Application Information (Continued)
DS012593-46
FIGURE 31. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
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.
DS012593-48
FIGURE 32. Charge Pump for Generating a
Low Current, Negative Output Voltage
29
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