9

UCC1921

UCC2921

UCC3921

SDFLTCH & VSS, as optionally shown in Figure 6,

then the latched operating mode (described earlier)

will be invoked upon the occurrence of a fault.

3) To externally shutdown the HSPM, the SHUTDOWN

signal (typically held at LOCAL VDD) must be pulled

to LOCAL GND. Assuming SHUTDOWN is tied to

LOCAL GND, the P-channel level shift transistor will

conduct, driving SDFLTCH high (to roughly VDD plus

a diode). By sourcing > 250

µA into SDFLTCH for >

1ms the output to the external power FET will be dis-

abled. The current sourced into SDFLTCH must be

SAFETY RECOMMENDATIONS

Although the UCC3921 is designed to provide system

protection for all fault conditions, all integrated circuits

can ultimately fail short. For this reason, if the UCC3921

is intended for use in safety critical applications where

UL

®

or some other safety rating is required, a redundant

safety device such as a fuse should be placed in series

with the external power FET. The UCC3921 will prevent

the fuse from blowing for virtually all fault conditions, in-

creasing system reliability and reducing maintenance

cost, in addition to providing the hot swap benefits of the

device.

APPLICATION INFORMATION (continued)

Ω

Figure 7. Typical Telecommuications Application

(The “Negative Magnitude-Side” of the Supply is Switched in)

UDG-98053