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LT3050MPMSE

100mA, Linear Regulator with Precision Current Limit and Diagnostic Outputs

Linear Technology 

LT3050 Series

OPERATION

IMON Pin Operation (Current Monitor)

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

LT3050 output PNP at a ratio of 1:100 (see block diagram

on page 11). The current sourced by the IMON pin is

~1/100th of the current sourced by the OUT pin when the

IMON and OUT pin voltages are equal and the device is not

operating in dropout. If the IMON and OUT pin voltages

are not the same, the ratio deviates from 1/100 due to

the Early voltages of the IMON and OUT PNPs according

to the equation:

IIMON  1 v

IOUT IMONR

¥§¦1774004V4VIIN2 N 4VVI4MOUO4TN3´¶µ

Early Voltage Compensation

where the Early voltage of the PNPs is 70V and IMONR is

a variable which represents the IOUT to IMON current ratio.

IMONR varies with VIN to VOUT voltage according to the

empirically derived equation:

IMONR = 97 + 5 • log10 (1+VIN – VOUT) for (VIN – VOUT)

≥0.5

IMONR = 96 + 2 • (VIN – VOUT) for (VIN – VOUT) < 0.5

The IMON pin current can be converted into a voltage for

use by monitoring circuitry simply by connecting the IMON

pin to a resistor.

Connecting a resistor from IMON to GND converts the

IMON pin current into a voltage that can be monitored by

circuitry such as an ADC.

For example, a 1.2k resistor results in a IMON pin voltage

of 1.2V for an output current of 100mA and an output

voltage of 1.2V.

The output current of the device can be calculated from

the IMON pin voltage by the following equation:

IOUT



I{ MONR

IOUT Ratio

IMON

v

VIMON

R1I2MO3N

IIMON

v

¥ 70  VIN VOUT ´

§¦1740 4V4IN24VI4MO4N3¶µ

Early VoltageCompensation

A small decoupling capacitor (22nF minimum) from IMON

to GND is required to improve IMON pin power supply

rejection. If the current monitor is not needed, it must be

tied to GND.

Open Circuit Detection (IMIN Pin)

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

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

diagram on page 11). The IMIN fault comparator asserts

the FAULT pin if the IMIN pin voltage is below 0.6V. This

low output current fault threshold voltage (IOPEN) is set

by attaching a resistor from IMIN to GND.

RIMIN

=

119.85

–

(1.68

– 36.8

IOPEN

• IOPEN

)

•

VOUT

This equation is empirically derived and partially

compensates for early voltage effects in the IMIN 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 ±2%. Unit

values are Amps, Volts, and Ohms.

If the open circuit detection function is not needed, the IMIN

pin must be left floating (unconnected). A small decoupling

capacitor (10nF minimum) from IMIN to GND is required

to improve IMIN pin power supply rejection and to prevent

FAULT pin glitches.

See the Typical Performance Characteristics section for

additional information.

3050fa

15

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