LT3012B datasheet(8/16 Pages) LINER

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LT3012B

3012bf

The LT3012B is a 250mA high voltage low dropout regu-

lator with micropower quiescent current. The device is

capable of supplying 250mA at a dropout voltage of

400mV. Operating quiescent current is only 40

µA. In

addition to the low quiescent current, the LT3012B incor-

porates 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 LT3012B acts like it has a diode in series with its output

and prevents reverse current flow.

Adjustable Operation

The LT3012B 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.

The adjustable device is tested and specified with the ADJ

pin tied to the OUT pin and a 5

µADCload(unlessotherwise

specified) for an output voltage of 1.24V. Specifications

for output voltages greater than 1.24V will be proportional

APPLICATIO S I FOR ATIO

Figure 1. Adjustable Operation

to the ratio of the desired output voltage to 1.24V; (VOUT/

1.24V). For example, load regulation for an output current

change of 1mA to 250mA is –7mV typical at VOUT = 1.24V.

At VOUT = 12V, load regulation is:

(12V/1.24V) • (–7mV) = –68mV

Output Capacitance and Transient Response

The LT3012B 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 3.3

µF with an ESR of 3Ω or less is

recommended to prevent oscillations. The LT3012B 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 LT3012B, 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 di-

electrics used are specified with EIA temperature charac-

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

dielectrics are good for providing high capacitances in a

small package, but they tend to have strong voltage and

temperature coefficients as shown in Figures 2 and 3.

When used with a 5V regulator, a 16V 10

µF Y5V capacitor

VIN

3012B F01

VOUT

VOUT = 1.24V

VADJ = 1.24V

IADJ = 30nA AT 25°C

OUTPUT RANGE = 1.24V TO 60V

+ (IADJ)(R2)

1 +

()

LT3012B

OUT

ADJ

GND

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