Datasheet ADM4073 (Analog Devices) - 10

ADM4073 THEORY OF OPERATION
The current from the source flows through RSENSE, which gen- To measure lower currents accurately, use as large a sense erates a voltage drop, VSENSE, across the RS+ and RS− terminals resistor as possible to utilize the higher end of the sense voltage of the sense amplifier. The Input Stage Amplifier A1 regulates range. This reduces the effects of the offset voltage errors in the its inputs to be equal, thereby shunting a current proportional internal amplifier. to VSENSE/RG1 to the output current mirror. This current is then When currents are very large, it is important to take the I2R multiplied by a gain factor of b in the output stage current mir- power losses across the sense resistor into account. If the sense ror and flows through RGD to generate VOUT. Therefore, VOUT is resistor’s rated power dissipation is not sufficient, its value can related to VSENSE by the ratio of R G1 to RGD and the current gain drift, giving an inaccurate output voltage or it could fail alto- of b. gether. This, in turn, causes the voltage across the RS+ and RS− VOUT = AV × VSENSE pins to exceed the absolute maximum ratings. where: If the monitored supply rail has a large amplitude high AV = RGD/R G1 × b frequency component, choose a sense resistor with low A inductance. v is equal to different voltages depending upon the model of the device.
RSENSE INPUT COPPER PCB TRACE OUTPUT
• 20 V/V for ADM4073T. • 50 V/V for ADM4073F.
RS+ RS–
• 100 V/V for ADM4073H.
3V TO 28V VCC ILOAD 0.1µF OUT 2V TO 28V RSENSE VOUT ADM4073 RS+ RS– GND
4
I
00
3V TO 28V V RG1 CC
1- 13 05
0.1µF R R G1 G2
Figure 26. Using PCB Trace for Current Sensing
AV OUTPUT (OUT)
The output stage of the ADM4073 is a current source driving a pull-down resistance. To ensure optimum accuracy, care must
ADM4073
be taken not to load this output externally. To minimize output
CURRENT OUT
errors, ensure OUT is connected to a high impedance input
MIRROR VOUT
stage. If this is not possible, output buffering is recommended.
R I GD = 12kΩ RGD
The percent error introduced by output loading is determined 26 0 1-
GND
with the following formula: 13 05 Figure 25. Functional Block Diagram % Error = 100 (1− R / R + R LOAD ( OUT_INT LOAD )
RSENSE
where: The ADM4073 has the ability to sense a wide variety of currents RLOAD is the external load applied to OUT. by selecting a particular sense resistor. Select a suitable output ROUT_INT is the internal output resistance (12 kΩ). voltage for full-scale current, such as 10 V for 10 A. Then, select a gain model that gives the most efficient use of the sense volt- age range (150 mV max). In the example above, using the ADM4073H (gain of 100) gives an output voltage of 10 V when the sense voltage is 100 mV. Use the following equation to determine what value of sense resistor gives 100 mV with 10 A flowing through it: RSENSE = 100 mV/10 A RSENSE = 10 mΩ VOUT = (ILOAD × RSENSE) × AV Rev. A | Page 10 of 12 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM APPLICATION DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL CHARACTERISTICS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION RSENSE OUTPUT (OUT) OUTLINE DIMENSIONS ORDERING GUIDE