LT3029 LINER [Linear Technology], LT3029 Datasheet - Page 17

• Manufacturer Part Number: LT3029
• Description: Dual 500mA/500mA Low Dropout, Low Noise, Micropower Linear Regulator
• Manufacturer: LINER [Linear Technology]

22V. The output acts like an open circuit, and no current
fl ows from the output. However, current fl ows in (but
is limited by) the external resistor divider that sets the
output voltage.
The LT3029 incurs no damage if either ADJ pin is pulled
above or below ground by 9V. If the input is left open
circuit or grounded, the ADJ pins perform like an open
circuit down to –1.5V, and then like a 1.2k resistor down
to –9V when pulled below ground. When pulled above
ground, the ADJ pins perform like an open circuit up to
0.5V, then like a 5.7k resistor up to 3V, then like a 1.8k
resistor up to 9V.
In situations where an ADJ pin connects to a resistor
divider that would pull the pin above its 9V clamp volt-
age if the output is pulled high, the ADJ pin input current
must be limited to less than 5mA. For example, assume
a resistor divider sets the regulated output voltage to
1.5V, and the output is forced to 20V. The top resistor of
the resistor divider must be chosen to limit the current
APPLICATIONS INFORMATION
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
I
I
ADJ
OUT
Figure 5. Reverse Output Current
T
V
V
J
IN
ADJ
= 25°C
FLOWS INTO ADJ PIN TO GND PIN
FLOWS INTO OUT PIN TO IN PIN
1
= 0V
= V
OUT
2
OUTPUT VOLTAGE (V)
3
I
I
ADJ
OUT
4
5
into the ADJ pin to less than 5mA when the ADJ pin is
at 9V. The 11V difference between the OUT and ADJ pins
divided by the 5mA maximum current into the ADJ pin
yields a minimum top resistor value of 2.2k.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled
to ground, pulled to some intermediate voltage or is left
open-circuit. Current fl ow back into the output follows the
curve shown in Figure 5.
If either of the LT3029’s IN pins is forced below its cor-
responding OUT pin, or the OUT pin is pulled above its
corresponding IN pin, input current for that channel typi-
cally drops to less than 2μA. This occurs if the IN pin is
connected to a discharged (low voltage) battery, and either
a backup battery or a second regulator circuit holds up
the output. The state of that channel’s SHDN pin has no
effect on the reverse output current if the output is pulled
above the input.
6
7
8
3029 F05
9
LT3029
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