Dual synchronous Step-Down DC/DC controller with PMBus

March 31, 2015 // By Graham Prophet
LTC3887 is a dual output synchronous step-down DC/DC controller with I²C-based PMBus interface for digital power system management. The LTC3887 differs from the previously released LTC3880 with an enhanced feature set that includes a faster 70 msec power-up time, higher output voltage capability and a fast ADC mode that provides an 8 msec update rate for one parameter.

For applications including high current ASIC, FPGA and processor supplies in telecom, datacom, computing and storage markets, LTC3887 enables digital programming and read back for real-time control and monitoring of critical point-of-load converter functions. Programmable control parameters include output voltage, margining and current limits, input and output supervisory limits, power-up sequencing and tracking, switching frequency, and identification and traceability data. On-chip precision data converters and EEPROM allow for the capture and nonvolatile storage of regulator configuration settings and telemetry variables, including input and output voltages and currents, duty cycle, temperature and fault logging.

LTC3887 and LTC3880 share a common board footprint; LTC3887 combines analogue switching regulator performance with precision mixed signal data conversion for ease of power system design and management, supported by the LTpowerPlay software development system with its graphical user interface (GUI).

The LTC3887 can regulate two independent outputs or can be configured for a two phase single output. Up to 6 phases can be interleaved and paralleled for accurate sharing among multiple ICs, minimising input and output filtering requirements for high current and/or multiple output applications. An integrated amplifier provides true differential remote output voltage sensing, enabling high accuracy regulation, independent of board IR voltage drops.

Configurations for the LTC3887 can be saved to internal EEPROM over the device’s I²C serial interface using Linear Technology’s LTpowerPlay GUI-based development software. With configurations stored on-chip, the controller can power up autonomously without burdening the host processor. Default settings can be optionally configured by external resistor dividers for output voltage, switching frequency, phase and device address. Multiple designs can be calibrated and configured in firmware to optimise a single hardware design for a range of applications. The converter loop gain does not change as the power supply parameters are modified, so compensation remains optimised for multiple configurations.

The LTC3887 features high current integrated gate drivers to drive all N-channel power MOSFETs from input voltages ranging from 4.5V to 24V, and it can produce ± 0.50% accurate output voltages from 0.5V to 5.5V with output currents up to 30A per phase over the full operating temperature range. Highest efficiency is achieved by sensing the voltage drop across the output inductor (DCR) to sense current, or an external sense resistor can optionally be used. Programmable DCR temperature compensation cancels the temperature coefficient of the copper inductor to maintain an accurate and constant current limit over a broad temperature range.

The device’s minimum on-time of 90 nsec suits the LTC3887 for compact high frequency/high step-down ratio applications. Accurate timing across multiple chips and event-based sequencing allow the optimisation of power-up and power-down of complex, multiple rail systems. Features include constant frequency current mode control with cycle-by-cycle current limit, adjustable soft start, a synchronisable switching frequency, and programmable GPIO pins to indicate part status and to provide autonomous recovery from faults.

The LTC3887 is available in a 40-pin 6 x 6 mm QFN packages with an operating junction temperature range from -40°C to 125°C, for $5.47 (1000).

Linear Technology; www.linear.com/product/LTC3887

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