L DESIGN FEATURES Precise Current Sense Amplifiers Operate from 4V to 60V Introduction The LTC6103 and LTC6104 are versatile, precise high side current sense amplifiers with a wide operation range. The LTC6103 is a dual current sense amplifier, while the LTC6104 is a single, bi-directional current sense amplifier—it can source or sink an output current that is proportional to a bi-directional sense voltage. Due to the amplifiers’ wide supply range (60V), fast speed (1µs response time), low offset voltage (85µV typical), low supply current (275µA/channel typical) and user-configurable gains, they can be used in precision industrial and automotive sensing applications, as well as current-overload protection circuits. Other features include high PSRR, low input bias current and wide input sense voltage range. Both parts are available in an 8-lead MSOP. VBATT_A VBATT_B VSENSE ILOAD – VSENSE + + RSENSE LOAD 6 –INA IOUT = 5k 5k +INB 5k 5k + – ISB – + VSA VSB 10V 10V V– OUTA 1 OUTB 4 2 IOUT IOUT ROUT VOUT = VSENSE • ROUT ROUT RIN Figure 1. The LTC6103 block diagram and typical connection – RIN 7 +INA 6 VS –INA 10V 5k 5 –INB 5k – 5k + – V+ A +INB 10V 5k + V– ILOAD + RSENSE RIN 8 VSENSE V+ B V– (ILOAD + IS ) RSENSE RIN flows through RIN. The high impedance inputs of the sense amplifier do not conduct this input current, so the current flows through an internal MOSFET to the OUT pin. In most application cases, IS << ILOAD, so IOUT 5 –INB LTC6103 Theory of Operation Figure 1 shows a block diagram of the LTC6103 in a basic current sense circuit. A sense resistor, RSENSE, is added in the load path, thereby creating a small voltage drop proportional to the load current. An internal sense amplifier loop forces –IN to have the same potential as +IN. Connecting an external resistor, RIN, between –IN and VBATT forces a potential across RIN that is the same as the sense voltage across RSENSE. A corresponding current LOAD RIN 7 +INA ISA ILOAD – RSENSE RIN 8 by Jun He I •R ≈ LOAD SENSE RIN 10V V– OUT 1 4 VOUT R VOUT = VSENSE • OUT + VREF RIN ROUT + – VREF Figure 2. The LTC6104 block diagram and typical connection Linear Technology Magazine • December 2006 DESIGN FEATURES L The output current can be transformed into a voltage by adding a resistor from OUT to V–. The output voltage is then Sources of Current Sensing Error As the output voltage is defined by ILOAD • R SENSE • ROUT RIN VOUT = (V–) + (IOUT • ROUT) VOUT = LTC6104 Theory of Operation any error of the external resistors contributes to the ultimate output error. If current flowing through the sense resistor is high, Kelvin connection of the –IN and +IN inputs to the sense resistor is necessary to avoid error introduced by interconnection and trace resistance on the PCB. Besides external resistors, the dominant error source is the offset voltage of the sense amplifier. Since this is a level independent error, Figure 2 shows a block diagram of the LTC6104 in a basic current sense circuit. Similar to the operation of the LTC6103, the LTC6104 can transfer a high side current signal into a ground-referenced readout signal. The difference is that the LTC6104 can sense the input signal in both polarities. Only one amplifier is active at a time in the LTC6104. If the current direction activates the “B” amplifier, the “A” amplifier is inactive. The signal current goes into the –INB pin, through the MOSFET, and then into a current mirror. The mirror reverses the polarity of the signal so that current flows into the “OUT” pin, causing the output voltage to change polarity. The magnitude of the output is VOUT = 10µF 63V VLOGIC 3 FAULT RS LT1910 1µF 1 –IN V– 5 LOAD 10k VOUT VLOGIC (3.3V TO 5V) 7 RSENSE(LO) 100mΩ RSENSE(HI) 10mΩ 3 1 VS 6 + – LTC1540 8 Q1 CMPT5551 4.7k 6 1.74M 5 HIGH RANGE INDICATOR (ILOAD > 1.2A) 619k HIGH CURRENT RANGE OUT 250mV/A 4 7.5k BAT54C VLOGIC V– R5 7.5k ROUT 5 301Ω 7 2 4 VIN 40.2k 301Ω + – OUT FOR RS = 5mΩ, VO = 2.5V AT IL = 10A (FULL SCALE) CMPZ4697 LTC6103 LTC6103 VO = 49.9 • RS • IL Figure 4. Automotive smart-switch with current readout 8 IS IL 6103 TA06 ILOAD +IN VO 4.99k M1 Si4465 RIN OUT SUB85N06-5 VBATT RT 1/2 LTC6103 +IN 6 2 RSENSE RT 100Ω 1% –IN 8 4 OFF ON Keep in mind that the OUT voltage cannot swing below V–, even though it is sinking current. A proper VREF and ROUT need to be chosen so that the designed OUT voltage swing does not go beyond the specified voltage range of the output. LOAD 14V 47k VSENSE • ROUT + VREF RIN ILOAD maximizing the input sense voltage improves the dynamic range of the system. If practical, the offset voltage error can also be calibrated out. Care should be taken when designing the printed circuit board layout. As shown in Figure 3, supply current flows through the +IN pin, which is also the positive amplifier input pin (for the LTC6104, this applies to the +INB pin only). The supply current can cause an equivalent additional input offset voltage if trace resistance between RSENSE and +IN is significant. Trace resistance to the -IN terminals is added to the value of RIN. In addition, the internal device resistance adds approximately 0.3Ω to RIN. (VLOGIC + 5V) ≤ VIN ≤ 60V 0A ≤ ILOAD ≤ 10A 6103 F03b LOW CURRENT RANGE OUT 250mV/A 6103 F04 Figure 3. Error Due to PCB trace resistance Linear Technology Magazine • December 2006 Figure 5. The LTC6103 allows high-low current ranging L DESIGN FEATURES ICHARGE The LTC6103 supplies a current output, rather than a voltage output, in proportion to the sense resistor voltage drop. The load resistor for the LTC6103 may be located at the far end of an arbitrary length connection, thereby preserving accuracy even in the presence of ground-loop voltages. 0.01Ω CHARGER IDISCHARGE 249Ω 8 7 +INA ILOAD 249Ω 6 –INA 5 –INB +INB + – LOAD VS LTC6104 VOUT 2.5V±2V (±10A FS) – + A B CURRENT MIRROR OUT 1 + VS High-Low Range Current Measurement Figure 5 shows LTC6103 used in a multi-range configuration where a low current circuit is added to a high current circuit. A comparator (LTC1540) is used to select the range, and transistor M1 limits the voltage across RSENSE(LO). V– 4 2.5V 6 4.99k LT1790-2.5 1µF 1 2 4 3V TO 18V 1µF Figure 6. The LTC6104 bi-direction current sense circuit with combined charge/discharge output Applications The LTC6103 and LTC6104 operate from 4V to 60V, with a maximum supply voltage of 70V. This allows them to be used in applications that require high operating voltages, such as motor control and telecom supply monitoring, or where it must survive in the face of high-voltages, such as with automotive load dump conditions. The accuracy is preserved across this supply range by a high PSRR of 120dB (typical). Fast response time makes the LTC6103 and LTC6104 the perfect choice for load current warnings and shutoff protection control. With very low supply current, they are suitable for power sensitive applications. The gain of the LTC6103 and LTC6104 is completely controlled by external resistors, making them flexible enough to fit a wide variety of applications. Monitor the Current of Automotive Load Switches With its 60V input rating, the LTC6103 is ideally suited for directly monitoring currents on automotive power systems without need for additional supply conditioning or surge protection components. Figure 4 shows an LT1910-based intelligent automotive high side switch with an LTC6103 providing an analog current indication. The LT1910 high Battery Charge/Discharge Current Monitor Figure 6 shows the LTC6104 used in monitoring the charge and discharge current of a battery. The voltage reference LT1790 provides a 2.5V offset so that the output can swing above side switch controls an N-channel MOSFET that drives a controlled load and uses a sense resistor to provide overload detection. The sense resistor is shared by the LT6103 to provide the current measurement. continued on page 28 VBATTERY (6V–60V) + VSENSE(A) – 10mΩ 10mΩ 200Ω 8 7 +INA LTC6104 6 –INA 5 –INB +INB – + A B CURRENT MIRROR OUT VS V– 1 4 VOUT ±2.5V (±10A FS) VEE (–5V) 4.99k M VSENSE(B) – 200Ω + – VS + DC MOTOR OR PELTIER DEVICE P –+ ILOAD P M Figure 7. Current monitoring for an H-bridge application Linear Technology Magazine • December 2006 L DESIGN IDEAS V1 rises above 10.8V. The transition from the V2 to V1 is accomplished by slowly (10ms) turning off Q2 and Q3 allowing the Q1 to turn on rapidly when VS matches V1. The H1 output is open until the E1 input drops below the VREF voltage level. The V1 VFAIL is determined by: R2A + R2C R2C 158k + 24.9k = 1.222V • 24.9kk = 8.98 V VFAIL = VETH • input drops to 12V and the V2 path is enabled. Finally, the load will be removed from the input supply when the voltage drops below 5V. Undervoltage R1A + R1C R1C 75k + 24.3k = 1.222V • 24.3k = 4.99 V VFAIL = VETH • VRESTORE = VETH • VRESTORE = VETH (R2A + (R2C R2E)) • = 1.222V • R2C R2E ( 1558k + 24.9k 105k = 1.222V • ) 24.9k 105k = 10.81V Undervoltage and Overvoltage Shutdown Figure 2 shows an application that disables the power to the load when the input voltage gets too low or too high. When VIN starts from zero volts, the load to the output is disabled until VIN reaches 5.5V. The V1 path is enabled and the load remains on the input until the supply exceeds 13.5V. At that voltage, the V2 path is disabled. As the input falls, the voltage source is reconnected to the load when the LTC6103/LTC6104, continued from page and below this point. Make sure that the lowest expected output level is higher than pin 4 (V–) by at least 0.3V to ensure that negative going output swings remain linear. H-Bridge Load Current Monitor The H-bridge power-transistor topology remains popular as a means of driving motors and other loads bi-directionally from a single supply potential. In most cases, monitoring the current delivered to the load allows for real-time operational feedback to a control system. 28 Conclusion (R1A + (R1C R1D)) Determine V1 VRESTORE by: R1C R1D ( 755k + 24.3k 182k lockout by using only one of the voltage paths and eliminating the components from the other. Only one PFET is required in this case. The LTC4416-1 should be used in this configuration rather than the LTC4416 because the LTC4416-1 turns off rapidly if an over or undervoltage condition is detected. ) 24.3k 182k = 5.497 V Overvoltage R2A + R2C || R2E R2C || R2E 221k + 24.9k || 187k = 1.222V • 24.9k || 187k VFAIL = VETH • = 13.51V R2A + R2C R2C 221k + 24.9k = 1.222V • 244.9k = 12.07 V VRESTORE = VETH • The over and undervoltage lockout circuits are shown here working in tandem. It is possible to configure the circuit for either over or undervoltage Figure 7 shows the LTC6104 used in monitoring the load current in an H-bridge. In this case, the LTC6104 operates with dual supplies. The output resistance is connected directly to ground instead of connected to a voltage reference. The output ranges from 0V to 2.5V for VSENSE_A = 0mV to 100mV, and from 0V to –2.5V for VSENSE_B = 0mV to 100mV. Conclusion The LTC6103 and LTC6104 are precise high side current sensing solutions. The parts can operate to 60V, making The LTC4416 provides power supply switchover solutions that cannot be easily generated using off-theshelf components. The LTC4416 also provides power efficiencies not available with traditional NFET Hot Swap controllers. These efficiencies reduce the IDD of the solution by not having active switching gate drivers. The power losses are also reduced by decreasing the voltage drop across the PFETs to 25mV. The LTC4416 provides a smoother transition between the backup and the secondary power supplies. The LTC4416-1 dual gate drivers provide a single controller solution to not only protect loads from overvoltage conditions, but also undervoltage conditions. The user can externally program the overvoltage and undervoltage thresholds using simple external resistor networks. These resistor networks also provide hysterisis to prevent chattering between the power source and the load. L them ideal for high voltage applications such as those found in automotive, industrial and telecom systems. Low DC offset allows the use of a small shunt resistor and large gain-setting resistors. The fast response time makes them suitable for overcurrentprotection circuits. Configurable gain means design flexibility. In addition, the open-drain output architecture provides an advantage for remotesensing applications. L Authors can be contacted at (408) 432-1900 Linear Technology Magazine • December 2006