Datasheet LTC6115 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | High Voltage High Side Current and Voltage Sense |
| Pages / Page | 20 / 10 — APPLICATIONS INFORMATION: CURRENT SENSE. Theory of Operation. Figure 2. … |
| Revision | B |
| File Format / Size | PDF / 1.4 Mb |
| Document Language | English |
APPLICATIONS INFORMATION: CURRENT SENSE. Theory of Operation. Figure 2. Table 1. Useful Gain Configurations. RIN. ROUT. VSENSE AT
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link to page 9 LTC6115
APPLICATIONS INFORMATION: CURRENT SENSE
The LTC6115 high side current sense amplifier (Figure 1, First, the power dissipation in the resistor should be con- Amplifier A1) provides accurate monitoring of current sidered. The system load current will cause both heat and through an external user-selected sense resistor. The voltage drop in RSENSE. As a result, the sense resistor sense voltage is amplified by a user-selected gain and should be as small as possible while still providing the level shifted from the positive power supply to a ground- input dynamic range required by the measurement. Note referred output current. The output signal is analog and that input dynamic range is the difference between the may be used as is or processed with an output filter. maximum input signal and the minimum accurately repro- duced signal, and is limited primarily by input DC offset
Theory of Operation
of the internal current sense amplifier. In addition, RSENSE An internal sense amplifier loop forces –IN to have the must be small enough that VSENSE does not exceed the same potential as +IN. Connecting an external resistor, maximum VSENSE input voltage specified by the LTC6115 R current sense amplifier, even under peak load condi- IN, between –IN and V+ forces a potential across RIN that is the same as the sense voltage across R tions. As an example, an application may require that the SENSE. A cor- responding current, V maximum sense voltage be 100mV. If this application is SENSE/RIN, will flow through RIN. The high impedance inputs of the sense amplifier will expected to draw 2A at peak load, RSENSE should be no not conduct this input current, so it will flow through the more than 50mΩ. internal MOSFET to the output pin. Once the maximum RSENSE value is determined, the mini- mum sense resistor value will be set by the resolution or dynamic range required. The minimum signal that can V+ be accurately represented by this sense amp is limited SUPPLY1 5V TO 100V by the input offset. As an example, the LTC6115 current RIN –IN sense amplifier has a typical input offset of 85µV. If the VSENSE RSENSE A1 minimum current is 20mA, a sense resistor of 5mΩ will +IN OUTI set VSENSE to 100µV. This is only slightly higher than the input offset. A larger sense resistor will reduce the error LOAD ROUT GND LTC6115 due to offset by increasing the sense voltage for a given 6115 F02 load current.
Figure 2.
Choosing a 50mΩ RSENSE will maximize the dynamic range and provide a system that has 100mV across the The output current can be transformed into a voltage by sense resistor at peak load (2A), while an input offset of adding a resistor from OUTI to GND. The output voltage 85μV causes an error equivalent to only 1.7mA of load is then VOUTI = IOUTI • ROUTI. current. Peak power dissipation in the sense resistor will be 200mW. If a 5mΩ sense resistor is employed, then
Table 1. Useful Gain Configurations
the effective current error is 17mA, while the peak sense
RIN ROUT VSENSE AT IOUT AT GAIN (Ω) (Ω) VOUT = 5V VOUT = 5V
voltage is reduced to 10mV at 2A, dissipating only 20mW. 20 499 10k 250mV 500µA The low offset and corresponding large dynamic range 50 200 10k 100mV 500µA of the LTC6115 make it more flexible than other solu- 100 100 10k 50mV 500µA tions in this respect. The 85µV typical offset gives 60dB of dynamic range for a sense voltage that is limited to
Selection of External Current Sense Resistor
85mV maximum, and over 75dB of dynamic range if the rated input maximum of 500mV is allowed. The external sense resistor, RSENSE, has a significant effect on the function of a current sensing system and must be chosen with care. Rev. B 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Current Sense Typical Performance Characteristics Voltage Sense Typical Performance Characteristics Pin Functions Block Diagram Applications Information: Current Sense Applications Information: Voltage Sense Typical Applications Package Description Revision History Typical Application Related Parts
APPLICATIONS INFORMATION: CURRENT SENSE
The LTC6115 high side current sense amplifier (Figure 1, First, the power dissipation in the resistor should be con- Amplifier A1) provides accurate monitoring of current sidered. The system load current will cause both heat and through an external user-selected sense resistor. The voltage drop in RSENSE. As a result, the sense resistor sense voltage is amplified by a user-selected gain and should be as small as possible while still providing the level shifted from the positive power supply to a ground- input dynamic range required by the measurement. Note referred output current. The output signal is analog and that input dynamic range is the difference between the may be used as is or processed with an output filter. maximum input signal and the minimum accurately repro- duced signal, and is limited primarily by input DC offset
Theory of Operation
of the internal current sense amplifier. In addition, RSENSE An internal sense amplifier loop forces –IN to have the must be small enough that VSENSE does not exceed the same potential as +IN. Connecting an external resistor, maximum VSENSE input voltage specified by the LTC6115 R current sense amplifier, even under peak load condi- IN, between –IN and V+ forces a potential across RIN that is the same as the sense voltage across R tions. As an example, an application may require that the SENSE. A cor- responding current, V maximum sense voltage be 100mV. If this application is SENSE/RIN, will flow through RIN. The high impedance inputs of the sense amplifier will expected to draw 2A at peak load, RSENSE should be no not conduct this input current, so it will flow through the more than 50mΩ. internal MOSFET to the output pin. Once the maximum RSENSE value is determined, the mini- mum sense resistor value will be set by the resolution or dynamic range required. The minimum signal that can V+ be accurately represented by this sense amp is limited SUPPLY1 5V TO 100V by the input offset. As an example, the LTC6115 current RIN –IN sense amplifier has a typical input offset of 85µV. If the VSENSE RSENSE A1 minimum current is 20mA, a sense resistor of 5mΩ will +IN OUTI set VSENSE to 100µV. This is only slightly higher than the input offset. A larger sense resistor will reduce the error LOAD ROUT GND LTC6115 due to offset by increasing the sense voltage for a given 6115 F02 load current.
Figure 2.
Choosing a 50mΩ RSENSE will maximize the dynamic range and provide a system that has 100mV across the The output current can be transformed into a voltage by sense resistor at peak load (2A), while an input offset of adding a resistor from OUTI to GND. The output voltage 85μV causes an error equivalent to only 1.7mA of load is then VOUTI = IOUTI • ROUTI. current. Peak power dissipation in the sense resistor will be 200mW. If a 5mΩ sense resistor is employed, then
Table 1. Useful Gain Configurations
the effective current error is 17mA, while the peak sense
RIN ROUT VSENSE AT IOUT AT GAIN (Ω) (Ω) VOUT = 5V VOUT = 5V
voltage is reduced to 10mV at 2A, dissipating only 20mW. 20 499 10k 250mV 500µA The low offset and corresponding large dynamic range 50 200 10k 100mV 500µA of the LTC6115 make it more flexible than other solu- 100 100 10k 50mV 500µA tions in this respect. The 85µV typical offset gives 60dB of dynamic range for a sense voltage that is limited to
Selection of External Current Sense Resistor
85mV maximum, and over 75dB of dynamic range if the rated input maximum of 500mV is allowed. The external sense resistor, RSENSE, has a significant effect on the function of a current sensing system and must be chosen with care. Rev. B 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Current Sense Typical Performance Characteristics Voltage Sense Typical Performance Characteristics Pin Functions Block Diagram Applications Information: Current Sense Applications Information: Voltage Sense Typical Applications Package Description Revision History Typical Application Related Parts
