Datasheet MAX471 (Maxim) - 10

Precision, High-Side Current-Sense Amplifiers MAX471/MAX472 Peak Sense Current
Table 1 shows suggested component values and indi- The MAX471’s maximum sense current is 3ARMS. For cates the resulting scale factors for various applications power-up, fault conditions, or other infrequent events, required to sense currents from 100mA to 10A. larger peak currents are allowed, provided they are Higher or lower sense-current circuits can also be built. short—that is, within a safe operating region, as shown Select components and calculate circuit errors using in Figure 5. the guidelines and formulas in the following section. 50
RSENSE
Small 45 DIP Outline Choose RSENSE based on the following criteria: fuse T 40 A = +25°C time fuse a)
Voltage Loss:
A high RSENSE value will cause the time 35 power-source voltage to degrade through IR loss. 30 For least voltage loss, use the lowest RSENSE value. 25 b)
Accuracy:
A high RSENSE value allows lower currents to be measured more accurately. This is 20 because offsets become less significant when the SENSE CURRENT (A) 15 sense voltage is larger. 10 c)
Efficiency and Power Dissipation:
At high current 5 levels, the I2R losses in RSENSE may be significant. 0 Take this into consideration when choosing the 10µ 100µ 1m 10m resistor value and power dissipation (wattage) rat- PULSE WIDTH (sec) ing. Also, if the sense resistor is allowed to heat up DIP safe Small Outline safe excessively, its value may drift. operating region operating region Figure 5. MAX471 Pulse Current Safe Operation for 10,000 d)
Inductance:
If there is a large high-frequency com- Pulses and Fuse Time for Continuous Current. Pulse tests done ponent to ISENSE, you will want to keep inductance with 250mW average power dissipation. low. Wire-wound resistors have the highest induc- tance, while metal film is somewhat better. Low-
MAX472
inductance metal-film resistors are available. Instead of being spiral wrapped around a core, as in metal- RSENSE, RG1, and RG2 are externally connected on film or wire-wound resistors, these are a straight the MAX472. VCC can be connected to either the band of metal. They are made in values under 1Ω. load/charge or power-source/battery side of the sense resistor. Connect VCC to the load/charge side of e)
Cost:
If the cost of RSENSE becomes an issue, you RSENSE if you want to include the MAX472 current drain may want to use an alternative solution, as shown in in the measured current. Figure 6. This solution uses the PC board traces to create a sense resistor. Because of the inaccuracies
Suggested Component Values
of the copper “resistor,” you will need to adjust the
for Various Applications
full-scale current value with a potentiometer. Also, The general circuit of Figure 4 is useful in a wide variety the resistance temperature coefficient of copper is of applications. It can be used for high-current applica- fairly high (approximately 0.4%/°C), so systems that tions (greater than 3A), and also for those where the full- experience a wide temperature variance should take scale load current is less than the 3A of the MAX471. this into account.
Table 1. Suggested Component Values for the MAX472 CURRENT- SCALE FULL-SCALE GAIN-SETTING OUTPUT FULL-SCALE SENSE FACTOR, TYPICAL ERROR AT X% LOAD RESISTORS, RESISTOR, OUTPUT RESISTOR, V OF FULL LOAD (%) CURRENT, RG1 = RG2 R VOLTAGE, OUT/ISENSE R OUT (V/A) I SENSE SENSE (A) (
Ω
) (k
Ω
) V (m
Ω
) OUT (V) 1% 10% 100%
0.1 500 200 10 2.5 25 14 2.5 0.9 1 50 200 10 2.5 2.5 14 2.5 0.9 5 10 100 5 2.5 0.5 13 2.0 1.1 10 5 50 2 2 0.2 12 2.0 1.6
_______________________________________________________________________________________ 9
Document Outline not-recomm-data-sheet-cover.pdf Not Recommended for New Designs