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FAN4010IL6X Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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FAN4010IL6X Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 10 page © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN4010 Rev. 1.0.1 7 Application Information Detailed Description The FAN4010 measures the voltage drop (VSENSE) across an external sense resistor located in the high voltage side of the circuit. VSENSE is converted to a linear current via an internal operational amplifier and precision 100Ω resistor. The value of this current is VSENSE/100Ω (internal). Output current flows from the IOUT pin to an external resistor ROUT to generate an output voltage proportional to the current flowing to the load. Use the following equations to scale a load current to an output voltage: VSENSE = ILoad * RSENSE EQ.1 VOUT = 0.01 x VSENSE x ROUT EQ.2 Figure 13. Functional Circuit Selecting RSENSE Selection of RSENSE is a balance between desired accu- racy and allowable voltage loss. Although the FAN4010 is optimized for high accuracy with low VSENSE values, a larger RSENSE value provides additional accuracy. How- ever, larger values of RSENSE create a larger voltage drop, reducing the effective voltage available to the load. This can be troublesome in low-voltage applications. Because of this, the maximum expected load current and allowable load voltage should be well understood. Although higher values of VSENSE can be used, RSENSE should be chosen to satisfy the following condition: 10mV < VSENSE < 200mV EQ. 3 For low-cost applications where accuracy is not as important, a portion of the printed circuit board (PCB) trace can be used as an RSENSE resistor. Figure 14 shows an example of this configuration. The resistivity of a 0.1 inch wide trace of two-ounce copper is about 30mΩ/ft. Unfortunately, the resistance temperature coefficient is relatively large (approximately 0.4% / C), so systems with a wide temperature range may need to compensate for this effect. Additionally, self heating due to load currents introduces a nonlinearity error. Care must be taken not to exceed the maximum power dissipation of the copper trace. Figure 14. Using PCB Trace for RSENSE Selecting ROUT ROUT can be chosen to obtain the output voltage ran- ge required for the particular downstream application. For example, if the output of the FAN4010 is intended to drive an analog-to-digital convertor (ADC), ROUT should be chosen such that the expected full-scale output current produces an input voltage that matches the input range of the ADC. For instance, if expected loading current ranges from 0 to 1A, a RSENSE resistor of 1Ω produces an output current that ranges from 0 to 10mA. If the input voltage range of the ADC is 0 to 2V, a ROUT value of 200Ω should be used. The input voltage and full-scale output current (IOUT_ FS) needs to be taken into account when setting up the output range. To ensure sufficient operating head- room, choose: (ROUT * IOUT_FS) such that VIN - VSENSE - (ROUT * IOUT_FS) > 1.2V EQ. 4 Output current accuracy for the recommended VSENSE levels between 10mV and 200mV are typically much better than 1%. As a result, the absolute output volta- ge accuracy is dependent upon the precision of the output resistor. Make sure the input impedance of the circuit con- nected to VOUT is much higher than ROUT to ensure accurate VOUT values. Since the FAN4010 provides a trans-impedance func- tion, it is ideal for applications involving current rather than voltage sensing. ROUT 2 3 5 4 IOUT VIN Load 1 GND NC VOUT VIN RSENSE VSENSE RLoad 100 + – R OUT 2 3 5 4 I OUT Load VIN 1 GND NC V OUT R SENSE INPUT LOAD 0.3in Copper 0.3in Copper 0.1in Copper |
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