Application Note 1777 Author: Tim Lok Low Side Precision Current Sense Amplifier Introduction The ISL28134ISENSEV1Z evaluation board is designed specifically for low side current sensing up to 10A of current. The evaluation board uses a 1mΩ current sense resistor that is capable of handling 4W of power dissipation. The ISL28134 Precision Low Noise Zero Drift Amplifier gains the current sensing input signal and can be used to directly drive ADC inputs. The ISL28134ISENSEV1Z evaluation board is optimized to operate at +5V. An ISL21090 Precision Low Noise 2.5V voltage reference sets a zero current reading of 2.5V at the amplifier output to interface with ADCs operating at 5V. The voltage reference also raises the common mode input by approximately 40mV above GND at zero current flow. This connection enables bi-directional current sensing, allowing the sense voltage to be positive or negative relative to the common mode voltage. This is helpful for applications such as the charging/discharging current from an Electric Vehicle battery or motors that switch polarity. With the input common mode at 40mV above ground and a 1mΩ sense resistor, this allows ±40A current sensing before amplifier saturation. However, due to PCB copper resistance causing error at high currents, the recommended current sense is ±10A. CONNECT TO LOAD ISENSE = POSITIVE CURRENT FLOW (VOUT - 2.5) 0.0635 connection with the sense voltage pick up points at the center of the current density distribution. Amplifier U3 gains up the voltage drop developed across the sense resistor from the current flow. Voltage Reference sets the zero current reading output of the amplifier at 2.5V. Quick Setup Guide 1. Connect single supply +5VDC to +5V and GND terminals. 2. Connect low side load to terminal lugs. Current flow from top to bottom of board. 3. At no load, VOUT is 2.5V. 4. With 1mΩ sense resistance and amplifier gain of 63.5V/V, the sense current reading is: (VOUT - 2.5V)/0.0635 in Amps. REF PART NUMBER U1 ISL21090 U2 DNP U3 ISL28134 Not populated on board Ultra Low Noise Zero Drift Amplifier CURRENT SENSE SPECIFICATIONS Supply Voltage Range 3.0V to 5.0V Optimized for 5V Operation Max Sense Current ±10A recommended ±40A Before Amplifier Saturation AMP Abs Max Sense Current VOUT COMMENTS Ultra Low Noise 2.5V Voltage Reference 60 Amps Thermally Limited by Sense Resistor Current Sense Resolution ±5mA Current Sense Accuracy 2% Voltage to Current Conversion ISENSE = (VOUT - 2.5V) / 0.0635 Amps Amplifier Bandwidth 10kHz Measuring Very High Current +5V AMPLIFIER GND CONNECT TO SYSTEM GND FIGURE 1. ISL28134ISENSEV1Z EVALUATION BOARD Board Layout The evaluation board is laid out with heavy duty screw lug terminals that allow connections of wires up to 6 AWG. The 1mΩ sense resistance is formed from four 4mΩ 2512 style resistors to increase power dissipation capacity of the sense circuitry. The total power dissipation critically allowed is 4W. The PCB board is made using 2oz copper PCB with the current path routed on the top and bottom layer, minimizing trace resistance to maintain accurate current sense resistance. To further reduce sense errors, the current sensing uses a Kelvin September 12, 2012 AN1777.0 1 The reference design is capable of sensing currents greater than ±40A, however, it is limited mechanically from the temperature rise of the copper PCB layout and current sense resistor and limited electrically by amplifier saturation. One must consider the temperature rise of the PCB trace from the power dissipated under high currents, which may cause the copper trace to delaminate. The high current carrying PCB trace is made with 2oz copper on FR4 board both top and bottom layers. The trace dimension is approximately 25mm width and 40mm length. Substantial vias are used to connect the planes for lower thermal impedance. The current sense resistance is capable of 4W dissipation max. CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. Copyright Intersil Americas Inc. 2012. All Rights Reserved. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries. All other trademarks mentioned are the property of their respective owners. Application Note 1777 Bill of Materials PART NUMBER REFERENCE DESIGNATOR DESCRIPTION MANUFACTURER ISL21090BFB825Z U1 Low Noise 2.5V Voltage Reference, SOIC-8 DNP U2 DO NOT POPULATE ISL28134FHZ U3 H1045-00101-50V5-T MANUFACTURER PART INTERSIL ISL21090BFB825Z-TK Low Noise Zero Drift Amplifier, SOT-23 INTERSIL ISL28134FHZ C8,C9 CAP, SMD, 0603, 100pF, 50V, 5%, C0G, ROHS GENERIC H1045-00102-50V5-T C4 CAP, SMD, 0603, 1000pF, 50V, 5%, C0G, ROHS GENERIC H1045-00103-50V10-T C1, C3, C5, C6, C11 CAP, SMD, 0603, 0.01µF, 50V, 10%, X7R, ROHS GENERIC H1045-00104-50V10-T C2 CAP, SMD, 0603, 0.1µF, 50V, 10%, X7R, ROHS GENERIC H1045-DNP C7 CAP, SMD, 0603, DNP-PLACE HOLDER, ROHS H1065-00475-50V10-T C10 CAP, SMD, 1206, 4.7µF, 50V, 10%, X5R, ROHS GENERIC BAV99LT1G-T S1 DIODE-SWITCHING, SMD, SOT23, 70V, 0.2A, ROHS ON SEMICONDUCTOR BAV99LT1G H2511-00R00-1/10W-T R7 RES, SMD, 0603, 0Ω, 1/10W, TF, ROHS GENERIC H2511-01623-1/10W1-T R6, R9 RES, SMD, 0603, 162k, 1/10W, 1%, TF, ROHS GENERIC H2511-02551-1/10W1-T R5, R10 RES, SMD, 0603, 2.55k, 1/10W, 1%, TF, ROHS GENERIC H2511-DNP R8 RES, SMD, 0603, DNP-PLACE HOLDER, ROHS H2515-0R004-1W1-T R1-R4 RES, SMD, 2512, 0.004Ω, 1W, 1%, TF, ROHS GENERIC C9 100pF R9 C3 10nF 162kΩ 2.55kΩ S1 +5V 4 R10 TERMINAL LUGS R1 4mΩ R2 4mΩ R3 4mΩ 5 C4 1nF R4 4mΩ U3 5 Ld SOT23 2 R5 0Ω 1 ISL28134 VOUT R7 3 +5V U1 1 2 3 4 C8 100pF +2.5V 7 162kΩ 6 R6 5 0.01µF R8 DNP C6 +5V TERMINAL 3 C10 4.7µF U2 DNP ISL21070 2.5V VREF SOT-23 C11 10nF GND TERMINAL 2 0.1µF C2 1 10nF C1 8 ISL21090 2.5V VREF SOIC-8 C7 OPEN 2.55kΩ 10nF C5 FIGURE 2. CURRENT SENSE AMPLIFIER SCHEMATIC 2 AN1777.0 September 12, 2012 Application Note 1777 FIGURE 3. CURRENT SENSE AMPLIFIER ASSEMBLY DRAWING FIGURE 4. PCB TOP LAYER FIGURE 5. PCB BOTTOM LAYER Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 3 AN1777.0 September 12, 2012