Datasheet LT6106 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | Low Cost 36V High Side Current Sense Amplifier |
| Pages / Page | 14 / 7 — APPLICATIONS INFORMATION. Introduction. Theory of Operation. Table 1. … |
| File Format / Size | PDF / 396 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Introduction. Theory of Operation. Table 1. Useful Gain Configurations. GAIN. RIN. ROUT
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Text Version of Document
LT6106
APPLICATIONS INFORMATION Introduction
must be small enough that VSENSE does not exceed the The LT6106 high side current sense amplifier (Figure 1) pro- maximum input voltage specified by the LT6106, even vides accurate monitoring of current through a user-selected under peak load conditions. As an example, an application sense resistor. The sense voltage is amplified by a user- may require that the maximum sense voltage be 100mV. selected gain and level shifted from the positive power sup- If this application is expected to draw 2A at peak load, ply to a ground-referred output. The output signal is analog RSENSE should be no more than 50mΩ. and may be used as is, or processed with an output filter. Once the maximum RSENSE value is determined, the mini- mum sense resistor value will be set by the resolution or
Theory of Operation
dynamic range required. The minimum signal that can be An internal sense amplifier loop forces –IN to have the accurately represented by this sense amplifier is limited by same potential as +IN. Connecting an external resistor, the input offset. As an example, the LT6106 has a typical R input offset of 150µV. If the minimum current is 20mA, a IN, between –IN and V+ forces a potential across RIN that is the same as the sense voltage across R sense resistor of 7.5mΩ will set V SENSE. A SENSE to 150µV. This is corresponding current, V the same value as the input offset. A larger sense resis- SENSE/RIN, will flow through RIN. The high impedance inputs of the sense amplifier will not tor will reduce the error due to offset by increasing the conduct this current, so it will flow through an internal sense voltage for a given load current. Choosing a 50mΩ PNP to the output pin as I R OUT. SENSE will maximize the dynamic range and provide a system that has 100mV across the sense resistor at peak The output current can be transformed into a voltage by load (2A), while input offset causes an error equivalent to adding a resistor from OUT to V–. The output voltage is only 3mA of load current. Peak dissipation is 200mW. If a then VO = V– + IOUT • ROUT. 5mΩ sense resistor is employed, then the effective current
Table 1. Useful Gain Configurations
error is 30mA, while the peak sense voltage is reduced to
GAIN
10mV at 2A, dissipating only 20mW.
RIN ROUT VSENSE at VOUT = 5V IOUT at VOUT = 5V
20 499Ω 10k 250mV 500µA The low offset and corresponding large dynamic range of 50 200Ω 10k 100mV 500µA the LT6106 make it more flexible than other solutions in 100 100Ω 10k 50mV 500µA this respect. The 150µV typical offset gives 60dB of dy-
GAIN RIN ROUT VSENSE at VOUT = 2.5V IOUT at VOUT = 2.5V
20 249Ω 5k 125mV 500µA namic range for a sense voltage that is limited to 150mV 50 100Ω 5k 50mV 500µA maximum, and over 70dB of dynamic range if the rated 100 50Ω 5k 25mV 500µA input maximum of 0.5V is allowed.
Selection of External Current Sense Resistor Sense Resistor Connection
The external sense resistor, RSENSE, has a significant effect Kelvin connection of the –IN and +IN inputs to the sense on the function of a current sensing system and must be resistor should be used in all but the lowest power ap- chosen with care. plications. Solder connections and PC board interconnec- First, the power dissipation in the resistor should be tions that carry high current can cause significant error considered. The system load current will cause both heat in measurement due to their relatively large resistances. and voltage loss in R One 10mm × 10mm square trace of one-ounce copper SENSE. As a result, the sense resis- tor should be as small as possible while still providing is approximately 0.5mΩ. A 1mV error can be caused by the input dynamic range required by the measurement. as little as 2A flowing through this small interconnect. Note that input dynamic range is the difference between This will cause a 1% error in a 100mV signal. A 10A load the maximum input signal and the minimum accurately current in the same interconnect will cause a 5% error measured signal, and is limited primarily by input DC offset for the same 100mV signal. By isolating the sense traces of the internal amplifier of the LT6106. In addition, R from the high current paths, this error can be reduced SENSE 6106fb For more information www.linear.com/LT6106 7
APPLICATIONS INFORMATION Introduction
must be small enough that VSENSE does not exceed the The LT6106 high side current sense amplifier (Figure 1) pro- maximum input voltage specified by the LT6106, even vides accurate monitoring of current through a user-selected under peak load conditions. As an example, an application sense resistor. The sense voltage is amplified by a user- may require that the maximum sense voltage be 100mV. selected gain and level shifted from the positive power sup- If this application is expected to draw 2A at peak load, ply to a ground-referred output. The output signal is analog RSENSE should be no more than 50mΩ. and may be used as is, or processed with an output filter. Once the maximum RSENSE value is determined, the mini- mum sense resistor value will be set by the resolution or
Theory of Operation
dynamic range required. The minimum signal that can be An internal sense amplifier loop forces –IN to have the accurately represented by this sense amplifier is limited by same potential as +IN. Connecting an external resistor, the input offset. As an example, the LT6106 has a typical R input offset of 150µV. If the minimum current is 20mA, a IN, between –IN and V+ forces a potential across RIN that is the same as the sense voltage across R sense resistor of 7.5mΩ will set V SENSE. A SENSE to 150µV. This is corresponding current, V the same value as the input offset. A larger sense resis- SENSE/RIN, will flow through RIN. The high impedance inputs of the sense amplifier will not tor will reduce the error due to offset by increasing the conduct this current, so it will flow through an internal sense voltage for a given load current. Choosing a 50mΩ PNP to the output pin as I R OUT. SENSE will maximize the dynamic range and provide a system that has 100mV across the sense resistor at peak The output current can be transformed into a voltage by load (2A), while input offset causes an error equivalent to adding a resistor from OUT to V–. The output voltage is only 3mA of load current. Peak dissipation is 200mW. If a then VO = V– + IOUT • ROUT. 5mΩ sense resistor is employed, then the effective current
Table 1. Useful Gain Configurations
error is 30mA, while the peak sense voltage is reduced to
GAIN
10mV at 2A, dissipating only 20mW.
RIN ROUT VSENSE at VOUT = 5V IOUT at VOUT = 5V
20 499Ω 10k 250mV 500µA The low offset and corresponding large dynamic range of 50 200Ω 10k 100mV 500µA the LT6106 make it more flexible than other solutions in 100 100Ω 10k 50mV 500µA this respect. The 150µV typical offset gives 60dB of dy-
GAIN RIN ROUT VSENSE at VOUT = 2.5V IOUT at VOUT = 2.5V
20 249Ω 5k 125mV 500µA namic range for a sense voltage that is limited to 150mV 50 100Ω 5k 50mV 500µA maximum, and over 70dB of dynamic range if the rated 100 50Ω 5k 25mV 500µA input maximum of 0.5V is allowed.
Selection of External Current Sense Resistor Sense Resistor Connection
The external sense resistor, RSENSE, has a significant effect Kelvin connection of the –IN and +IN inputs to the sense on the function of a current sensing system and must be resistor should be used in all but the lowest power ap- chosen with care. plications. Solder connections and PC board interconnec- First, the power dissipation in the resistor should be tions that carry high current can cause significant error considered. The system load current will cause both heat in measurement due to their relatively large resistances. and voltage loss in R One 10mm × 10mm square trace of one-ounce copper SENSE. As a result, the sense resis- tor should be as small as possible while still providing is approximately 0.5mΩ. A 1mV error can be caused by the input dynamic range required by the measurement. as little as 2A flowing through this small interconnect. Note that input dynamic range is the difference between This will cause a 1% error in a 100mV signal. A 10A load the maximum input signal and the minimum accurately current in the same interconnect will cause a 5% error measured signal, and is limited primarily by input DC offset for the same 100mV signal. By isolating the sense traces of the internal amplifier of the LT6106. In addition, R from the high current paths, this error can be reduced SENSE 6106fb For more information www.linear.com/LT6106 7
