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LT3008 Series

3008fc

APPLICATIONS INFORMATION

The LT3008 is a low dropout linear regulator with ultra-

low quiescent current and shutdown. Quiescent current

is extremely low at 3μA and drops well below 1μA in

shutdown.Thedevicesuppliesupto20mAofoutputcurrent.

Dropout voltage at 20mA is typically 300mV. The LT3008

incorporates several protection features, making it ideal for

use in battery-powered systems. The device protects itself

against both reverse-input and reverse-output voltages.

In battery backup applications, where a backup battery

holds up the output when the input is pulled to ground,

the LT3008 acts as if a blocking diode is in series with its

output and prevents reverse current ﬂow. In applications

where the regulator load returns to a negative supply, the

output can be pulled below ground by as much as 50V

without affecting start-up or normal operation.

Adjustable Operation

The LT3008 has an output voltage range of 0.6V to 44.5V.

Figure 2 shows that output voltage is set by the ratio of two

external resistors. The IC regulates the output to maintain

the ADJ pin voltage at 600mV referenced to ground. The

current in R1 equals 600mV/R1 and the current in R2 is

the current in R1 minus the ADJ pin bias current. The ADJ

pin bias current, typically 400pA at 25°C, ﬂows out of the

pin. Calculate the output voltage using the formula in Figure

2. An R1 value of 619k sets the divider current to 0.97μA.

Do not make R1’s value any greater than 619k to minimize

output voltage errors due to the ADJ pin bias current and

to insure stability under minimum load conditions. In

shutdown, the output turns off and the divider current is

zero. Curves of ADJ Pin Voltage vs Temperature and ADJ

Pin Bias Current vs Temperature appear in the Typical

Performance Characteristics.

Speciﬁcations for output voltages greater than 0.6V are

proportional to the ratio of the desired output voltage to

0.6V: VOUT/0.6V. For example, load regulation for an output

current change of 100μA to 20mA is –0.5mV typical at

VOUT = 0.6V. At VOUT = 5V, load regulation is:

0.6V

•(

−0.5mV) =−4.17mV

Table 1 shows resistor divider values for some com-

mon output voltages with a resistor divider current of

about 1μA.

Figure 2. Adjustable Operation

Table 1. Output Voltage Resistor Divider Values

VOUT

604k

402k

1.2V

590k

1.5V

590k

887k

1.8V

590k

1.18M

2.5V

590k

1.87M

590k

2.37M

3.3V

619k

2.8M

590k

4.32M

SHDN

3008 F02

OUT

VIN

VOUT = 600mV • (1 + R2/R1) – (IADJ • R2)

VADJ = 600mV

IADJ = 0.4nA at 25°C

OUTPUT RANGE = 0.6V to 44.5V

ADJ

GND

LT3008

VOUT

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