LM2598 datasheet(29/33 Pages) NSC

Part #	LM2598

Description	SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator, with Features

PDF	33 Pages

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Manufacturer	NSC [National Semiconductor (TI)]

Direct Link	http://www.national.com

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29 / 33 page

Application Information (Continued)

NEGATIVE VOLTAGE CHARGE PUMP

Occasionally a low current negative voltage is needed for

biasing parts of a circuit. A simple method of generating a

negative voltage using a charge pump technique and the

switching waveform present at the OUT pin, is shown in

Figure 32. This unregulated negative voltage is approxi-

mately equal to the positive input voltage (minus a few volts),

and can supply up to a 200 mA of output current. There is a

requirement however, that there be a minimum load of sev-

eral hundred mA on the regulated positive output for the

charge pump to work correctly. Also, resistor R1 is required

to limit the charging current of C1 to some value less than

the LM2598 current limit (typically 1.5A).

This method of generating a negative output voltage without

an additional inductor can be used with other members of

the Simple Switcher Family, using either the buck or boost

topology.

DS012593-46

FIGURE 31. Undervoltage Lockout With

Hysteresis for an Inverting Regulator

DS012593-48

FIGURE 32. Charge Pump for Generating a

Low Current, Negative Output Voltage

LM2598 Datasheet(PDF) 29 Page - National Semiconductor (TI)