μF with an ESR of 3Ω or

less is recommended to prevent oscillations. The LT3011

is a micropower device and output transient response

will be a function of output capacitance. Larger values

of output capacitance decrease the peak deviations and

provide improved transient response for larger load current

changes. Bypass capacitors, used to decouple individual

components powered by the LT3011, will increase the

effective output capacitor value.

Extra consideration must be given to the use of ceramic

capacitors. Ceramic capacitors are manufactured with a

variety of dielectrics, each with different behavior across

temperature and applied voltage. The most common

dielectrics used are specified with EIA temperature char-

acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and

Y5V dielectrics are good for providing high capacitances

3011 F01

R2

R1

+

R2

R1

OUTPUT RANGE = 1.24V TO 60V

1 +

IN

LT3011

OUT

ADJ

GND

Figure 1. Adjustable Operation