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MAX471ESA Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX471ESA Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page Peak Sense Current The MAX471’s maximum sense current is 3ARMS. For power-up, fault conditions, or other infrequent events, larger peak currents are allowed, provided they are short—that is, within a safe operating region, as shown in Figure 5. MAX472 RSENSE, RG1, and RG2 are externally connected on the MAX472. VCC can be connected to either the load/charge or power-source/battery side of the sense resistor. Connect VCC to the load/charge side of RSENSE if you want to include the MAX472 current drain in the measured current. Suggested Component Values for Various Applications The general circuit of Figure 4 is useful in a wide variety of applications. It can be used for high-current applica- tions (greater than 3A), and also for those where the full- scale load current is less than the 3A of the MAX471. Table 1 shows suggested component values and indi- cates the resulting scale factors for various applications required to sense currents from 100mA to 10A. Higher or lower sense-current circuits can also be built. Select components and calculate circuit errors using the guidelines and formulas in the following section. RSENSE Choose RSENSE based on the following criteria: a) Voltage Loss: A high RSENSE value will cause the power-source voltage to degrade through IR loss. For least voltage loss, use the lowest RSENSE value. b) Accuracy: A high RSENSE value allows lower currents to be measured more accurately. This is because offsets become less significant when the sense voltage is larger. c) Efficiency and Power Dissipation: At high current levels, the I2R losses in RSENSE may be significant. Take this into consideration when choosing the resistor value and power dissipation (wattage) rat- ing. Also, if the sense resistor is allowed to heat up excessively, its value may drift. d) Inductance: If there is a large high-frequency com- ponent to ISENSE, you will want to keep inductance low. Wire-wound resistors have the highest induc- tance, while metal film is somewhat better. Low- inductance metal-film resistors are available. Instead of being spiral wrapped around a core, as in metal- film or wire-wound resistors, these are a straight band of metal. They are made in values under 1 Ω. e) Cost: If the cost of RSENSE becomes an issue, you may want to use an alternative solution, as shown in Figure 6. This solution uses the PC board traces to create a sense resistor. Because of the inaccuracies of the copper “resistor,” you will need to adjust the full-scale current value with a potentiometer. Also, the resistance temperature coefficient of copper is fairly high (approximately 0.4%/°C), so systems that experience a wide temperature variance should take this into account. Precision, High-Side Current-Sense Amplifiers _______________________________________________________________________________________ 9 Table 1. Suggested Component Values for the MAX472 FULL-SCALE LOAD CURRENT, ISENSE (A) CURRENT- SENSE RESISTOR, RSENSE (m Ω) GAIN-SETTING RESISTORS, RG1 = RG2 ( Ω) OUTPUT RESISTOR, ROUT (k Ω) FULL-SCALE OUTPUT VOLTAGE, VOUT (V) SCALE FACTOR, VOUT/ISENSE (V/A) 0.1 500 200 10 2.5 25 1 50 200 10 2.5 2.5 5 10 100 5 2.5 0.5 10 5 50 2 2 0.2 0.9 0.9 1.1 1.6 14 2.5 14 2.5 13 2.0 12 2.0 TYPICAL ERROR AT X% OF FULL LOAD (%) 1% 100% 10% 50 45 0 10 µ 5 40 PULSE WIDTH (sec) DIP safe operating region Small Outline safe operating region 100 µ 1m 10m 30 35 15 20 25 10 DIP fuse time Small Outline fuse time TA = +25°C Figure 5. MAX471 Pulse Current Safe Operation for 10,000 Pulses and Fuse Time for Continuous Current. Pulse tests done with 250mW average power dissipation. |
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