IL2575-xx

Figure 16. Inverting Buck–Boost

Regulator with Delayed Startup

It has been already mentioned above, that in some situations, the delayed startup or the undervoltage

lockout features could be very useful. A delayed startup circuit applied to a buck–boost converter is shown in

Figure 16.

Figure 22 in the “Undervoltage Lockout” section describes an undervoltage lockout feature for the same

converter topology.

NOTE: This picture does not show the complete circuit.

Figure 17. Inverting Buck–Boost Regulator Shut Down

Circuit Using an Optocoupler

With the inverting configuration, the use of the ON/OFF pin requires some level shifting techniques.

This is caused by the fact, that the ground pin of the converter IC is no longer at ground. Now, the ON/OFF

pin threshold voltage (1.4 V approximately) has to be related to the negative output voltage level. There are

many different possible shut down methods, two of them are shown in Figures 17 and 18.

Rev. 00