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LT3050

3050f

External Programmable Current Limit (IMAX Pin)

The IMAX pin is the collector of a PNP which mirrors the

LT3050 output at a ratio of approximately 1:200 (see Block

Diagram). The IMAX pin is also the input to the precision

current limit ampliﬁer. If the output load increases to the

point where it causes the IMAX pin voltage to reach 0.6V,

the current limit ampliﬁer takes control of output regulation

so that the IMAX pin clamps at 0.6V, regardless of the

output voltage. The current limit threshold (ILIMIT) is set

by attaching a resistor (RIMAX) from IMAX to GND:

119.22 0.894 V

IMAX

OUT

LIMIT

−

•

This equation is empirically derived and partially

compensates for early voltage effects in the IMAX current

mirror. It is valid for an input voltage range from 0.6V

above the output to 10V above the output. It is valid for

output voltages up to 12V. The accuracy of this equation

for setting the resistor value is approximately ±1%. Unit

values are Amps, Volts, and Ohms.

In cases where the IN to OUT voltage exceeds 10V, fold-

back current limit will lower the internal current level limit,

possibly causing it to preempt the external programmable

current limit. See the Internal Current Limit vs VIN – VOUT

graph in the Typical Performance Characteristics section.

If the external programmable current limit is not needed,

the IMAX pin must be connected to GND. The IMAX pin

OPERATION

requires a 10nF decoupling capacitor.

See the Typical Performance Characteristics section for

additional information.

FAULT Pin Operation

The FAULT pin is an open collector logic pin which asserts

during internal current limit, precision current limit, thermal

limit, or a minimum current fault. There is no internal

pull-up on the FAULT pin; an external pull-up resistor is

required. The FAULT pin provides drive for up to 100μA

of pull-down current. Off state logic may be as high as

45V, regardless of the input voltage used. When asserted,

the FAULT pin drive circuitry adds 50μA (nominal) of GND

pin current.

Depending on the IMIN capacitance, BYP capacitance,

and OUT capacitance, the FAULT pin may assert during

startup. Consideration should be given to masking the

FAULT signal during startup. The FAULT pin circuitry is

inactive (not asserted) during shutdown and when the

OUT pin is pulled above the IN pin.

Operation in Dropout

The LT3050 contains circuitry which prevents the PNP

output power device from saturating in dropout. This also

keeps the IMON, IMIN, and IMAX current mirrors functioning

accurately, even in dropout. However, this anti-saturation

circuitry becomes less active at lower output currents, so

there is some degradation of current mirror function for

output currents less than 10mA.

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