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LT3580

3580fg

APPLICATIONS INFORMATION

source voltage conditions. UVLO prevents the regulator

from operating at source voltages where these problems

might occur.

The shutdown pin comparator has voltage hysteresis with

typical thresholds of 1.32V (rising) and 1.29V (falling).

Resistor RUVLO2 is optional. RUVLO2 can be included

to reduce the overall UVLO voltage variation caused by

variations in SHDN pin current (see the Electrical Character-

istics). A good choice for RUVLO2 is ≤10k ±1%. After

choosing a value for RUVLO2, RUVLO1 can be determined

from either of the following:

UVLO1 =

+ −1.32V

1.32V

UVLO2

⎛

⎝

⎜

⎞

⎠

⎟+11.6μA

UVLO1 =

− −1.29V

1.29V

UVLO2

⎛

⎝

⎜

⎞

⎠

⎟+11.6μA

where VIN+ and VIN– are the VIN voltages when rising or

falling respectively.

For example, to disable the LT3580 for VIN voltages below

3.5V using the single resistor configuration, choose:

UVLO1 =

3.5V

−1.29V

1.29V

∞

⎛

⎝

⎜

⎞

⎠

⎟+11.6μA

=190.5k

Figure 7. Configurable UVLO

To activate the LT3580 for VIN voltage greater than

4.5V using the double resistor configuration, choose

RUVLO2 = 10k and:

UVLO1 =

4.5V

−1.32V

1.32V

10k

⎛

⎝

⎜

⎞

⎠

⎟+11.6μA

=22.1k

Internal Undervoltage Lockout

The LT3580 monitors the VIN supply voltage in case VIN

drops below a minimum operating level (typically about

2.3V). When VIN is detected low, the power switch is

deactivated, and while sufficient VIN voltage persists, the

soft-start capacitor is discharged. After VIN is detected

high, the power switch will be reactivated and the soft-start

capacitor will begin charging.

Thermal Considerations

For the LT3580 to deliver its full output power, it is imperative

that a good thermal path be provided to dissipate the heat

generated within the package. This is accomplished by

taking advantage of the thermal pad on the underside of

the IC. It is recommended that multiple vias in the printed

circuit board be used to conduct heat away from the IC and

into a copper plane with as much area as possible.

Thermal Lockout

If the die temperature reaches approximately 165°C, the

part will go into thermal lockout, the power switch will be

turned off and the soft-start capacitor will be discharged.

The part will be enabled again when the die temperature

has dropped by ~5°C (nominal).

Thermal Calculations

Power dissipation in the LT3580 chip comes from four

primary sources: switch I2R loss, NPN base drive (AC), NPN

base drive (DC), and additional input current. The following

formulas can be used to approximate the power losses.

These formulas assume continuous mode operation,

RUVLO2

(OPTIONAL)

1.3V

RUVLO1

3580 F07

VIN

ACTIVE/

LOCKOUT

GND

11.6μA

AT 1.3V

–

SHDN

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