LTC6104

FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ LTC6104 High Voltage, High Side, Bi-Directional Current Sense Amplifier DESCRIPTION The LTC®6104 is a versatile, high voltage, high side, bidirectional current sense amplifier. Design flexibility is provided by the excellent device characteristics: ±450µV maximum offset and only 520µA of current consumption (typical at 12V). The LTC6104 operates on supplies from 4V to 60V. The LTC6104 monitors bi-directional current via the voltage across an external sense resistor (shunt resistor). This sense voltage is then translated into a ground referenced signal. Gain is set with three external resistors and can be separately configured for both directions. Low DC offset allows the use of a small shunt resistor and large gain-setting resistors. As a result, power loss in the shunt is minimal. The wide operating supply range and high accuracy make the LTC6104 ideal for a wide variety of automotive, industrial and power management applications. A maximum input sense voltage of 500mV allows a wide range of currents to be monitored. The fast response makes the LTC6104 the perfect choice for load current warnings and shutoff protection control. With very low supply current, the LTC6104 is suitable for power sensitive applications. The LTC6104 is available in an 8-lead MSOP package. Wide Supply Range: 4V to 60V with 70V Absolute Maximum Low Offset Voltage: ±450µV Maximum Fast Response: 1µs Response Time Gain Configurable with External Resistors; Each Direction is Gain Configurable Low Input Bias Current: 170nA Maximum PSRR: 110dB Minimum Output Current: ±1mA Maximum Low Supply Current: 520µA, VS = 12V Specified for –40°C to 125°C Temperature Range Available in an 8-Lead MSOP Package APPLICATIONS ■ ■ ■ ■ Current Shunt Measurement Battery Monitoring Remote Sensing Power Management , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION 16-Bit Resolution Bi-Directional Output into LTC1286 ADC ILOAD TO CHARGER/LOAD – VSENSE + + 12V RSENSE RIN RIN 100Ω 100Ω Step Response VSENSE– ∆VSENSE = 100mV 8 +INA 7 –INA 6 –INB 5 +INB 6V +– A VS –+ B VS VREF CURRENT MIRROR VREF +IN VCC CS LTC1286 C2 0.1µF –IN LT®1004-2.5 GND 6104 TA01a R1 2.3k C1 1µF + 5V VOUT 1.5V 1V TO µP IOUT = 100µA IOUT = 0µA TIME (1µs/DIV) 6104 G15 LTC6104 OUT 1 V– 4 CLK DOUT TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 1V ROUT 2.5k 6104f 1 LTC6104 ABSOLUTE MAXIMUM RATINGS Total Supply Voltage (+INB(VS) to V–) ......................70V Maximum Applied Output Voltage (OUT) ....................9V Input Current........................................................±10mA Output Short-Circuit Duration (to V–)............... Indefinite Operating Temperature Range LTC6104C ............................................ –40°C to 85°C LTC6104I ............................................. –40°C to 85°C LTC6104H .......................................... –40°C to 125°C Specified Temperature Range (Note 2) LTC6104C ................................................ 0°C to 70°C LTC6104I ............................................. –40°C to 85°C LTC6104H .......................................... –40°C to 125°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C (Note 1) PACKAGE/ORDER INFORMATION TOP VIEW OUT NC NC V– 1 2 3 4 8 7 6 5 +INA –INA –INB +INB/VS MS8 PACKAGE 8-LEAD PLASTIC MSOP TJMAX = 150°C, θJA = 300°C/W ORDER PART NUMBER LTC6104CMS8 LTC6104IMS8 LTC6104HMS8 MS8 PART MARKING* LTCMP LTCMP LTCMP Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. ELECTRICAL CHARACTERISTICS SYMBOL VS VOS ΔVOS/ΔT IB VSENSE(MAX) (Note 3) PSRR PARAMETER Supply Range Output Offset Voltage The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. RIN = 100Ω, ROUT = 5k, 4V ≤ +INB(VS) ≤ 60V, V– = 0V, VREF = 2V for VS ≥ 6V, VREF = 0.75V for VS = 4V, unless otherwise noted. CONDITIONS ● MIN 4 TYP ±85 MAX 60 ±450 ±600 ±700 170 245 UNITS V µV µV µV µV/°C nA nA mV mV VSENSE = ±5mV, LTC6104 VSENSE = ±5mV, LTC6104C, LTC6104I VSENSE = ±5mV, LTC6104H VSENSE = ±5mV RIN = 1M for –INA and –INB 6V ≤ VS ≤ 60V, RIN = 1k, ROUT = 2k, VREF = 2V VS = 4V, RIN = 1k, ROUT = 1k, VREF = 0.5V when VSENSE = 500mV, VREF = 1V when VSENSE = –500mV ● ● ● ● ● ● Input Offset Voltage Drift Input Bias Current Input Sense Voltage Full Scale ±1.5 100 ±500 ±500 116 112 110 105 110 105 105 100 8 3 1 0.3 0.3 0.25 140 120 133 Power Supply Rejection Ratio VS = 6V to 60V, VSENSE = 5mV VS = 6V to 60V, VSENSE = –5mV VS = 4V to 60V, VSENSE = 5mV VS = 4V to 60V, VSENSE = –5mV ● ● ● ● ● ● ● ● ● ● dB dB dB dB dB dB dB dB V V V V V V 115 VOUT(MAX) Maximum Output Voltage 12V ≤ VS ≤ 60V, VSENSE = 90mV, VREF = 4V VS = 6V, VSENSE = 75mV, VREF = 1.8V, ROUT = 2k VS = 4V, VSENSE = 35mV, VREF = 0.75V, ROUT = 1k 12V ≤ VS ≤ 60V, VSENSE = –80mV, VREF = 4V VS = 6V, VSENSE = –90mV, VREF = 1.8V, ROUT = 2k VS = 4V, VSENSE = –75mV, VREF = 0.75V, ROUT = 1k VOUT(MIN) Minimum Output Voltage 6104f 2 LTC6104 ELECTRICAL CHARACTERISTICS SYMBOL IOUT(MAX) IOUT-GAINERR IOUT-OSERR tr PARAMETER Maximum Output Current Current Mirror Gain Error Current Mirror Offset Error Input Step Response (ΔVOUT = to 50% on a 5V Output Step) The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. RIN = 100Ω, ROUT = 5k, 4V ≤ +INB(VS) ≤ 60V, V– = 0V, VREF = 2V for VS ≥ 6V, VREF = 0.75V for VS = 4V, unless otherwise noted. CONDITIONS 6V ≤ VS ≤ 60V, VSENSE = ±110mV, VREF = 2V, ROUT = 1k VS = 4V, VSENSE = ±27.5mV, VREF = 0.75V, ROUT = 1k VINB– > VINB+ and VINA– < VINA+ (Note 4) VINB– > VINB+ and VINA– < VINA+ (Note 4) 8V ≤ VS ≤ 60V, VREF = 1V, VSENSE = 0mV to 100mV Transient 8V ≤ VS ≤ 60V, VREF = 6V, VSENSE = –100mV to 0mV Transient 8V ≤ VS ≤ 60V, VREF = 4V, VSENSE = –50mV to 50mV Transient Input Step Response (ΔVOUT = to 50% on a 0.5V Output Step) VS = 4V, ROUT = 500Ω, Gain = 5, VREF = 0.5V, VSENSE = 0mV to 100mV Transient VS = 4V, ROUT = 500Ω, Gain = 5, VREF = 1V, VSENSE = –100mV to 0mV Transient VS = 4V, ROUT = 500Ω, Gain = 5, VREF = 0.75V, VSENSE = –50mV to 50mV Transient ● ● MIN ±1 ±0.25 TYP MAX UNITS mA mA ±0.2 ±0.2 1 1 3 1.2 1.2 3.2 140 140 200 200 ● ● ● ● ● ±0.75 % µA µs µs µs µs µs µs kHz kHz kHz kHz BW Signal Bandwidth IOUT = 200µA, ROUT = 5k IOUT = –200µA, ROUT = 5k IOUT = 1mA, ROUT = 5k IOUT = –1mA, ROUT = 5k VS = 4V, IOUT = 0, RIN = 1M VS = 6V, IOUT = 0, RIN = 1M VS = 12V, IOUT = 0, RIN = 1M VS = 60V, IOUT = 0, RIN = 1M LTC6104I, LTC6104C LTC6104H IS Supply Current 0.45 0.5 0.52 0.64 0.73 0.825 0.79 1 0.81 1 1.04 1.1 1.2 mA mA mA mA mA mA mA mA mA Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC6104C is guaranteed to meet specified performance from 0°C to 70°C. The LTC6104C is designed, characterized and expected to meet specified performance from –40°C to 85°C but are not tested or QA sampled at these temperatures. LTC6104I is guaranteed to meet specified performance from –40°C to 85°C. The LTC6104H is guaranteed to meet specified performance from –40°C to 125°C. Note 3: VSENSE(MAX) is tested by applying 550mV and verifying the gain error is less than 1%. The 1% limit is set by the accuracy of high speed test equipment. Gain error is typically dominated by external resistor tolerance. Note 4: When amplifier A is active and amplifier B is inactive, the gain error is entirely due to the external resistors RIN and ROUT. When amplifier A is inactive and amplifier B is active, there is an additional gain error from the LTC6104 current mirror circuit. This error term is the gain error term, IOUT-GAINERR plus the offset error term, IOUT-OSERR. 6104f 3 LTC6104 TYPICAL PERFORMANCE CHARACTERISTICS Input VOS vs Temperature 100 80 60 INPUT OFFSET (µV) 40 20 0 –20 –40 –60 –80 –100 –40 –20 0 RIN = 100Ω ROUT = 5k VIN = 5mV VOS OF INTERNAL AMPLIFIER A VOS OF INTERNAL AMPLIFIER B 20 40 60 80 TEMPERATURE (°C) 100 120 6104 G01 Input VOS vs Supply Voltage 50 40 30 MAXIMUM VSENSE (V) INPUT OFFSET (µV) 20 10 0 –10 –20 –30 –40 –50 4 11 18 25 32 39 VS (V) 46 53 60 TA = 25°C RIN = 100Ω ROUT = 5k VIN = 5mV VOS OF INTERNAL AMPLIFIER A VOS OF INTERNAL AMPLIFIER B TWO REPRESENTATIVE UNITS 2.5 Input Sense Range vs Supply Voltage TA = –40°C TA = 25°C 2.0 TA = 0°C TA = 70°C 1.5 TA = 85°C TA = 125°C 1.0 TWO REPRESENTATIVE UNITS 0.5 RIN = 5k ROUT = 2.5k TA = 25°C 4 10 20 NO LIMITS FOR VSENSE IF VSENSE < 0V AND VS ≥ 4V 30 40 VS (V) 50 60 6104 G03 0 6104 G02 VOUT Maximum vs Temperature 12 VS = 60V 10 VS = 12V MAXIMUM OUTPUT (V) MINIMUM OUTPUT (V) 8 6 4 2 0 –40 –20 0.090 0.085 VOUT Minimum vs Temperature 6 4 MAXIMUM IOUT (mA) 2 0 IOUT Maximum vs Temperature 0.080 VS = 60V 0.075 VS = 4V, 6V, 12V 0.070 0.065 0.060 0.055 VS = 60V VS = 12V –2 –4 –6 –8 VS = 6V VS = 4V VS = 6V VS = 4V 0 20 40 60 80 TEMPERATURE (°C) 100 120 6104 G04 0.050 –40 –20 0 20 40 60 80 TEMPERATURE (°C) 100 120 6104 G05 –10 –40 –20 0 20 40 60 80 TEMPERATURE (°C) 100 120 6104 G06 Gain vs Frequency 40 35 30 25 GAIN (dB) 20 15 10 5 0 –5 –10 1k 40 TA = 25°C RIN = 100Ω ROUT = 5k 10k 100k FREQUENCY (Hz) 1M 6104 G08 Input Bias Current vs Temperature 160 IOUT = ±1mA 140 VS = 6V TO 100V VS = 4V SUPPLY CURRENT (µA) 120 900 800 700 600 500 400 300 200 100 0 0 20 40 60 80 TEMPERATURE (°C) 100 120 6104 G09 Supply Current vs Supply Voltage TA = 85°C TA = 125°C TA = 70°C IOUT = ±200µA IB (nA) 100 80 60 TA = 25°C TA = 0°C TA = –40°C 20 0 –40 –20 VIN = 0V ROUT = 1M 4 10 20 30 40 SUPPLY VOLTAGE (V) 50 60 6104 G10 6104f 4 LTC6104 TYPICAL PERFORMANCE CHARACTERISTICS Step Response 0mV to 10mV VS VS –10mV 2.5V TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 2V VOUT 2V VOUT TIME (10µs/DIV) 6104 G11 Step Response 0mV to –10mV VS + 10mV VS 2V VS + 5mV VSENSE– VS – 5mV 2.25V Step Response –5mV to 5mV VSENSE– VSENSE– 2V TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 2V TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 2V TIME (10µs/DIV) 6104 G12 6104 G13 1.75V VOUT 1.5V TIME (10µs/DIV) Step Response –50mV to 50mV VS + 50mV VS – 50mV VSENSE 6.5V 6V CLOAD 1000pF 4V CLOAD 10pF TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 4V TIME (10µs/DIV) 6104 G14 Step Response Rising Edge VSENSE– ∆VSENSE = 100mV Step Response Rising Edge VSENSE– 6V 5.5V ∆VSENSE– = 100mV IOUT = 0µA IOUT = –100µA VOUT 1.5V 1V IOUT = 100µA IOUT = 0µA TIME (1µs/DIV) 1.5V VOUT TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 1V 1V 0.5V VOUT TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 6V TIME (1µs/DIV) 6104 G15 6104 G16 Step Response Rising Edge VS + 50mV VS – 50mV 7V VSENSE– 6.5V 5V Step Response Falling Edge VSENSE– ∆VSENSE– = 100mV VOUT 4.5V TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 4.5V TIME (1µs/DIV) 6104 G17 2V VOUT 1.5V 1V TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 1V TIME (1µs/DIV) IOUT = 100µA IOUT = 0µA 6104 G18 6104f 5 LTC6104 TYPICAL PERFORMANCE CHARACTERISTICS Step Response Falling Edge VSENSE– 6V 5.5V IOUT = 0µA IOUT = –100µA 4.5V TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 6V TIME (1µs/DIV) 6104 G19 Step Response Falling Edge VS + 50mV VS – 50mV 7V VSENSE– VOUT PSRR (dB) 160 140 120 100 80 PSRR vs Frequency ∆VSENSE– = 100mV V+ = 4V V+ = 12V 1V 0.5V VOUT 2V TA = 25°C VS = 12V RIN = 100Ω ROUT = 5k VSENSE+ = VS VREF = 4.5V TIME (1µs/DIV) 6104 G20 60 R = 100Ω IN ROUT = 5k 40 C OUT = 5pF GAIN = 50 20 I OUTDC = 100µA VINAC = 50mVP-P 0 0.1 1 10 100 1k 10k FREQUENCY (Hz) 100k 1M 6104 G20 PIN FUNCTIONS OUT (Pin 1): Current Output. OUT will source or sink a current that is proportional to the sense voltage into an external resistor. A voltage reference is required to provide the proper positive offset voltage so that the output can swing both positive and negative. V– (Pin 4): Negative Supply (or Ground for Single-Supply Operation). +INB/VS (Pin 5): The positive input of the internal sense amplifier B. It also works as the positive supply input. Supply current is drawn through this pin. –INB (Pin 6): The negative input of the internal sense amplifier B. The internal sense amplifier will drive –INB to the same potential as +INB when VSENSE is negative. A resistor (RIN) tied from one end of RSENSE to –INB sets the output current IOUT = VSENSE/RIN. VSENSE is the voltage developed across the external RSENSE (Figure 1). –INA (Pin 7): The negative input of the internal sense amplifier A. The internal sense amplifier will drive –INA to the same potential as +INA when VSENSE is positive. A resistor (RIN) tied from one end of RSENSE to –INA sets the output current IOUT = VSENSE/RIN. +INA (Pin 8): The positive input of the internal sense amplifier A. 6104f 6 LTC6104 BLOCK DIAGRAM – RINB VSENSE RSENSE RINA ILOAD + 8 +INA 7 –INA 10V VS 10V 6 –INB 5 +INB 5k 5k 5k 5k + V– A – V+ V+ – B + V– 10V OUT 1 VOUT R VOUT = VSENSE • OUT + VREF RIN ROUT 4 V– 6104 F01 + – VREF Figure 1. LTC6104 Block Diagram THEORY OF OPERATION When VSENSE is positive, an internal sense amplifier loop forces –INA to have the same potential as +INA. Connecting an external resistor, RINA, in series with –INA causes a current, VSENSE/RINA, to flow through RINA. The high impedance inputs of the sense amplifier will not conduct this input current, so the current will flow through an internal MOSFET to the OUT pin. The output current can be transformed into a voltage by adding a resistor from OUT to a reference voltage (VREF). The output voltage is then VOUT = (VSENSE/RINA) • ROUT + VREF. When operating on a dual supply, ROUT can be tied to ground. The output voltage is then VOUT = (VSENSE/RINA) • ROUT. Only one amplifier is active at a time in the LTC6104. If the load current direction (VSENSE is negative) activates the “B” amplifier, the “A” amplifier will be inactive. The signal current goes into the –INB pin, through the MOSFET, and into the 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 then VSENSE • ROUT/RINB + VREF . Keep in mind that the OUT voltage cannot swing below V–, even though it’s 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. Supply current is drawn from +INB pin. The user may choose to include this current in the monitored current through RSENSE by careful choice of connection polarity. 6104f 7 LTC6104 APPLICATIONS INFORMATION Selection of External Current Sense Resistor The external sense resistor, RSENSE, has a significant effect on the function of a current sensing system and must be chosen with care. First, the power dissipation in the resistor should be considered. The system load current will cause both heat and voltage loss in RSENSE. As a result, the sense resistor should be as small as possible while still providing the input dynamic range required by the measurement. Note that input dynamic range is the difference between the maximum input signal and the minimum accurately reproduced signal, and is limited primarily by input DC offset of the internal amplifier of the LTC6104. In addition, RSENSE must be small enough that VSENSE does not exceed the maximum input voltage specified by the LTC6104, even under peak load conditions. As an example, an application may require that the maximum sense voltage be ±100mV. If this application is expected to draw ±2A at peak load, RSENSE should be no more than 50mΩ. RSENSE = VSENSE 100mV = = 50mΩ IPEAK 2A The low offset and corresponding large dynamic range of the LTC6104 make it more flexible than other solutions in this respect. The ±85µV typical offset gives 60dB of dynamic range for a sense voltage that is limited to ±85mV max, and over 75dB of dynamic range if the rated input maximum of ±500mV is allowed. Sense Resistor Connection Kelvin connection of the –INA/–INB and +INA/+INB 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. One 10mm × 10mm square trace of one-ounce copper is approximately 0.5mΩ. A 1mV error can be caused by as little as 2A flowing through this small interconnect. This will cause a 1% error in a 100mV signal. A 10A load current in the same interconnect will cause a 5% error for the same 100mV signal. 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. Figure 2 illustrates the recommended method. ILOAD Once the maximum RSENSE value is determined, the minimum sense resistor value will be set by the resolution or dynamic range required. The minimum signal that can be accurately represented by this sense amp is limited by the input offset. As an example, the LTC6104 has a typical input offset of ±85µV. If the minimum current is ±20mA, a sense resistor of 4.25mΩ will set VSENSE to ±85µV. This is the same value as the input offset. A larger sense resistor will reduce the error due to offset by increasing the sense voltage for a given load current. Choosing a 50mΩ RSENSE will maximize the dynamic range and provide a system that has ±100mV across the sense resistor at peak load (±2A), while input offset causes an error equivalent to only ±1.7mA of load current. Peak dissipation in the sense resistor is 200mW in this example. If instead a 5mΩ sense resistor is employed, then the effective current error is ±17mA, while the peak sense voltage is reduced to ±10mV at ±2A, dissipating only 20mW. – VSENSE RSENSE RIN + + RIN TO CHARGER/LOAD 8 +INA 7 –INA 6 –INB 5 +INB +– A VS –+ B VS LTC6104 OUT 1 CURRENT MIRROR 4 ROUT V– 6104 F02 + VOUT + – VREF – Figure 2. Kelvin Input Connections Preserve Accuracy Despite Large Load Currents 6104f 8 LTC6104 APPLICATIONS INFORMATION Selection of External Input Resistor, RIN The external input resistor, RIN, controls the transconductance of the current sense circuit. Since IOUT = VSENSE 1 , transconductance gm = RIN RIN VSENSE or 100Ω –IN terminals will increase the effective RIN value, causing a gain error, especially for small RIN values. In addition, internal device resistance will add approximately 0.3Ω to RIN. Trace and interconnect impedances to the +INB terminal will have an effect on offset error. These errors are described in more detail later in this data sheet. Selection of External Output Resistor, ROUT The output resistor, ROUT, determines how the output current is converted to voltage. VOUT is simply IOUT • ROUT + VREF. In choosing an output resistor, the maximum output voltage range must first be considered. If the circuit that is driven by the output does not limit the output voltage range, then ROUT must be chosen such that the maximum output voltage range does not exceed the LTC6104 maximum output voltage range (see Electrical Characteristics). If the following circuit is a buffer or ADC with limited input range, then ROUT must be chosen so that VOUT is in the allowed maximum input range of this circuit. In addition, the output impedance is determined by ROUT. If the circuit to be driven has high enough input impedance, then almost any useful output impedance will be acceptable. However, if the driven circuit has relatively low input impedance, or draws spikes of current, such as an ADC might do, then a lower ROUT value may be required in order to preserve the accuracy of the output. As an example, if the input impedance of the driven circuit is 100 times ROUT, then the accuracy of VOUT will be reduced by 1% since: VOUT – VREF = IOUT • ROUT • RIN(DRIVEN) ROUT + RIN(DRIVEN) 100 = 0.99 • IOUT • ROUT 101 For example, if RIN = 100Ω, then IOUT = IOUT = ±1mA for VSENSE = ±100mV. RIN should be chosen to allow the required resolution while limiting the output current. At low supply voltage, IOUT may be as much as ±1mA. By setting RIN such that the largest expected sense voltage gives IOUT = ±1mA, then the maximum output dynamic range is available. Output dynamic range is limited by both the maximum allowed output current and the maximum allowed output voltage, as well as the minimum practical output signal. If less dynamic range is required, then RIN can be increased accordingly, reducing the maximum output current and power dissipation. If low sense currents must be resolved accurately in a system that has very wide dynamic range, a smaller RIN than the maximum current specification allows may be used if the maximum current is limited in another way, such as with a Schottky diode across RSENSE (Figure 3). 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 occasional large burst currents may be ignored. Care should be taken when designing the printed circuit board layout to minimize input trace resistance (to Pins 5, 6, 7 and 8). Trace and interconnect impedances to the RSENSE LOAD BATTERY = IOUT • ROUT • Selection of External Voltage Reference, VREF DSENSE 6104 F03 Selection of external reference voltage should be considered together with selection of ROUT. Example: Given the conditions: IOUT = –1mA to 1mA, VS = 12V. 6104f Figure 3. Shunt Diodes Limit Maximum Input Voltage to Allow Better Low Input Resolution Without Overranging 9 LTC6104 APPLICATIONS INFORMATION From the Electrical Characteristics of the LTC6104, the output voltage range is 0.3V to 8V. If the circuit that is driven by the output limits the maximum output voltage to ≈5V, to achieve maximum dynamic range, VOUT should be 0.3V for –1mA IOUT and 5V for 1mA IOUT. ROUT = 5V – 0.3V = 2.35k, 2mA 5 – 0.3 = 2.65V 2 Output Error, EOUT, Due to the Amplifier DC Offset Voltage, VOS EOUT( VOS) = VOS • ROUT RIN VREF = 0.3 + The DC offset voltage of the amplifier adds directly to the value of the sense voltage, VSENSE. This is the dominant error of the system and it limits the available dynamic range. The section, Selection of External Current Sense Resistor, provides details. Output Error, EOUT, Due to the Bias Currents, IB+ and IB– The bias current IB+ flows into the positive input of the internal op amp. IB– flows into the negative input. ⎛ ⎞ R EOUT(IBIAS) = ROUT ⎜IB + • SENSE – IB – ⎟ RIN ⎝ ⎠ Since IB+ ≈ IB– = IBIAS, if RSENSE