A simplified approach to digitally managing power in high-availability systems

High-end servers, telecommunications, and networking equipment use power management controllers to measure, track, and control the different power supplies on each board and report the measurement, tracking, and control information. This is called "managing power digitally." Digital management of high-availability power supplies is promising, but this digital management often comes at the expense of complex, multi-chip circuit solutions with high costs. For example, an application with voltage-current monitoring and supply margining capabilities may require many chips, such as low-drift references, multi-channel differential input ADCs with at least 12-bit resolution, 8-bit DACs, and dedicated microcontrollers. In addition, implementing margining algorithms, voltage and current monitor functions requires considerable software development work. Add to this the cost, complexity, board space requirements, and design debug time, and even the most professional power supply designers may be deterred from attempting to manage power digitally.

Designed to digitally manage power in high-availability systems, the LTC 2970 dual I2C power monitor and margin controller achieves the convergence of digital and analog power. The I2C digital interface, 14-bit ADC, high-accuracy reference, and current output DAC (IDAC) meet the needs of digital power designers. The LTC2970 can be used with most power supplies, allowing designers to select the best DC/DC converter with an analog control loop that smoothly controls the output voltage and fast transient response. The on-chip reference and 14-bit delta-sigma ADC ensure accurate measurement of supply voltage, load current, or temperature. Two voltage-buffered 8-bit IDACs adjust the feedback signal of the DC-DC converter. The adjustment range and resolution can be configured with only two resistors per channel, and the IDAC can also be programmed with a slow linear voltage servo loop to accurately fine-tune the converter output and control its margin.

Figure 1 LTC2790 application circuit using TRIM pin to implement DC/DC converter

The selection of DC/DC modules for high-availability systems is exactly the same as the selection of IC-based converters. Whether the DC/DC converter provides a TRIM pin or a feedback node, the LTC2970 is suitable. Figure 1 is a typical application circuit that uses the TRIM pin to control the output voltage margin of the DC/DC converter. After power is applied, the VOUT0 pin of the LTC2970 enters a high-impedance state in the default setting. If the soft connection function is used, the LTC2970 automatically finds the IDAC code closest to the TRIM pin open circuit voltage before starting the IDAC voltage buffer.

Precise voltage control

The LTC2970's ADC is a second-order delta-sigma modulator followed by a sinc2 digital filter that converts the modulator's serial data into 14-bit parallel data at a 30Hz conversion rate. One advantage of delta-sigma ADCs over conventional ADCs is the ability to implement on-chip digital filtering, which, combined with a large oversampling ratio (OSR = 512), makes the LTC2970 noise-free when sampling the supply voltage. The LTC2970's sinc2 digital filter provides high rejection at frequencies other than integer multiples of the modulator sampling frequency (fS = 0.72kHz). A simple RC low-pass filter at the input of the ADC reduces ripple components that can cause DC aliasing.

The ADC's differential inputs can monitor the supply voltage and sense resistor voltage at the load point. The differential and common-mode input range is -0.3V to 6V. The ADC has a resolution of 500mV/LSB and can resolve voltage over a wide range of load currents with sense resistor values ​​of only a few millivolts.

If the voltage of the DC/DC converter deviates by more than ±0.1%, the LTC2970 is configured to servo the converter to 1V. The LTC2970 is able to maintain the output voltage within 1V ±1mV when both the LTC2970 and the DC/DC converter are heated from -50°C to 100°C. When the LTC2970 is isolated from the DC/DC converter, the output voltage remains between 1.002V and 1.0055V over the same temperature range.

Tracking and Sequencing with the LTC2970-1

The LTC2970-1 adds just a few external components to enable supply tracking and sequencing. A special global address and synchronization command allows multiple LTC2970-1s to track and sequence multiple pairs of supplies.

Figure 2 Using the LTC2970 to track two power supplies

A typical LTC2970-1 tracking application circuit is shown in Figure 2. The GPIO_0 and GPIO_1 pins are connected directly to the RUN/SS pins of their respective DC/DC converters. Since GPIO_CFG is pulled high to VDD, the LTC2970-1 automatically delays the start-up of the DC/DC converter after power-up by asserting the open-drain outputs GPIO_0 and GPIO_1 low. When GPIO_CFG is high, N-channel FETs Q10/11 and diodes D10/11 form unidirectional range switches around resistors R30A/31A. These range switches allow the VOUT0 and VOUT1 pins of the LTC2970-1 to drive the output of the converter to or from ground through resistors R30B/31B. When GPIO_CFG is pulled low, N-channel FETs Q10 and Q11 are turned off. R30A/31A and R30B/31B are then connected in series to achieve the usual margining operation. A 100k/0.1mF low-pass filter is connected in series with the gate of Q10/11 to minimize the charge injected into the DC/DC converter feedback node when GPIO_CFG is pulled low.

Digital Communications

All communication operations of the chip are performed through the I2C bus, meeting all SMBus setup time, hold time and timeout requirements. The ALERT pin can be used to indicate that one or more of the 14 configurable fault tolerance limits have been reached. Each fault can be individually masked.

Conclusion

The LTC2970 forms a highly accurate digital power solution for digital communication, advanced power monitoring and control of power supplies in high availability systems. Considering the complexity, software development and extensive debugging time requirements, the LTC2970 is quite simple compared to discrete component solutions. User-configurable multiple fault monitoring functions and built-in servo algorithms reduce the burden on system computing resources and shorten software development time. ■