MAX49921 datasheet(11/14 Pages) MAXIM

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Applications Information

Recommended Component Values

Ideally, the maximum load current develops the full-scale sense voltage across the current-sense resistor. Choose the

gain needed to yield the maximum output voltage required for the application using the following equation:

VOUT = VFS x AV

where VFS is the full-scale sense voltage, 100mV for gain of 20V/V, 90mV for gain of 50V/V, and AV is the gain of the

device.

The MAX49921 senses a wide variety of currents with different sense-resistor values. In applications monitoring a high

current, ensure that RSENSE is able to dissipate its own I2R loss. If the resistor’s power dissipation exceeds the nominal

value, its value may drift or it may fail altogether.

Choosing the Sense Resistor

Choose RSENSE based on the following criteria:

Voltage Loss

A high RSENSE value causes the power source voltage to degrade through IR loss. For minimal voltage loss, use the

lowest RSENSE value.

Accuracy

A high RSENSE value allows lower currents measured more accurately. This is due to offsets becoming less significant

when the sense voltage is larger. For best performance, select RSENSE to provide approximately 100mV (gain of 20V/

V), 90mV (gain of 50V/V) of sense voltage for the full-scale current in each application.

Efficiency and Power Dissipation

At high current levels, the I2R losses in RSENSE can be significant. Consider this when choosing the resistor value and

its power dissipation (wattage) rating. In addition, the sense resistor’s value might drift if it heats up excessively.

Stray Inductance

The stray inductance due to package parasitics in the current sense resistor should be kept to a minimum. The

unwanted voltage error produced by the stray inductance is proportional to the magnitude of the load current. Wire-wound

resistors have the highest inductance, while metal film is comparably better. Low-inductance, metal-film resistors are also

available. Instead of being spiral wrapped around a core, as in metal-film or wire-wound resistors, they are straight bands

of metal and are available in values under 100mΩ.

Important Considerations

Due to the high currents that may flow through RSENSE, be sure to eliminate solder and parasitic trace resistance to

keep from causing errors in the sense voltage. Either use a four-terminal current sense resistor or use Kelvin (force and

sense) PCB layout techniques.

Power-Supply Bypassing

Power-supply bypass capacitors are recommended for best performance and should be placed as close as possible

to the supply VDD and ground terminals of the device. A typical value for this supply bypass capacitor is 0.1μF (NP0/

C0G type) close to the VDD/GND pins. The capacitors should be rated for at least twice the maximum expected applied

voltage. Applications with noisy or high-impedance power supplies may require additional decoupling capacitors to reject

power-supply noise.

MAX49921

0 to 70V, High-Precision Current-Sense Amplifier

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