Datasheet

LM2598
www.ti.com SNVS125C –MARCH 1998–REVISED APRIL 2013
Figure 46. Undervoltage Lockout for a Buck Regulator
Figure 47 and Figure 48 apply the same feature to an inverting circuit. Figure 47 features a constant threshold
voltage for turn on and turn off (zener voltage plus approximately one volt). Since the SD /SS pin has an internal
7V zener clamp, R2 is needed to limit the current into this pin to approximately 1 mA when Q1 is on. If hysteresis
is needed, the circuit in Figure 48 has a turn ON voltage which is different than the turn OFF voltage. The
amount of hysteresis is approximately equal to the value of the output voltage.
Figure 47. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
Figure 48. 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 49. This unregulated negative voltage is approximately 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 several 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).
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