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LT8580

Boost/SEPIC/Inverting DC/DC Converter with 1A, 65V Switch, Soft-Start and Synchronization

Linear Technology 

LT8580

Applications Information

Configurable Undervoltage Lockout

Figure 8 shows how to configure an undervoltage lock-

out (UVLO) for the LT8580. Typically, UVLO is used in

situations where the input supply is current-limited, has

a relatively high source resistance, or ramps up/down

slowly. A switching regulator draws constant power from

the source, so source current increases as source voltage

drops. This looks like a negative resistance load to the

source and can cause the source to current-limit or latch

low under low 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 1.31V (rising) and 1.27V (falling). Re-

sistor RUVLO2 is optional. RUVLO2 can be included to reduce

the overall UVLO voltage variation caused by variations

in SHDN pin current (see the Electrical Characteristics).

A good choice for RUVLO2 is ≤10k ±1%. After choosing a

value for RUVLO2, RUVLO1 can be determined from either

of the following:

RUVLO1

=

⎛

⎜

⎝

VIN+ −

1.31V

RUVLO2

1.31V

⎞

⎟ + 12µA

⎠

or

RUVLO1

=

⎛

⎜

⎝

VIN− −

1.27V

RUVLO2

1.27V

⎞

⎟ + 12µA

⎠

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

falling, respectively.

For example, to disable the LT8580 for VIN voltages below

3.5V using the single resistor configuration, choose:

RUVLO1

=

⎛

⎝⎜

3.5V − 1.27V

1.27V

∞

⎞

⎠⎟

+

12µA

=

187k

To activate the LT8580 for VIN voltages greater than

4.5V using the double resistor configuration, choose

RUVLO2 = 10k and:

RUVLO1

=

⎛

⎝⎜

4.5V − 1.31V

1.31V

10k

⎞

⎠⎟

+

12µA

=

22.1 k

Internal Undervoltage Lockout

The LT8580 monitors the VIN supply voltage in case VIN

drops below a minimum operating level (typically about

2.35V). 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 LT8580 to deliver its full output power, it is impera-

tive 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.

VIN

VIN

RUVLO1

1.3V –

SHDN

+

RUVLO2

(OPTIONAL)

12µA

AT 1.3V

GND

ACTIVE/

LOCKOUT

8580 F08

Figure 8. Configurable UVLO

For more information www.linear.com/LT8580

8580f

17

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