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LT3045-1

30451fa

For more information www.linear.com/LT3045-1

APPLICATIONS INFORMATION

The maximum junction temperature equals the maxi-

mum ambient temperature plus the maximum junction

temperature rise above ambient:

TJMAX = 85°C + 35°C = 120°C

Overload Recovery

LikemanyICpowerregulators,theLT3045-1incorporates

safe-operating-area(SOA)protection.TheSOAprotection

activates at input-to-output differential voltages greater

than 12V. The SOA protection decreases the current limit

as the input-to-output differential increases and keeps

the power transistor inside a safe operating region for all

values of input-to-output voltages up to the LT3045-1’s

absolute maximum ratings. The LT3045-1 provides some

level of output current for all values of input-to-output dif-

ferentials. Refer to the Current Limit curves in the Typical

Performance Characteristics section. When power is first

appliedandinputvoltagerises,theoutputfollowstheinput

and keeps the input-to-output differential low to allow the

regulator to supply large output current and start-up into

high current loads.

Due to current limit foldback, however, at high input volt-

ages a problem can occur if the output voltage is low and

the load current is high. Such situations occur after the

removal of a short-circuit or if the EN/UV pin is pulled high

after the input voltage has already turned ON. The load

line in such cases intersects the output current profile at

two points. The regulator now has two stable operating

points. With this double intersection, the input power

supply may need to be cycled down to zero and brought

back up again to make the output recover. Other linear

regulators with foldback current limit protection (such as

the LT1965 and LT1963A) also exhibit this phenomenon,

so it is not unique to the LT3045-1.

Protection Features

The LT3045-1 incorporates several protection features for

battery-powered applications. Precision current limit and

thermal overload protection protect the LT3045-1 against

overload and fault conditions at the device’s output. For

normal operation, do not allow the junction temperature

to exceed 125°C (E-grade, I-grade).

To protect the LT3045-1’s low noise error amplifier, the

SET-to-OUTSprotectionclamplimitsthemaximumvoltage

between SET and OUTS with a maximum DC current of

20mA through the clamp. So for applications where SET

is actively driven by a voltage source, the voltage source

must be current limited to 20mA or less. Moreover, to limit

the transient current flowing through these clamps during

a transient fault condition, limit the maximum value of the

SET pin capacitor (CSET) to 22µF.

The LT3045-1 also incorporates reverse input protection

whereby the IN pin withstands reverse voltages of up to

–20V without causing any input current flow and without

developing negative voltages at the OUT pin. The regulator

protects both itself and the load against batteries that are

plugged-in backwards.

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

GND, pulled to some intermediate voltage, or left open-

circuit. In all of these cases, the reverse-current protection

circuitry prevents current flow from output to the input.

Nonetheless, due to the OUTS-to-SET clamp, unless the

SET pin is floating, current can flow to GND through the

SET pin resistor as well as up to 15mA to GND through

the output overshoot recovery circuitry. This current flow

through the output overshoot recovery circuitry can be

significantly reduced by placing a Schottky diode between

OUTS and SET pins, with its anode at the OUTS pin.

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