APPLICATION NOTE 105: Current Sense Circuit Collection High Current (100mA to Amps) Sensing high currents accurately requires excellent control of the sensing resistance, which is typically a very small value to minimize losses, and the dynamic range of the measurement circuitry Shunt Diode Limits Maximum Input Voltage to Allow Better Low Input Resolution Without Over-Ranging the LTC6101 V+ To see other chapters in this Application Note, return to the Introduction. RSENSE Kelvin Input Connection Preserves Accuracy Despite Large Load Currents LOAD If low sense currents must be resolved accurately in a system that has very wide dynamic range, more gain can be taken in the sense amplifier by using a smaller value for resistor RIN. This can result in an operating current greater than the max current spec allowed unless the max current is limited in another way, such as with a Schottky diode across RSENSE. This will reduce the high current measurement accuracy by limiting the result, while increasing the low current measurement resolution. This approach can be helpful in cases where an occasional large burst of current may be ignored. V+ RSENSE RIN 4 3 + – LOAD 2 DSENSE 6101 F03a 5 1 VOUT LTC6101 ROUT Kelvin Sensing 6101 F02 DIRECTION OF CHARGING CURRENT Kelvin connection of the IN– and IN+ inputs to the sense resistor should be used in all but the lowest power applications. Solder connections and PC board interconnections that carry high current can cause significant error in measurement due to their relatively large resistances. By isolating the sense traces from the high current paths, this error can be reduced by orders of magnitude. A sense resistor with integrated Kelvin sense terminals will give the best results. RSENSE 4008 F12 CSP BAT In any high current, >1Amp, application, Kelvin contacts to the sense resistor are important to maintain accuracy. This simple illustration from a battery charger application shows two voltage-sensing traces added to the pads of the current sense resistor. If the voltage is sensed with high impedance amplifier inputs, no IxR voltage drop errors are developed. High Current (100mA to Amps)-1 APPLICATION NOTE 105: Current Sense Circuit Collection 0A to 33A High Side Current Monitor with Filtering 4.4V TO 48V SUPPLY Single Supply RMS Current Measurement 3V 2 LT6100 VCC 7 6 A4 A2 + 8 VS VOUT 5 RSENSE 3mΩ – 1 VS LOAD VEE FIL 4 3 VOUT = 2.5V ISENSE = 33A 6100 TA01a 220pF CONFIGURED FOR GAIN = 25V/V The LT1966 is a true RMS-to-DC converter that takes a single-ended or differential input signal with rail-to-rail range. The output of a pcb mounted current sense transformer can be connected directly to the converter. Up to 75A of AC current is measurable without breaking the signal path from a power source to a load. The accurate operating range of the circuit is determined by the selection of the transformer termination resistor. All of the math is built in to the LTC1966 to provide a dc output voltage that is proportional to the true rms value of the current. This is valuable in determining the power/energy consumption of ac powered appliances. High current sensing on a high voltage supply rail is easily accomplished with the LT6100. The sense amplifier is biased from a low 3V supply and pin-strapped to a gain of 25V/V to output a 2.5V full scale reading of the current flow. A capacitor at the FIL pin to ground will filter out noise of the system (220pF produces a 12KHz low pass corner frequency). Dual LTC6101’s Allow High-Low Current Ranging VLOGIC (3.3V TO 5V) CMPZ4697 7 10k 3 M1 Si4465 VIN RSENSE HI 10m ILOAD VOUT 301 RSENSE LO 100m 4 + – 2 + – LTC6101 Q1 CMPT5551 40.2k 6 301 301 301 4.7k 1.74M 4 8 5 3 4 5 2 VIN 1 3 + – 2 5 619k 1 LTC6101 LTC1540 1 HIGH RANGE INDICATOR (ILOAD > 1.2A) HIGH CURRENT RANGE OUT 250mV/A 7.5k VLOGIC BAT54C R5 7.5k (VLOGIC +5V) ≤ VIN ≤ 60V 0 ≤ ILOAD ≤ 10A High Current (100mA to Amps)-2 LOW CURRENT RANGE OUT 2.5V/A 6101 F03b APPLICATION NOTE 105: Current Sense Circuit Collection Using two current sense amplifiers with two values of sense resistors is an easy method of sensing current over a wide range. In this circuit the sensitivity and resolution of measurement is 10 times greater with low cur- rents, less than 1.2 Amps, than with higher currents. A comparator detects higher current flow, up to 10 Amps, and switches sensing over to the high current circuitry. LDO Load Balancing VIN 1.8V TO 20V IN + 10µF BALLAST RESISTANCE: IDENTICAL LENGTH THERMALLY MATED WIRE OR PCB TRACE OUT LT1763 0.01µF 10µF SHDN BYP FB R2 2k IN R1 2k ⎛ R1⎞ VOUT = 1.22V ⎜1 + ⎟ ⎝ R2⎠ OUT LT1763 0.01µF 10µF SHDN BYP 100Ω ILOAD FB LOAD 2k 2k 1k 0.1µF – A 10k IN + OUT LT1763 0.01µF 10µF SHDN BYP 100Ω FB 2k 0 ≤ ILOAD ≤ 1.5A 1.22V ≤ VOUT ≤ VDD LDO LOADS MATCH TO WITHIN 1mA WITH 10mΩ OF BALLAST RESISTANCE (2 INCHES OF AWG 28 GAUGE STRANDED WIRE) A, B: LTC6078 2k 0.1µF 1k VDD – B 10k As system design enhancements are made there is often the need to supply more current to a load than originally expected. A simple way to modify power amplifiers or voltage regulators, as shown here, is to parallel devices. When paralleling devices it is desired that each device shares the total load current equally. In this circuit two adjustable “slave” regulator output voltages are sensed + 60789 TA09 and servo’ed to match the master regulator output voltage. The precise low offset voltage of the LTC6078 dual op amp (10uV) balances the load current provided by each regulator to within 1mA. This is achieved using a very small 10mΩ current sense resistor in series with each output. This sense resistor can be implemented with pcb copper traces or thin gauge wire. High Current (100mA to Amps)-3 APPLICATION NOTE 105: Current Sense Circuit Collection Sensing Output Current VCC 0V TO 1V 12V VCSRC VCSNK +IN –IN EN VCC V+ ISRC ISNK RS 0.2Ω TSD OUT LT1970 SENSE+ – SENSE FILTER – V VEE RLOAD COMMON R4 255k LT1787 RG RF VS– –12V VS+ 20k VEE –12V BIAS –12V R1 60.4k R2 10k 12V – + VOUT 2.5V ±5mV/mA LT1880 R3 20k 1kHz FULL CURRENT BANDWIDTH –12V 0V TO 5V A/D 1970 F10 OPTIONAL DIGITAL FEEDBACK The LT1970 is a 500mA power amplifier with voltage programmable output current limit. Separate DC voltage inputs and an output current sensing resistor control the maximum sourcing and sinking current values. These control voltages could be provided by a D-to-A Converter High Current (100mA to Amps)-4 in a microprocessor controlled system. For closed loop control of the current to a load an LT1787 can monitor the output current. The LT1880 op amp provides scaling and level shifting of the voltage applied to an A-to-D Converter for a 5mV/mA feedback signal.