Datasheet MAX4173 (Maxim) - 7

MAX4173
Low-Cost, SOT23, Voltage-Output, High-Side Current-Sense Amplifier Detailed Description
Set the full-scale output range by selecting RSENSE and the appropriate gain version of the MAX4173. The MAX4173 high-side current-sense amplifier fea- tures a 0 to +28V input common-mode range that is
Applications Information
independent of supply voltage. This feature allows the monitoring of current out of a battery in deep discharge
Recommended Component Values
and also enables high-side current sensing at voltages The MAX4173 senses a wide variety of currents with greater than the supply voltage (VCC). different sense resistor values. Table 1 lists common resistor values for typical operation of the MAX4173. The MAX4173 operates as follows: Current from the source flows through RSENSE to the load (Figure 1). Since
Choosing RSENSE
the internal-sense amplifier’s inverting input has high To measure lower currents more accurately, use a high impedance, negligible current flows through RG2 value for RSENSE. The high value develops a higher (neglecting the input bias current). Therefore, the sense voltage that reduces offset voltage errors of the sense amplifier’s inverting-input voltage equals internal op amp. VSOURCE - (ILOAD)(RSENSE). The amplifier’s open-loop In applications monitoring very high currents, R gain forces its noninverting input to the same voltage as SENSE must be able to dissipate the I2R losses. If the resistor’s the inverting input. Therefore, the drop across RG1 rated power dissipation is exceeded, its value may drift equals (ILOAD)(RSENSE). Since IRG1 flows through RG1, or it may fail altogether, causing a differential voltage IRG1 = (ILOAD)(RSENSE) / RG1. The internal current mirror across the terminals in excess of the absolute maxi- multiplies IRG1 by a current gain factor, β, to give mum ratings. IRGD = β
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IRG1. Solving IRGD = β
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(ILOAD)(RSENSE) / RG1. Assuming infinite output impedance, VOUT = (IRGD) If ISENSE has a large high-frequency component, mini- (RGD). Substituting in for IRGD and rearranging, VOUT = mize the inductance of RSENSE. Wire-wound resistors β
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(RGD / RG1)(RSENSE
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ILOAD). The parts gain equals have the highest inductance, metal-film resistors are β
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RGD / RG1. Therefore, VOUT = (GAIN) (RSENSE) somewhat better, and low-inductance metal-film resis- (ILOAD), where GAIN = 20 for MAX4173T, GAIN = 50 for tors are best suited for these applications. MAX4173F, and GAIN = 100 for MAX4173H.
Using a PCB Trace as RSENSE
If the cost of RSENSE is an issue and accuracy is not critical, use the alternative solution shown in Figure 2. ILOAD RSENSE TO LOAD BATTERY This solution uses copper PC board traces to create a VSOURCE sense resistor. The resistivity of a 0.1-inch-wide trace of 0 TO +28V 2-ounce copper is approximately 30m RS+ RS- Ω/ft. The resis- tance-temperature coefficient of copper is fairly high IRG1 (approximately 0.4%/°C), so systems that experience a +3V TO +28V RG1 RG2 VCC wide temperature variance must compensate for this effect. In addition, do not exceed the maximum power dissipation of the copper trace. A1 For example, the MAX4173T (with a maximum load cur- rent of 10A and an RSENSE of 5mΩ) creates a full-scale VSENSE of 50mV that yields a maximum VOUT of 1V. RSENSE in this case requires about 2 inches of 0.1 inch- MAX4173 wide copper trace. VOUT CURRENT OUT
Output Impedance
MIRROR The output of the MAX4173 is a current source driving a IRGD 12kΩ resistance. Resistive loading added to OUT reduces the output gain of the MAX4173. To minimize RGD = 12k output errors for most applications, connect OUT to a GND high-impedance input stage. When output buffering is required, choose an op amp with a common-mode input range and an output voltage swing that includes ground when operating with a single supply. The op Figure 1. Functional Diagram Maxim Integrated 7