Datasheet

LM2597, LM2597HV
www.ti.com SNVS119C –MARCH 1998–REVISED APRIL 2013
Figure 50. Undervoltage Lockout for a Buck Regulator
Figure 51 and Figure 52 apply the same feature to an inverting circuit. Figure 51 features a constant threshold
voltage for turn on and turn off (zener voltage plus approximately one volt). If hysteresis is needed, the circuit in
Figure 52 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. 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.
Figure 51. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
Figure 52. 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 53. This unregulated negative voltage is approximately equal to the positive input voltage
(minus a few volts), and can supply up to a 100 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 LM2597
current limit (typically 800 mA).
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.
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