IL2576-xx
Because of the boosting function of this type of regulator, the switch current is relatively high, especially at low input
voltages.
Output load current limitations are a result of the maximum current rating of the switch. Also, boost regulators can not
provide current limiting load protection in the event of a shorted load, so some other means (such as a fuse) may be necessary.
UNDERVOLTAGE LOCKOUT
In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold. An
undervoltage lockout circuit which accomplishes this task is shown in Figure 7 while Figure 8 shows the same circuit applied to
a buck-boost configuration. These circuits keep the regulator off until the input voltage reaches a predetermined level.
VTH =VZ1 + 2VBE(Q1)
Note: Complete circuit not shown.
FIGURE 7. Undervoltage Lockout for Buck Circuit
Note: Complete circuit not shown (see Figure 10).
FIGURE 8. Undervoltage Lockout for Buck-Boost Circuit
DELAYED STARTUP
The ON /OFF pin can be used to provide a delayed startup feature as shown in Figure 9. With an input voltage of 20V and for
the part values shown, the circuit provides approximately 10 ms of delay time before the circuit begins switching.
Increasing the RC time constant can provide longer delay times. But excessively large RC time constants can cause problems
with input voltages that are high in 60 Hz or 120 Hz ripple, by coupling the ripple into the ON /OFF pin.
Note: Complete circuit not shown.
FIGURE 9. Delayed Startup
Rev. 02
IKSEMICON [ IK SEMICON CO., LTD ]
