design features 3mm × 3mm QFN IC Directly Monitors 0V to 80V Supplies: Features I2C Interface, Peak Value Tracking and Runs from Any Supply CY Lai Power monitoring in combination with control mechanisms can significantly boost system energy efficiency and reliability. The LTC2945 is a highly integrated digital power monitoring solution that is compact, rugged and easy-to-use. It is designed to fit applications requiring power monitoring with a minimal number of components. MONITOR POWER ON ANY SUPPLY •The secondary supply exists, but is not readily accessible—it is inconveniently located on the printed circuit board, complicating the routing of a power line The LTC2945’s internal current sense amplifier features a common mode range of 0V to 80V to suit a wide variety of high side and low side current sensing applications. Most wide range supply monitors available today require a low voltage secondary supply for operation, which can be undesirable for several reasons: The LTC2945 avoids these problems by integrating a high voltage linear regulator that can be powered directly from 4V to 80V supplies. The output of the linear regulator (INTVCC) powers the LTC2945, and can be externally bypassed to prevent supply noise from corrupting the signal integrity of internal circuitry. The linear regulator is capable of supplying a 10m A load, saving the cost of a dedicated high voltage linear regulator needed to power circuits such as opto-couplers in some applications. •There is no suitable secondary supply •The secondary supply, often loaded with noisy digital circuits, must be sufficiently filtered or bypassed due to the finite power supply rejection ratio of the supply monitor at higher frequencies Figure 1. Functional block diagram of the LTC2945 SENSE+ VDD SENSE– ADR0 ADR1 ALERT DECODER + VSTBY 20X 5.7V VSTBY GEN INTV CC 2 IC – Figure 1 shows the LTC2945’s functional block diagram. All basic elements required for power monitoring are integrated, including a precision current sense amplifier, precision resistive dividers, an analog-to-digital converter (ADC) and an I2C interface for communicating with the host controller. Only an external current sense resistor is required. The host can periodically poll the LTC2945 for available power data, minimum and maximum values are stored, and an alert can be sent from the LTC2945 to interrupt the host when measured values exceed their preprogrammed limits. 735k SDAO/SDAO (LTC2945 / LTC2945-1) VREF = 2.048V 735k LOGIC SDAI VSTBY 12 MUX 6.3V 15k 6.4V 12-BIT ADC SCL REGISTERS 15k 6.4V ADIN GND July 2013 : LT Journal of Analog Innovation | 13 The LTC2945 is a highly integrated power monitor that easily fits into a wide range of systems. It offers a 0V to 80V common mode range, 2.7V to 80V operating range, ±0.75% accurate voltage and current measurements, and an on-chip digital multiplier that computes power. RSNS VIN 4V TO 80V SENSE+ RSNS VIN 0V TO 80V VOUT SENSE+ SENSE– VDD SENSE– SENSE+ LTC2945 VOUT SENSE– INTVCC 2.7V TO 5.9V VDD 4V TO 80V RSNS VIN 0V TO 80V VOUT VDD LTC2945 LTC2945 C2 INTVCC C2 INTVCC C2 GND GND Figure 2a. The LTC2945 deriving power from the monitored supply GND Figure 2b. The LTC2945 deriving power from a wide ranging secondary supply Figure 2c. The LTC2945 deriving power from a low voltage secondary supply GND RSNS VIN >80V SENSE+ VOUT SENSE+ SENSE– >80V INTVCC VDD RSNS VIN 0V TO 80V RSHUNT RSHUNT VOUT SENSE– VDD VDD INTVCC VDD INTVCC LTC2945 GND LTC2945 C1 GND C2 INTVCC C2 LTC2945 GND LTC2945 C2 VNEG >–80V RSHUNT Figure 3a. The LTC2945 deriving power through a high side shunt regulator Figure 3b. The LTC2945 deriving power through a low side shunt regulator in a high side current sense topology Figure 2a shows a typical LTC2945 application monitoring a 4V to 80V supply and deriving power off the same supply. The bus voltage is measured at the SENSE+ pin through an internal resistive divider and a sense resistor is used to measure the load current on the high side. If the bus voltage to be monitored is below 2.7V, the power for the LTC2945 can be derived from a wide range secondary supply 14 | July 2013 : LT Journal of Analog Innovation SENSE– GND GND SENSE– SENSE+ RSNS VOUT Figure 3c. The LTC2945 deriving power through a low side shunt regulator in a low side current sense topology VNEG –4V TO –80V SENSE+ RSNS VOUT Figure 3d. The LTC2945 deriving power from the monitored supply in a low side current sense topology as shown in Figure 2b or a low voltage secondary supply as shown in Figure 2c. standing off the bus voltage and supplying LTC2945’s operating current will work. The LTC2945 also integrates a 6.3V, 35m A shunt regulator at the INTVCC pin for operation beyond 80V. Figure 3a shows the LTC2945 used in one such application with its ground floated at 6.3V below the bus voltage. The bulk of the bus voltage is dropped across an external shunt resistor; in practice any current source capable of Figure 4 shows how to measure the bus voltage in this configuration using a matched PNP pair and some resistors. The resistor values shown are optimized for VIN of 165V ±10%. The LTC2945’s shunt regulator can also be configured as shown in Figure 3b when the only design features LTC2945 integrates an oversampling ∆∑ ADC that inherently averages the measured voltage over the conversion cycle to effectively reject noise due to transient spikes and AC power line. Bus voltage, sense voltage and ADIN are measured with total error of less than ±0.75% at full scale over the full industrial temperature range. Figure 4. Application circuit for measuring the bus voltage in a high side shunt regulator configuration ±0.75% TOTAL ERROR MEASUREMENT ACCURACY VIN R1 2k R2 2k NST30010MXV6 INTVCC LTC2945 integrates an oversampling DS ADC that inherently averages the measured voltage over the conversion cycle to effectively reject noise due to transient spikes and AC power line harmonics. Bus voltage, sense voltage and ADIN are measured with total error of less than ±0.75% at full scale over the full industrial temperature range. LTC2945 ADIN R3 1150k R4 5.76k GND UP TO 8mA LOAD (OPTOS, ETC.) RSHUNT* 15k *THREE 1W, 5k RESISTORS IN SERIES secondary supply available exceeds 80V in high side current sensing applications. If the output of the power supply is negative such as in –48V distributed power systems for networking, communications and high end computing equipment, low side current sensing is generally preferred, as shown in Figures 3c and 3d. Figure 3c shows a shunt resistor and LTC2945’s shunt regulator limiting INTVCC to 6.3V above a negative supply that exceeds 80V. More commonly, the negative supply is below 80V and instead the internal linear regulator can be used to power the LTC2945 directly, as shown in Figure 3d. In this configuration the VDD pin measures the bus voltage through an internal resistive divider. Measuring bus voltage in excess of 80V in low side current sensing applications such as Figure 3c can be done by connecting a resistive divider to the ADIN pin as shown in Figure 5. Figure 5. ADIN senses the bus voltage in low side shunt regulator configuration. The 12-bit DS ADC provides a full-scale voltage of 102.4mV (25µV/LSB) for sense voltage, 102.4V (25mV/LSB) for bus voltage and 2.048V (0.5mV/LSB) for ADIN. Typical integral linearity error (INL) of the ADIN voltage and the sense voltage are both well within ±0.5LSB, as shown in Figures 6 and 7. The LTC2945 is also ideal in applications where accuracy is important at the low end of the measurements since its specified offset voltages are as low as ±1.1LSB for ADIN and ±3.1LSB for current sense voltage in the worst case. Figure 6. ADIN INL curve Figure 7. SENSE INL curve 0.3 0.2 GND 0.2 INTVCC LTC2945 ADIN R2 GND 0.1 ADC INL (LSB) R1 ADC INL (LSB) RSHUNT VNEG > –80V 0.4 0.0 –0.1 0.0 –0.2 –0.2 –0.3 0 1024 2048 CODE 3072 4096 –0.4 0 1024 2048 CODE 3072 4096 July 2013 : LT Journal of Analog Innovation | 15 Opto-isolation is common in high voltage systems where the high voltage sections must be galvanically isolated for safety reasons. The LTC2945 accommodates isolated applications by splitting the SDA signal on the I2C interface into an SDAI pin and an SDAO pin (for LTC2945-1, SDAO) for applications with an opto-isolator interface. PEAK VALUES TRACKING AND OVER/ UNDERVALUE ALERTS the monitor needs to be monotonic and of high resolution in order to minimize stability issues. The LTC2945 generates 24-bit power data by digitally multiplying the 12-bit sense voltage and 12-bit bus voltage data without truncating the result. Keeping track of the minimum and maximum measurement values is important in many power monitoring systems because it could be used to study usage behavior for more efficient resource allocation and is often an indicator of system health. Previously, gathering such information required periodic polling of the power monitor by the system’s microprocessor, which wasted precious computation time and potentially tied up the I2C interface. The LTC2945 solves this problem by storing the minimum and maximum values for power, voltage, current, and ADIN. The Page Read feature on the LTC2945 allows these data to be read with just a single I2C read transaction. An ALERT pin can also be configured to signal overvalue or undervalue limit violations for power, voltage, current and ADIN. OPTO-ISOLATION AND SHUTDOWN The LTC2945 can be shut down via the serial I2C interface, reducing the typical quiescent current to 20µ A—especially important for battery-powered applications. Opto-isolation is common in high voltage systems where the high voltage sections must be galvanically isolated for safety reasons. The LTC2945 accommodates isolated applications by splitting the SDA signal on the I2C interface into an SDAI pin and an SDAO pin (for LTC2945-1, SDAO) for applications with an optoisolator interface as shown in Figure 8. For limited amounts of current, the internal linear regulator or shunt regulator can be used to supply the pull-up resistors on the I2C bus. In situations where it is undesirable to tap off the internal regulator and a low voltage UNTRUNCATED 24-BIT POWER DATA For applications where a digital servo loop is used to regulate the power output of a system, the power data read back from Figure 8. Opto-isolation of a 10kHz I2C interface between the LTC2945 and a microcontroller supply is not available, the LTC2945-1 allows the pull-up resistors to connect directly to high voltages. The SCL and the SDAI pins are limited to safe voltages by internal 6.3V, 3m A clamps. The SDAO pin is inverted (to SDAO) so that it can be clamped by the anode of the input diode of an optoisolator as shown in Figure 9. SUPPLY TRANSIENTS The wide operating range of the LTC2945 is advantageous even in applications where the bus voltage is normally well below 80V. Transient voltage surges due to inductive kickbacks in automotive load dump situations and hot swap output shorts are just two possible scenarios where a rugged power monitoring solution is required in order to withstand overvoltage conditions far in excess of the normal operating voltage. The 100V absolute maximum rating of the LTC2945 makes it easy to guard against these types of voltage surges since there is a wide range of transient surge suppressor (TVS) diodes from which to choose. In certain applications a large Figure 9 Opto-isolation of a 1.5kHz I2C interface between the LTC2945-1 and a microcontroller 3.3V 3.3V 5V SDAI R4 10k R5 0.82k R6 0.51k 48V R7 10k R4 20k µP 1/2 MOCD207M LTC2945-1 GND SDA SDAO GND SDAO 16 | July 2013 : LT Journal of Analog Innovation µP 1/2 MOCD207M GND 1/2 MOCD207M R7 10k VDD SDA GND R6 0.51k SDAI VDD LTC2945 R5 7.5k 1/2 MOCD207M design features The 100V absolute maximum rating of the LTC2945 makes it easy to guard against voltage surges since there is a wide range of transient surge suppressor (TVS) diodes from which to choose. In certain applications a large MOSFET power device can break down to clip the inductive spike safely, and in most 12V and 24V systems the break-down voltage of these power devices is less than 100V, potentially negating the requirement for a TVS diode. RSNS 0.02Ω VIN SENSE + VOUT SENSE– INTVCC VDD R12 100Ω R4 1k LTC2945 Q1 PZTA42 R3 0.51k R1 1k C2 1µF R2 1k FGND 3.3V R7 0.51k R8 0.51k R9 1k R10 1k R11 10k VDD VCC T1 SMAJ78A SCL C1 0.1µF SCL SDAI SDA GND HCPL-063L FGND µP 3.3V VCC ADR1 ADR0 SDAO ADIN INT ALERT GND GND GND HCPL-063L FGND M1 BSP149 Figure 10. Ruggedized 4V to 80V high side power monitor with surge protection up to 200V MOSFET power device can break down to clip the inductive spike safely, and in most 12V and 24V systems the breakdown voltage of these power devices is less than 100V, potentially negating the requirement for a TVS diode. Hard-clamping the voltage with TVS diode or MOSFET break down may not be practical when the inductive energy is too high or unpredictable. Figure 10 shows a LTC2945-based power monitor that can ride through a 200V surge where its high voltage pins are clamped by T1 to less than 80V. In normal operation, M1 operates in the triode region with the device ground R5 1Ω at a few mV above the system ground. During the surge, the device ground is lifted by T1 and the balance of the surge voltage is dropped across M1. The BSP149 has a 200V break down and the surge duration is limited by its safe operating area—for example at room temperature it can survive a 200V surge for 1ms at VIN . multiplier that computes power. Digital watchdog functions such as peak and valley values and window comparators are available for power, voltage, current and an external voltage. Opto-isolation is simplified with a split SDA pin. The LTC2945 is available in space-saving 3mm × 3mm QFN and 12-pin MSOP packages. CONCLUSION Visit www.linear.com/LTC2945 for data sheets, demo boards and other applications information. n The LTC2945 is a highly integrated power monitor that easily fits into a wide range of systems. It offers a 0V to 80V common mode range, 2.7V to 80V operating range, ±0.75% accurate voltage and current measurements, and an on-chip digital July 2013 : LT Journal of Analog Innovation | 17