DEMO CIRCUIT 1495A DC-1495A QUICK START GUIDE LT2940 Power Monitor DESCRIPTION Demonstration circuit 1495A showcases the tor can monitor either the power output, PMON, or LT2940 Power Monitor, configured to measure up the current output, IMON. to 30W over a range of 8V to 40V and 0A to 3A. Design files for this circuit board are available. LEDs indicate when the load power exceeds Call the LTC factory. 12.5W. Power is indicated at PMON with a scaling L, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are factor of 10W/V IMON indicates current at 1A/V. registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, Module, Micropower All scaling factors and the voltage and current SwitcherCAD, Multimode Dimming, No Latency , No Latency Delta-Sigma, No R , Operational Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SENSE measurement ranges may be reconfigured by SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, UltraFast and VLDO are trade- changing a few resistors. The LT2940s compara- marks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. PERFORMANCE SUMMARY Specifications are at TA = 25C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Input Supply Range 8 40 V IN V V Supply Range (When Separately Powered) 12 80 V CC CC PMON Load Power for Full Scale Output PMON = 3V 27 30 33 W IMON Load Current for Full Scale Output IMON = 3V 2.85 3 3.15 A P Overpower Threshold Red LED Turns On 11 12.5 14 W TRIP OVERVIEW The LT2940 is a four-quadrant multiplier designed A distinction must be drawn between the features to measure current and voltage, multiply them, and and performance of the LT2940, and the features produce an output proportional to power. A sec- and performance of DC-1495A. The LT2940 has ond output is proportional to the measured cur- the following important features: rent. All inputs and outputs are bipolar the LT2940 l Four quadrant power measurement with bidi- measures positive or negative power, voltage and rectional power and current outputs current. The power and current outputs operate in l V -independent, high-side current sense in- current mode, with a full scale of 200A. To fa- CC put cilitate use in a practical application, the current sense pins (I+ and I) are designed to operate over l 4V to 80V current sense input operating a 4V to 80V common mode range, independent of range the supply pin, V . This permits the LT2940 to CC l 6V to 80V V operating range CC measure, for example, a 48V supply while operat- ing on a 12V supply. The LT2940 includes an aux- l 100V absolute maximum rating iliary comparator with a fixed 1.25V reference and complementary outputs. 1 DC-1495A l Auxiliary comparator with complementary R9B or tie the LATCH turret high (>2.5V) for latch- outputs ing behavior. Grounding the LATCH turret clears the comparator after latching. As configured, DC-1495A features: RJ-2, R2A, R1A, R1C, R2B, R1B: divider compo- l Single quadrant power and current meas- nents for voltage sense pins V+ and V. As urement with unidirectional outputs stuffed, R2A and R1A form a 5:1 divider that moni- l 8V to 40V, 0A to 3A measurement range tors the output voltage, with a 40V full scale value. By re-arranging the components it is possible to l 30W full scale power monitor output, 10W/V achieve a variety of configurations for single-ended l 3A full scale current monitor output, 1A/V and differential voltage measurements. l 75V maximum input, limited by clamp C3, R3, RJ-4, RJ-5, R4B, R4A, C4, R5B, R5A, C5: configuration and scaling resistors and integration l V connected to INPUT turret CC or filter capacitors for PMON, IMON and the com- l LEDs indicate load power above (red) or be- parator input, CMP+. DC-1495A is stuffed with low (green) 12.5W 15k load resistors for PMON and IMON (R4A DC-1495A includes provision for optional compo- and R5A), and RJ-4 connects PMON to CMP+. nents, allowing the board to be reconfigured for Positions are provided for 3 sense resistors, allow- different scaling and a variety of applications. ing for high current operation or for combining two Components or three resistors in parallel to achieve a specific value. Sufficient copper is present on the circuit Components are divided into four basic groups: board to handle more than 10A beyond this point R12A, R12B, R12C, C1, D1, D2: these compo- DC-1495A can be connected to an off-board sense nents are associated with V (D1 supplies con- resistor or shunt using the INPUT and OUTPUT CC stant current to the LEDs). A zero ohm jumper terminals. In this case remove RS1, RS2, and installed in any one of the R12 positions selects RS3. The I+ and I terminals allow precise Kelvin the source of power for V : input, output, or ex- examination of the current sense signal, as seen by CC ternal supply connected to the V turret. Use a CC the LT2940. resistor if filtering is desired. As built R12B is Multiplier Operation stuffed with 10 and V is bypassed with 100nF, CC serving both as a V filter and also as a snubber The LT2940 has two differential inputs which we CC for the input. will call V and V . V is the voltage across the V+ V I V and V voltage input pins, and V is the voltage I An SMAT70A clamp diode, D2, is connected di- across the I+ and I current sense input pins. The rectly across the input to ground. It has a maxi- voltages at these inputs are multiplied together by mum dc standoff rating of 75V. Do not exceed a four-quadrant Gilbert cell, producing a current at 75Vdc or D2 will be permanently damaged. The the PMON output proportional to the product of V V LT2940 can tolerate up to 100V input absolute and V . Specifically, I maximum on the V , I+, I and LATCH pins. Nev- CC 2 ertheless, when connecting to supplies greater PMON Output Current = V V 500A/V V I than 50V it is possible to exceed 100V owing to 2 Where 500A/V is the gain or transfer function of ringing. D2 is included to prevent destruction of 2 the multiplier, and a V V product of 0.4V V I the LT2940 while performing bench tests. drives the PMON output to 200A full scale. R9A, C2, R9B: configuration resistors and noise Although the multiplier core can handle a maxi- filter for the LATCH pin. As stuffed the comparator 2 mum V V product of just 0.4V , the voltage V I operates in flow-through mode install 47k at 2