Datasheet

LM2599

SNVS123C –APRIL 1998–REVISED APRIL 2013

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Figure 46. Undervoltage Lockout for a Buck Regulator

Figure 47 and 30 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). 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. 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 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 600 mA of output current. There is a requirement however, that there

be a minimum load of 1.2A 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 LM2599 current limit (typically

4.5A).

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.

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