APPLICATION NOTE 105: Current Sense Circuit Collection Bidirectional Bidirectional current sensing monitors current flow in both directions through a sense resistor. Practical H-Bridge Current Monitor Offers Fault Detection and Bidirectional Load Information To see other chapters in this Application Note, return to the Introduction. – BATTERY BUS DIFF OUTPUT TO ADC + Bidirectional Current Sensing with Single Ended Output ROUT LTC6101 RIN RIN RS RS LTC6101 + VS B ROUT FOR IM RANGE = ±100A, DIFF OUT = ±2.5V A B RS = 1mΩ RIN = 200Ω ROUT = 4.99k A LOAD IM RS 0.1 100Ω 100Ω 100Ω I DN374 F04 100Ω 4 3 5 5 – 3 4 – LTC6101 LTC6101 + + 2 1 2.5V REF 2.5k 1 2 5V + LT1490 2.5V TO 5V (CONNECTION A) 2.5V TO 0V (CONNECTION B) 0A TO 1A IN EITHER DIRECTION VOUT – 2.5k Two LTC6101’s are used to monitor the current in a load in either direction. Using a separate rail-to-rail op amp to combine the two outputs provides a single ended output. With zero current flowing the output sits at the reference potential, one-half the supply voltage for maximum output swing or 2.5V as shown. With power supplied to the load through connection A the output will move positive between 2.5V and Vcc. With connection B the output moves down between 2.5V and 0V. This circuit implements a differential load measurement for an ADC using twin unidirectional sense measurements. Each LTC6101 performs high side sensing that rapidly responds to fault conditions, including load shorts and MOSFET failures. Hardware local to the switch module (not shown in the diagram) can provide the protection logic and furnish a status flag to the control system. The two LTC6101 outputs taken differentially produce a bidirectional load measurement for the control servo. The ground-referenced signals are compatible with most ∆ΣADCs. The ∆ΣADC circuit also provides a “free” integration function that removes PWM content from the measurement. This scheme also eliminates the need for analog-to-digital conversions at the rate needed to support switch protection, thus reducing cost and complexity. Bidirectional-1 APPLICATION NOTE 105: Current Sense Circuit Collection Conventional H-Bridge Current Monitor BATTERY BUS Single Supply 2.5V Bidirectional Operation with External Voltage Reference and I/V Converter + 1 RS DIFF AMP IM 3 LT1787 ROUT 4 2.5V + VSENSE(MAX) VS+ 7 2.5V VBIAS 6 DNC – C1 1µF 8 FIL+ FIL– – 2 VS + ISENSE RSENSE TO CHARGER/ LOAD VEE C3 1000pF 5 VOUT – VOUT A A1 + 2.5V 1M 5% DN374 F03 The LT1787’s output is buffered by an LT1495 rail-to-rail op-amp configured as an I/V converter. This configuration is ideal for monitoring very low voltage supplies. The LT1787’s VOUT pin is held equal to the reference voltage appearing at the op amp’s non-inverting input. This allows one to monitor supply voltages as low as 2.5V. The op-amp’s output may swing from ground to its positive supply voltage. The low impedance output of the op amp may drive following circuitry more effectively than the high output impedance of the LT1787. The I/V converter configuration also works well with split supply voltages. Battery Current Monitor IL CHARGE RSENSE 0.1Ω DISCHARGE A2 1/2 LT1495 + A common monitoring approach in these systems is to amplify the voltage on a “flying” sense resistor, as shown. Unfortunately, several potentially hazardous fault scenarios go undetected, such as a simple short to ground at a motor terminal. Another complication is the noise introduced by the PWM activity. While the PWM noise may be filtered for purposes of the servo law, information useful for protection becomes obscured. The best solution is to simply provide two circuits that individually protect each half-bridge and report the bidirectional load current. In some cases, a smart MOSFET bridge driver may already include sense resistors and offer the protection features needed. In these situations, the best solution is the one that derives the load information with the least additional circuitry. LT1389-1.25 1787 F07 – Many of the newer electric drive functions, such as steering assist, are bidirectional in nature. These functions are generally driven by H-bridge MOSFET arrays using pulsewidth-modulation (PWM) methods to vary the commanded torque. In these systems, there are two main purposes for current monitoring. One is to monitor the current in the load, to track its performance against the desired command (i.e., closed-loop servo law), and another is for fault detection and protection features. LT1495 RA RA RA 2N3904 DISCHARGE OUT RB 12V 5V RA – A1 1/2 LT1495 + 2N3904 CHARGE OUT VO = IL () RB RSENSE RA RB FOR RA = 1k, RB = 10k VO = 1V/A IL 1495 TA05 One LT1495 dual op-amp package can be used to establish separate charge and discharge current monitoring outputs. The LT1495 features Over-the-Top operation allowing the battery potential to be as high as 36V with only a 5V amplifier supply voltage. Bidirectional-2 APPLICATION NOTE 105: Current Sense Circuit Collection The LT1995 is shown as a simple unity gain difference amplifier. When biased with split supplies the input current can flow in either direction providing an output voltage of 100mV per Amp from the voltage across the 100mΩ sense resistor. With 32MHz of bandwidth and 1000V/usec slew rate the response of this sense amplifier is fast. Adding a simple comparator with a built in reference voltage circuit such as the LT6700-3 can be used to generate an over-current flag. With the 400mV reference the flag occurs at 4A. Fast Current Sense with Alarm Bidirectional Current Sense with Separate Charge/Discharge Output IDISCHARGE ICHARGE RSENSE CHARGER RIN C 100 RIN D 100 RIN C 100 RIN D 100 4 L O A D + – 2 3 3 5 5 1 LTC6101 1 + ROUT D 4.99k + VOUT D VOUT C – – 4 – + VBATT 2 LTC6101 ROUT C 4.99k 6101 TA02 DISCHARGING: VOUT D = IDISCHARGE • RSENSE CHARGING: VOUT C = ICHARGE • RSENSE ( ( ) ROUT D WHEN IDISCHARGE ≥ 0 RIN D ) ROUT C WHEN ICHARGE ≥ 0 RIN C In this circuit the outputs are enabled by the direction of current flow. The battery current when either charging or discharging enables only one of the outputs. For example when charging, the VOUT D signal goes low since the output MOSFET of that LTC6101 turns completely off while the other LT6101, VOUT C, ramps from low to high in proportion to the charging current. The active output reverses when the charger is removed and the battery discharges into the load. Bidirectional-3 APPLICATION NOTE 105: Current Sense Circuit Collection Bidirectional Absolute Value Current Sense IDISCHARGE ICHARGE RSENSE CHARGER RIN C RIN D RIN C RIN D 4 L O A D + – 2 3 3 5 5 1 LTC6101 4 – + 1 LTC6101 + VOUT VBATT 2 ROUT – 6101 TA05 DISCHARGING: VOUT = IDISCHARGE • RSENSE CHARGING: VOUT = ICHARGE • RSENSE ( ( ) ROUT WHEN IDISCHARGE ≥ 0 RIN D ) ROUT WHEN ICHARGE ≥ 0 RIN C value of the magnitude of the current into or out of the battery. The direction or polarity of the current flow is not discriminated. The high impedance current source outputs of two LTC6101’s can be directly tied together. In this circuit the voltage at VOUT continuously represents the absolute Full-Bridge Load Current Monitor +VSOURCE 5V LT1990 900k 10k 8 7 – + 2 1M 3 1M 100k – RS 6 VOUT + VREF = 1.5V IL 4 OUT IN LT6650 GND FB –12V ≤ VCM ≤ 73V VOUT = VREF ± (10 • IL • RS) 10k 1nF 54.9k 40k 5 900k 40k 100k 20k 1 1990 TA01 1µF The LT1990 is a difference amplifier that features a very wide common mode input voltage range that can far exceed its own supply voltage. This is an advantage to reject transient voltages when used to monitor the current in a full bridge driven inductive load such as a motor. The LT6650 provides a voltage reference of 1.5V to bias up Bidirectional-4 the output away from ground. The output will move above or below 1.5V as a function of which direction the current in the load is flowing. As shown, the amplifier provides a gain of 10 to the voltage developed across resistor RS. APPLICATION NOTE 105: Current Sense Circuit Collection Low Power, Bidirectional 60V Precision Hi Side Current Sense Using a very precise zero-drift amplifier as a pre-amp allows for the use of a very small sense resistor in a high voltage supply line. A floating power supply regulates the voltage across the pre-amplifier on any voltage rail up to the 60V limit of the LT1787HV circuit. Overall gain of this circuit is 1000. A 1mA change in current in either direction through the 10mΩ sense resistor will produce a 10mV change in the output voltage. Split or Single Supply Operation, Bidirectional Output into A/D 1Ω 1% IS = ±125mA VSRCE ≈4.75V 1 – 2 VS 3 DNC VEE 4 VEE –5V VCC 5V 8 FIL+ FIL– LT1787 VS+ 7 10µF 16V VBIAS 6 20k VOUT 5 1 CONV VOUT (±1V) 2 OPTIONAL SINGLE SUPPLY OPERATION: DISCONNECT VBIAS FROM GROUND AND CONNECT IT TO VREF. REPLACE –5V SUPPLY WITH GROUND. OUTPUT CODE FOR ZERO CURRENT WILL BE ~2430 In this circuit, split supply operation is used on both the LT1787 and LT1404 to provide a symmetric bidirectional measurement. In the single-supply case, where the 7 6 AIN LTC1404 CLK VREF 5 DOUT GND 10µF 16V 4 8 CLOCKING CIRCUITRY 3 10µF 16V VEE –5V DOUT 1787 TA02 LT1787 pin 6 is driven by VREF, the bidirectional measurement range is slightly asymmetric due to VREF being somewhat greater than mid-span of the ADC input range. Bidirectional-5