The adual time depends on the output voltage and

current limit (approx 4.5A) are needed for at least 2 ms or more,

until the output reaches its nominal output voltage.

loads. Input currents as high as the LM2575 /LM2575HV

the size of the outputcapacitor. Input power sources that are

current limited or sources that can not deliver these currents

without getting loaded down, may not work correctly. Because

of the relatively high startup currents required by the inverting

tolology the delayed startup feature (C1, R

in

) shown the

25 is recommended. By delaying the regulator

of the high nput current needed for startup is now supplied by

gher votage before the switcher begins operating. A portion

startup, the input capacitor is allowed to charge up to a hi-

the input capacitor (C

). For severe start up conditions,

the input capacitor can be made much larger than normal.

Application Information (Continued)

Because of differences in the operation of the inverting

regulator, the standard design procedure is not used to

select the inductor value. In the majority of designs, a 33 µH,

3.5A inductor is the best choice. Capacitor selection can also

be narrowed down to just a few values. Using the values

shown in

Figure 25 will provide good results in the majority of

inverting designs.

This type of inverting regulator can require relatively large

amounts of input current when starting up, even with light

1and R2

IN

This circuit has hysteresis

Regulator starts switching at V

IN

= 13V

Regulator stops switching at VIN =8V

FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator

CIN

— 68 µF/25V Tant. Sprague 595D

470 µF/50V Elec. Panasonic HFQ

COUT

— 47 µF/20V Tant. Sprague 595D

220 µF/25V Elec. Panasonic HFQ

FIGURE 25. Inverting −5V Regulator with Delayed Startup

FIGURE 26. Inverting Regulator Typical Load Current

13

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LM2575

/LM2575HV

5.0

LM2575

/LM2575HV

5.0

Figure

2A Step-Down Voltage Regulator

UMW

R

UMW LM2575/LM2575HV

UMW LM2575/LM2575HV