Converters Supply Individual Outputs for Logic Applications

May 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

Two triple-output, isolated, plug-in power supplies from Texas Instruments provide regulated low-voltage outputs for mixed logic applications — such as DSPs, ASICs, and microprocessors. Besides saving board space, the 35W PT4820 and 75W PT4850 (photo) ease design and speed time to market. The modules require no external heatsink and are available in surface-mount or vertical or horizontal through-hole styles. The footprint of the PT4820 is 1.3 in.²; the PT4850 is 1.97 in.²(vertical).

Operating from a standard -48V central office supply, their output voltage combinations include 5V, 3.3V, 2.5V, and 1.8V to power logic and 1.5V or 1.2V for processor core voltages, as shown in the figure for the PT4820. They incorporate features to simplify system integration, including flexible dual logic enabled controls, input undervoltage lockout, and overtemperature protection. You can internally sequence all outputs to meet the power-up and -down requirements of DSPs, ASICs, and microprocessors.

To protect against load faults, all three outputs incorporate output short circuit protection. When the combined output current from the three outputs exceeds the current limit threshold, the module shuts down after a period of about 15 ms. The shutdown forces the output voltage at all three regulated outputs to simultaneously fall to zero. Following shutdown, the module periodically attempts to recover by executing a soft start power-up. This occurs at intervals of about 65 ms. If the load fault persists, the module will continually cycle through successive overcurrent trips, shutdowns, and restarts.

The converters have an internal temperature sensor that monitors the module's metal case temperature; if it exceeds a nominal 115°C, the converter will shut down. The converter will automatically restart when the sensed temperature returns to about 100°C.

Undervoltage lockout (UVLO) with about 2V of hysteresis stops converter operation whenever the input voltage is insufficient to maintain output regulation. This prevents oscillation with a slowly changing input voltage. Below the UVLO threshold the module is off and the EN1 and EN2 enable control inputs are inoperative.

Regulation control circuits are on the output side of the isolation barrier. The PT4850 series passes the control signals between the primary and secondary sides of the converter via a proprietary magnetic coupling scheme. This eliminates the need for optocouplers.

The power-up characteristic of the modules meets the requirements of microprocessor and DSP chipsets. All three converter outputs power up in unison. During power-up, Vo1 initially rises to about 0.8V, followed by Vo2 and Vo3 that promptly rise to the same voltage as Vo1. All three output voltages then rise together until each reaches their respective output voltage. Converters typically produce a fully regulated output within 60 ms of applying Vin.

During turn-off, all outputs drop rapidly due to the discharging effect of actively switched rectifiers. Discharge time is typically 100μs, but varies with the amount of external load capacitance.

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