L DESIGN FEATURES Versatile Current Sense Amplifiers Offer Rail-to-Rail Input, 150°C Operating Temperature by William Jett and Glen Brisebois Introduction 24 3V TO 36V RSENSE 0.02Ω RIN 100Ω +IN –IN + LOAD – V– V+ Q1 OUT LT6106 ROUT 1k VOUT 200mV/A Figure 1. Typical current sense application using the LT6106. Sense resistor is 20mΩ and gain is 10, so transfer function is VOUT/ILOAD = 0.2V/A. SOURCE –0.3V TO 44V RIN2 VS+ 100Ω 0.02Ω RIN1 100Ω VS– TO LOAD LT6105 +IN + –IN VOUT VOUT = 1V/A – V+ ROUT 4.99k V– ( ) VOUT = VS + − VS − • R OUT RIN RIN 1 = RIN 2 = RIN R OUT RIN AV = 2.85V TO 36V Figure 2. Typical LT6105 connection. Sense resistor is 20mΩ and gain is 50, so the transfer function is VOUT/ILOAD = 1V/A. Note the wide input source. 250µV and the power supply rejection of 106dB makes the accuracy almost independent of supply. The LT6107 is functionally identical to the LT6106, but provides guaranteed performance and specifications at junction temperatures up to 150°C. This suits it to under hood automotive and some industrial applications that can exceed 125°C for relatively short periods of time. Input offset voltage is less than 400µV over the entire temperature range of –40°C to 150°C. As with the LT6106, two external resistors set the amplifier gain. 0.6 0.4 ACCURACY (% OF FULL SCALE) Fast and accurate current measurement is required in an increasing number of electronics subsystems. The list of applications that call for current sensing includes diagnostic system assessment, fault detection, load protection and scaling, battery “gas gauge” monitoring, and impending component failure detection, to name just a few. The challenge is that there is no one-size-fits-all solution for current measurement. For example, protection circuits often emphasize measurement speed, while battery applications usually emphasize accuracy and low power. Nevertheless, design time can be reduced by using an accurate current measurement amplifier with features suited to the task at hand, such as the latest members of Linear Technology’s current sense amplifier family, the LT6105, LT6106 and LT6107. The LT6105 is distinguished by its rail-to-rail inputs. It is a perfect fit for automatic test equipment (ATE) systems and other systems that use a combination of fixed voltage supplies and programmable or switched supplies. The LT6105 features an input voltage range of –0.1V to 44V that is independent of the power supply voltage. The supply voltage for the LT6105 can be obtained from any convenient source within the supply range of 2.85V to 36V. For instance, a programmable supply with an output of 0V to 24V can be monitored while the LT6105 is powered by a fixed 5V supply. The LT6106 is distinguished by ease-of-use and accuracy in single supply environments. Just add two resistors and tie the supply to the sense resistor, and the device is configured to measure currents in supplies from 2.7V to 36V. Input offset voltage is a low LIMIT OVER TEMPERATURE 0.2 0 TYPICAL PART AT TA = 25°C –0.2 –0.4 –0.6 LIMIT OVER TEMPERATURE –0.8 5A FULL SCALE RIN = 100Ω –1.0 RSENSE = 0.02Ω ROUT = 1k AV = 10 V+ = 3V –1.2 0 1 3 2 LOAD CURRENT (A) 4 5 Figure 3. Measurement accuracy vs load current for the LT6106 and LT1607 Linear Technology Magazine • March 2008 DESIGN FEATURES L Table 1 summarizes the guaranteed performance of the LT6105, LT6106 and the LT6107. LT6106, LT6107 Theory of Operation LT6105 25°C LT6106 25°C LT6107 –40°C to 150°C Input Voltage Range –0.3V to 44V 2.7V to 36V 2.7V to 36V Supply Voltage Range 2.85V to 36V 2.7V to 36V 2.7V to 36V VOS 300µV/1000µV* 250µV 400µV Maximum Differential Input Voltage 44V 0.5V 0.5V CMRR 95dB N/A N/A PSRR 100dB 106dB 106dB Gain Error ±1%/±2.5%* –0.65% to 0% –0.65% to 0% Output Current 1mA 1mA 1mA Supply Current 340µA 95µA 125µA Package MS8, DFN TSOT-5 TSOT-5 * Input common mode voltage = 12V, 0V Referring to Figure 1, the current to be measured, ILOAD, passes through a sense resistor RSENSE, resulting in a voltage drop of VSENSE. Feedback from the amplifier causes a current to flow in RIN and Q1 such that the amplifier inputs are equal, V–IN = V+IN. The current in Q1 also flows through ROUT. The output voltage is therefore proportional to the load current and is given by VOUT = ILOAD • R SENSE TO LOAD RSENSE VS– LOAD The LT6105: Robust and Easy to Use The LT6105 tolerates negative voltages on its inputs of up to –9.5V. In addition, it can also be used to sense across fuses or MOSFETs as shown in Figure 4. The LT6105 has no problem when the fuse or MOSFET opens because it has high voltage PNP’s and a unique input topology that features full high impedance differential input swing capability to ±44V. This allows Linear Technology Magazine • March 2008 C1 0.1MF DC SOURCE (≤44V) +IN –IN + VS+ RIN2 RIN1 V+ C2 0.1MF 5V V– OUT R • OUT RIN The overall accuracy graph shown in Figure 3 combines the effects of gain error and input offset voltage to create a worst-case error band for the application circuit in Figure 1. A slight negative gain error, typically –0.25%, is due to the finite current gain of the PNP. FUSE – The LT6105, LT6106 and LT6107 use traditional external gain-setting resistors. This is actually an important feature in a current sense amplifier. Most current sense applications require a very small maximum sense shunt voltage (to minimize power loss), which must be amplified to match either a very specific comparator threshold or ADC input voltage span. The ability to carefully control the gain is paramount to optimize performance. Figures 1 and 2 show typical applications of the LT6106 and LT6105. Parameter + Flexible Gain Setting Table 1. Guaranteed performance specifications OUTPUT LT6105 ROUT Figure 4. The LT6105 can monitor across a fuse or switch. It’s inputs are undamaged even when split wide apart, and current is limited to about 3mA. V+ Q2 Q1 RIN2 VS+ VSENSE Q3 Q6 Q7 +IN VOUT RSENSE 0.02Ω + RIN1 VS– –IN – ROUT Q4 Q5 ILOAD Figure 5. Block diagram of the LT6105 25 L DESIGN FEATURES 24VDC 1N5818 – 0V/OFF VBAT = 3.6V ICPO = 200µA 5V/DIV CCPO = 2.2ΩF 200Ω 1% LT6105 –IN 5V/ON + 200Ω 1% 24V, 3W SOLENOID 1Ω 1% +IN 2N7000 10V/DIV 5VDC V+ 2V/DIV V– VOUT VOUT = 25mV/mA 4.99k 1% Figure 6. Simplest form of solenoid driver. The LT6105 monitors the current in both on and freewheel states. Lowest common mode voltage is 0V, while the highest is 24V plus the forward voltage of the Schottky diode. See waveforms, Figure 7. direct sensing of fuse or MOSFET voltage drops, without concern for an open circuit condition in the fuse or MOSFET. Another benefit of the LT6105 is that you can leave it connected to a battery even when it is unpowered. When the LT6105 loses power, or is intentionally powered down, both sense inputs remain high impedance. In fact, when powered down, the LT6105 inputs actually draw less current than when powered up. Powered up or down, it represents a benign load. The LT6105 extends the current sense measurement concept used in the LT6106 (and others) to accommodate an input voltage range that includes ground. For both the LT6105 and LT6106, the voltage developed across a sense resistor is translated into a current that appears in the output pin. The voltage gain is set by the ratio of the input and output resistors, (ROUT/RIN). The wide input range in the LT6105 is obtained by the use of two feedback paths to the input pins. 24VDC 1Ω 1% 24V/OFF 200Ω 1% – 1N914 24V, 3W SOLENOID 2k 1% –IN 5VDC TP0610L + 200Ω 1% 19V/ON 2k 1% LT6105 1N5818 Referring to Figure 5, when the input voltage VS+ is between 0V and 1.6V, devices Q1, Q2, and Q3 are active and devices Q4–Q7 are off. Feedback from the amplifier causes the current to flow in Q1, which equalizes the amplifier input voltages. Devices Q1 and Q3 are matched, so the collector current of Q3 will equal the collector current of Q1. The output voltage is then VOUT = ILOAD • R SENSE • ROUT RIN1 When the input voltage VS+ is greater than 1.6V, devices Q4, Q5, Q6, and Q7 are active and devices Q1–Q3 are off. Again, feedback from the amplifier causes the current to flow in Q4 which equalizes the amplifier input voltages. The current in Q4 is mirrored to the output through the matching of Q4 to Q5 and Q6 to Q7. The output voltage for this mode is given by ROUT RIN2 LT6105 Application: Solenoid Current Monitor VOUT VOUT = 25mV/mA 4.99k 1% Figure 8. Similar circuit to Figure 6 but with solenoid grounded, so freewheeling forces inputs negative. Providing resistive pullups keeps amplifier inputs from falling outside of their accurate input range. 26 Figure 7. Waveforms for solenoid driver. Top trace is the MOSFET gate, with high on. Middle trace is the bottom of the solenoid/ inductor. Bottom trace is the LT6105 output, representing solenoid current at 80mA/DIV. Glitches are useful indicators of solenoid plunger movement. VOUT = ILOAD • R SENSE • +IN V+ V– 50ms/DIV The large input common mode range of the LT6105 makes it suitable for monitoring currents in quarter, half, and full bridge inductive load driving applications. Figure 6 shows an example of a quarter bridge. The MOSFET pulls down on the bottom of the solecontinued on page 38 Linear Technology Magazine • March 2008 L NEW DEVICE CAMEOS (UV) and overvoltage (OV) conditions. It can monitor two supplies for UV or OV, or a single supply for UV and OV simultaneously. The adjustable trip thresholds are set with external resistive divider networks, giving users complete control over the trip voltage. An open-drain RST output is held low when any adjust supply is invalid or VCC is in undervoltage. When all the inputs are valid, the RST pin is released after a timeout delay, which can be set to 200ms, adjusted with an external capacitor, or configured for no-delay. When compared to the LTC2909, the LTC2919 provides two additional independent output pins to indicate the status of each adjustable input. When connected to the enable pins of power supplies these outputs can be used to implement start-up sequencing. In addition to providing a highly versatile, precise solution for supply monitoring, the low quiescent current of 50µA and the tiny DFN package makes the LTC2919 an ideal choice in space limited applications. With the addition of a single external current limiting resistor, the LTC2919’s onboard 6.5V shunt regulator permits operation from a high voltage supply. The IC is offered in 10-pin plastic MSOP and 3mm × 2mm DFN packages and is specified over the C, I, and H temperature ranges. L LT6105/6/7, continued from page 26 constraints of the absolute maximum ratings, is that the negative supply to the LT6105 be at least as negative as the supply it is monitoring. flexibility of external gain setting. The –0.1V to 44V input range of the LT6105 enables the current in switched supplies to be monitored from initial turn-on/turn-off to the steady state value. The LT6106 provides a simple but accurate solution for systems with a single supply. The LT6107 extends the temperature range of current measurements to 150°C. L noid to increase the solenoid current. It lets go to decrease current, and the solenoid voltage freewheels around the Schottky diode. Current measurement waveforms are shown in Figure 7. The small glitches occur due to the action of the solenoid plunger, and this provides an opportunity for mechanical system monitoring without an independent sensor or limit switch. Figure 8 shows another solenoid driver circuit, a high side drive approach with one end of the solenoid grounded and a P-Channel MOSFET pulling up on the other end. In this case, the inductor freewheels around ground, imposing a negative input common mode voltage of one Schottky diode drop. This voltage may exceed the input range of the LT6105. This does not endanger the device, but it degrades the accuracy. In order to avoid exceeding the input range, pullup resistors may be used as shown. Conclusion The LT6105, LT6106, and LT6107 provide simple, flexible solutions to high side (and low side) current sensing. Common to all the parts is the CURRENT FLOW +15V POSITIVE SUPPLY +IN 38 5VDC V+ –15V V– – 20mΩ 1% LT6105 100Ω 1% TO +15V LOAD –IN VOUT VOUT = 1V/A 4.99k 1% LT6105 LT6105 Application: Supply Monitor The input common mode range of the LT6105 also makes it suitable for monitoring either positive or negative supplies. Figure 9 shows one LT6105 applied as a simple positive supply monitor, and another LT6105 as a simple negative supply monitor. Note that the schematics are practically identical, and both have outputs conveniently referred to ground. The only requirement for negative supply monitoring, in addition to the usual + 100Ω 1% –15V V– 5VDC V+ –15V NEGATIVE SUPPLY VOUT = 1V/A 4.99k 1% –IN 100Ω 1% VOUT +IN – 20mΩ 1% + 100Ω 1% TO –15V LOAD CURRENT FLOW Figure 9. The LT6105 can monitor the current of either positive or negative supplies, without a schematic change. Just ensure that the current flow is in the correct direction. Linear Technology Magazine • March 2008