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Fault Protection

Unlike a “Buck” or step-down regulator it is not possible

to provide a boost regulator with short-circuit protection.

As revealed in the block diagram of Figure 1-1, inhibiting

the MOSFET switching action only disables the regulator’s

boost function. Therefore under a severe output impedance

fault the control circuit cannot disconnect the output from

the input source.

To prevent an output over-current or short-circuit fault

from propagating to the input bus, a fuse or equivalent

over-current protection is recommended at the input of

the module. Whatever form of protection is selected, it is

important to note that the impedance and/or voltage drop

of the series element will add to the regulator’s minimum

input voltage requirements. Power up may also be affected.

The combination of an input surge current with an im-

pedance in series with the regulator input may cause the

input voltage to momentarily dip back below the UVLO

threshold. Ensure that the fuse rating or input current

limit threshold are designed with a generous margin.

PT5540 Series