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Data Sheet

LT3097

APPLICATIONS INFORMATION

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Rev. 0 | 42 of 45

PROTECTION FEATURES

The LT3097 incorporates several protection features for sensitive

applications. Precision current-limit and thermal-overload protection

protect the LT3097 against overload and fault conditions at the

output of the device. For normal operation, do not allow the junction

temperature to exceed 125°C.

To protect the low-noise error amplifier of the positive regulator

of the LT3097, the SETP-to-OUTSP protection clamp limits the

maximum voltage between SETP and OUTSP with a maximum DC

current of 20 mA through the clamp. Therefore, for applications

where SETP is actively driven by a voltage source, the voltage

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

limit the transient current flowing through these clamps during a

transient fault condition, limit the maximum value of the SETP pin

capacitor (CSETP) to 22 μF.

To protect the low-noise error amplifier of the negative regulator

of the LT3097, the SETN-to-OUTSN protection clamp limits the

maximum voltage between SETN and OUTSN with a maximum DC

current of 10 mA through the clamp. Therefore, for applications

where SETN is actively driven by a voltage source, the voltage

source must be current-limited to 10 mA or less. Moreover, to

limit the transient current flowing through these clamps during a

transient fault condition, limit the maximum value of the SETN pin

capacitor (CSETN) to 22 μF.

The positive regulator of the LT3097 also incorporates reverse-input

protection whereby the INP pin withstands reverse voltages of up

to –20 V without causing any input-current flow and without devel-

oping negative voltages at the OUTP pin. The positive 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 and output conditions can occur for the positive regulator.

The positive output voltage can be held up while the positive input

is either pulled to GND, pulled to some intermediate voltage, or

left open-circuit. In all cases, the reverse-current protection circuitry

prevents current flow from the positive output to the positive input.

Nonetheless, due to the OUTSP-to-SETP clamp, unless the SETP

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

resistor as well as up to 15 mA to GND through the positive output

overshoot recovery circuitry. This current flow through the positive

output overshoot recovery circuitry can be significantly reduced by

placing a Schottky diode between the OUTSP and SETP pins, with

its anode at the OUTSP pin.

Pulling the negative output of the LT3097 above ground induces

no damage to the part. If INN is left open circuit or grounded,

OUTN can be pulled 20V above GND. In this condition, a maximum

current of 25mA flows into the OUTN pin and out of the GND pin

(pin 13). If INN is powered by a voltage source, OUTN sinks the

short circuit current of the negative regulator of the LT3097 and

protects itself by thermal limiting. In this case, however, grounding

the ENN/UVN pin turns off the negative regulator and stops OUTN

from sinking the short-circuit current.

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