Datasheet MAX471 (Maxim) - 7

Precision, High-Side Current-Sense Amplifiers ______________________________________________________________Pin Description PIN NAME FUNCTION MAX471 MAX472
Shutdown. Connect to ground for normal operation. When high, supply current is 1 1 SHDN less than 5µA. Battery (or power) side of the internal current-sense resistor. The “+” indicates direction of 2, 3 — RS+ flow for SIGN output only. Connect pins 2 and 3 together at the package. — 2 N.C. No Connect—no internal connection — 3 RG1 Gain Resistor. Connect to battery side of current-sense resistor through the gain resistor. 4 4 GND Ground or Battery Negative Terminal An open-collector logic output. For the MAX471, a low level indicates current is flowing from
MAX471/MAX472
5 5 SIGN RS- to RS+. For the MAX472, a low level indicates a negative VSENSE (see Figure 2). SIGN is high impedance when SHDN is high. Leave open if SIGN is not needed. Load side of the internal current-sense resistor. The “-” indicates direction of flow for SIGN 6, 7 — RS- output only. Connect pins 6 and 7 together at the package. — 6 RG2 Gain Resistor. Connect to load side of current-sense resistor through the gain resistor. — 7 VCC Power input for MAX472. Connect to sense resistor (RSENSE) junction with RG1. Current output that is proportional to the magnitude of the sensed current flowing through 8 8 OUT RSENSE. A 2kΩ resistor from this pin to ground will result in a voltage equal to 1V/Amp of sensed current in the MAX471.
_______________Detailed Description
IOUT = (ILOAD x RSENSE) / RG1 The MAX471 and MAX472 current-sense amplifier’s
Current Output
unique topology allows a simple design to accurately The output voltage equation for the MAX471/MAX472 is monitor current flow. The MAX471/MAX472 contain two given below. In the MAX471, the current-gain ratio has amplifiers operating as shown in Figures 1 and 2. The been preset to 500µA/A so that an output resistor battery/load current flows from RS+ to RS- (or vice (ROUT) of 2kΩ yields 1V/A for a full-scale value of +3V versa) through RSENSE. Current flows through either at ±3A. Other full-scale voltages can be set with differ- RG1 and Q1 or RG2 and Q2, depending on the sense- ent ROUT values, but the output voltage can be no resistor current direction. Internal circuitry, not shown in greater than VRS+ - 1.5V for the MAX471 or VRG_ - 1.5V Figures 1 and 2, prevents Q1 and Q2 from turning on at for the MAX472. the same time. The MAX472 is identical to the V MAX471, except that R OUT = (RSENSE x ROUT x ILOAD) / RG SENSE and gain-setting resistors RG1 and RG2 are external (Figure 2). where VOUT = the desired full-scale output voltage, I To analyze the circuit of Figure 1, assume that current LOAD = the full-scale current being sensed, RSENSE = the current-sense resistor, R flows from RS+ to RS- and that OUT is connected to OUT = the voltage-setting resistor, and RG = the gain-setting resistor (RG = RG1 GND through a resistor. In this case, amplifier A1 is = RG2). active and output current IOUT flows from the emitter of Q1. Since no current flows through RG2 (Q2 is off), the The above equation can be modified to determine the negative input of A1 is equal to VSOURCE - (ILOAD x ROUT required for a particular full-scale range: RSENSE). The open-loop gain of A1 forces its positive ROUT = (VOUT x RG) / (ILOAD x RSENSE) input to essentially the same level as the negative input. For the MAX471, this reduces to: Therefore, the drop across RG1 equals I LOAD x RSENSE. Then, since IOUT flows through Q1 and RG ROUT = VOUT / (ILOAD x 500µA/A) (ignoring the extremely low base currents), IOUT x RG1 OUT is a high-impedance current-source output that = ILOAD x RSENSE, or: can be connected to other MAX471/MAX472 OUT pins
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Document Outline not-recomm-data-sheet-cover.pdf Not Recommended for New Designs