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LT3029

3029fa

APPLICATIONS INFORMATION

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

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 ﬂow 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.

Overload Recovery

Like many IC power regulators, the LT3029 has safe

operating area (SOA) protection. The safe area protec-

tion decreases current limit as input-to-output voltage

increases and keeps the power transistor inside a safe

operating region for all values of input-to-output voltage.

The protective design provides some output current at

all values of input-to-output voltage up to the speciﬁed

maximum operational input voltage of 20V.

When power is ﬁrst applied, as input voltage rises, the

output follows the input, allowing the regulator to start-up

into very heavy loads. During start-up, as the input voltage

is rising, the input-to-output voltage differential is small,

allowing the regulator to supply large output currents.

With a high input voltage, an event can occur wherein

removal of an output short will not allow the output to

recover. The event occurs with a heavy output load when

the input voltage is high and the output voltage is low.

Common situations occur immediately after the removal

of a short-circuit or if the shutdown pin is pulled high after

the input voltage has already been turned on. The load line

intersects the output current curve at two points creating

two stable output operating points for the regulator. With

this double intersection, the input power supply may need

to be cycled down to zero and brought up again to make

the output recover.

Figure 5. Reverse Output Current

OUTPUT VOLTAGE (V)

3029 F05

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

TJ = 25°C

VIN = 0V

VADJ = VOUT

IADJ FLOWS INTO ADJ PIN TO GND PIN

IOUT FLOWS INTO OUT PIN TO IN PIN

0123

6789

IADJ

IOUT

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