FEATURES

· Precision Fault Threshold

· Programmable:

Average Power Limiting, Linear

Current Control, Overcurrent Limit

and Fault Time

· Fault Output Indication Signal

· Automatic Retry Mode or Latched

Operation Mode

· Shutdown Control

· Undervoltage Lockout

· 250ms Glitch Filter on the SDFLTCH

pin

· 8-Pin DIL and SOIC

DESCRIPTION

The UCC3921 family of negative floating hot swap power managers provides

complete power management, hot swap, and fault handling capability.

The IC is referenced to the negative input voltage and is powered

through an external resistor connected to ground, which is essentially a

current drive as opposed to the traditional voltage drive. The onboard

10V shunt regulator protects the IC from excess voltage and serves as a

reference for programming the maximum allowable output sourcing current

during a fault. All control and housekeeping functions are integrated

and externally programmable. These include the fault current level, maximum

output sourcing current, maximum fault time, selection of Retry or

Latched mode, soft start time, and average power limiting. In the event of

a constant fault, the internal timer will limit the on time from less than

0.1% to a maximum of 3% duty cycle. The duty cycle modulation depends

on the current into PL, which is a function of the voltage across

the FET, thus limiting average power dissipation in the FET. The fault

level is fixed at 50mV across the current sense amplifier to minimize total

dropout. The fault current level is set with an external

current sense resistor, while the maximum allowable

sourcing current is programmed with a voltage divider

from VDD to generate a fixed voltage on IMAX. The current

level, when the output acts as a current source, is

equal to VIMAX/RSENSE. If desired, a controlled current

start up can be programmed with a capacitor on IMAX.

When the output current is below the fault level, the output

device is switched on. When the output current exceeds

the fault level, but is less than the maximum

sourcing level programmed by IMAX, the output remains

switched on, and the fault timer starts charging CT. Once

CT charges to 2.5V, the output device is turned off and

performs a retry some time later (provided that the selected

mode of operation is Automatic Retry Mode).

When the output current reaches the maximum sourcing

current level, the output acts as a current source, limiting

the output current to the set value defined by IMAX.

Other features of the UCC3921 include undervoltage

lockout, 8-pin Small Outline (SOIC) and Dual-In-Line

(DIL) packages, and a Latched Operation Mode option,

in which the output is latched off once CT charges to

2.5V and stays off until either SDFLTCH is toggled (for

greater than 1ms) or the IC is powered down and then

back up.