Octal power-converter-control chip uses PMBus
By Paul Rako, Technical Editor -- EDN, August 10, 2009
Linear Technology Corp’s new LTC2978 monitoring-and-control chip supervises eight switching power converters. You communicate with the IC over the PMBus (power-management bus). The controlled supplies have output voltages of 0 to 6V. The device has an onboard EEPROM that stores settings when the chip is off. Eight 10-bit DACs use the power converter’s trim pin or resistor-feedback network to sequence, adjust, and margin the eight target converter chips. The device measures the target converter’s output voltage by multiplexing a 15-bit delta-sigma ADC across the eight converters. The ADC can also measure the part’s input voltage. A carefully trimmed onboard 1.232V reference provides a ±0.25% overall accurate measurement, including ADC errors. The device operates from a 4.5 to 12V power supply and also contains 2.5 and 3.3V linear regulators. It also has an internal temperature sensor. You can cascade multiple devices if you need to control more than eight converters.
The LTC2978 allows you to bring eight analog- or digital-power-converter chips under PMBus control. It can perform as a watchdog timer, a sequencer, and a supervisor to monitor both overvoltage and undervoltage events. You can also use the part to perform margining of the target power supplies during manufacturing to ensure that your system works properly under a predetermined range of converter supply voltages. The chip logs faults to the internal EEROM. The chip’s interface and command set comply with the PMBus specification and operate as fast as 400 kHz. You can explore all the product’s features with LTpowerPlay, a free software package that can communicate over the PMBus. The demo board kit includes a dongle to convert your computer’s USB port to an SMBus (system-management bus).
The LTC2978 comes in a 9×9-mm, 64-pin QFN package. The LTC2978CUP version operates at a 0 to +70°C junction temperature, and the LTC2978IUP version operates in the −40 to +85°C range. The commercial-temperature version sells for $10.95 (1000).
