LM2585 datasheet(20/32 Pages) TI

20 / 32 page

LM2585

SNVS120F – APRIL 2000 – REVISED APRIL 2013

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*The LM2585 will require a heat sink in these applications. The size of the heat sink will depend on the maximum

ambient temperature. To calculate the thermal resistance of the IC and the size of the heat sink needed, the “HEAT

SINK/THERMAL CONSIDERATIONS” in the Application Hints.

Figure 51. +24V to +48V Boost Regulator

Application Hints

Figure 52. Boost Regulator

PROGRAMMING OUTPUT VOLTAGE

(SELECTING R1 AND R2)

Referring to the adjustable regulator in Figure 52, the output voltage is programmed by the resistors R1 and R2

by the following formula:

VOUT = VREF (1 + R1/R2)

where

•

VREF = 1.23V

(1)

Resistors R1 and R2 divide the output voltage down so that it can be compared with the 1.23V internal reference.

With R2 between 1k and 5k, R1 is:

R1 = R2 (VOUT/VREF − 1)

where

•

VREF = 1.23V

(2)

For best temperature coefficient and stability with time, use 1% metal film resistors.

SHORT CIRCUIT CONDITION

Due to the inherent nature of boost regulators, when the output is shorted (Figure 52), current flows directly from

the input, through the inductor and the diode, to the output, bypassing the switch. The current limit of the switch

does not limit the output current for the entire circuit. To protect the load and prevent damage to the switch, the

current must be externally limited, either by the input supply or at the output with an external current limit circuit.

The external limit should be set to the maximum switch current of the device, which is 3A.

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