LT3009 Series
12
3009fd
APPLICATIONS INFORMATION
The LT3009 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 shut-
down. The device supplies up to 20mA of output current.
Dropout voltage at 20mA is typically 280mV. The LT3009
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 LT3009 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 22V
without affecting startup or normal operation.
Adjustable Operation
The LT3009 has an output voltage range of 0.6V to 19.5V.
Figure 1 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 300pA at 25°C, fl ows out
of the pin. Calculate the output voltage using the formula
in Figure 1. 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 condi-
tions. 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 –0.7mV typical at
V
OUT
= 0.6V. At V
OUT
= 5V, load regulation is:
5V
0.6V
•(−0.7mV)= −5.83mV
Table 1 shows resistor divider values for some com-
mon output voltages with a resistor divider current of
about 1μA.
Figure 1. Adjustable Operation
Table 1. Output Voltage Resistor Divider Values
V
OUT
R1 R2
1V 604k 402k
1.2V 604k 604k
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
Because the ADJ pin is relatively high impedance (de-
pending on the resistor divider used), stray capacitances
at this pin should be minimized. Special attention should
be given to any stray capacitances that can couple ex-
ternal signals onto the ADJ pin producing undesirable
output transients or ripple.
Extra care should be taken in assembly when using high
valued resistors. Small amounts of board contamination
can lead to signifi cant shifts in output voltage. Appro-
priate post-assembly board cleaning measures should
IN
SHDN
R2
R1
OUT
V
IN
V
OUT
= 600mV* (1 + R2/R1) – (I
ADJ
• R2)
V
ADJ
= 600mV
I
ADJ
= 0.3nA at 25°C
OUTPUT RANGE = 0.6V to 19.5V
ADJ
GND
L
T3009
V
OU
