Datasheet

LT3008 Series
12
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. The device supplies up to 20mA of output current.
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 fl ow. 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, fl ows 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.
Specifi cations for output voltages greater than 0.6V are
proportional to the ratio of the desired output voltage to
0.6V: V
OUT
/0.6V. For example, load regulation for an output
current change of 100µA to 20mA is –0.5mV typical at
V
OUT
= 0.6V. At V
OUT
= 5V, load regulation is:
5V
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
V
OUT
R1 R2
1V 604k 402k
1.2V 590k 590k
1.5V 590k 887k
1.8V 590k 1.18M
2.5V 590k 1.87M
3V 590k 2.37M
3.3V 619k 2.8M
5V 590k 4.32M
IN
SHDN
R2
R1
3008 F02
OUT
V
IN
V
OUT
= 600mV • (1 + R2/R1) – (I
ADJ
• R2)
V
ADJ
= 600mV
I
ADJ
= 0.4nA at 25°C
OUTPUT RANGE = 0.6V to 44.5V
ADJ
GND
L
T3008
V
OUT