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.
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