Remote Antenna Current-Sense

Amplifier and Switches

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9

Applications Information

Choosing the Sense Resistor

Ideally, the maximum load current develops the full-

scale sense voltage across the current-sense resistor.

The current-sense amplifier output voltage is given by:

so that the differential voltage across IN and SENS does

not exceed the minimum full-scale sense voltage (87mV):

maximum load current.

Use resistors specified for current-sensing applications

with a minimum resistance value of 0.65Ω, and the

maximum resistance value of 4.7Ω. Keep inductance

Wire-wound resistors have the highest inductance,

while metal film is somewhat 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 a straight band of metal and

are available in values under 1Ω. Because of the high

resistance from causing errors in the sense voltage.

Open-Load Threshold Selection

For the MAX16913A, a resistive divider between REF,

OLT, and GND sets the open-load threshold. See

Figure 3.

Use the following formula to set the desired open-load

threshold:

impedance of the internal reference (5kΩ) is negligible

effect on the accuracy of the adjusted open-load

threshold.

For example, to set the open-load threshold at 10mA,

using a 1Ω sense resistor, use the following method to

Input Capacitor

Connect a low-leakage ceramic capacitor from IN to

GND to limit the input voltage drop during momentary

output short-circuit conditions, and to protect the

device against transients due to inductance in the IN

line. For example, use at least a 0.1µF ceramic capaci-

tor if the input inductance (including any stray induc-

tance) is estimated to be 20µH. Larger capacitor values

reduce the voltage undershoot at the input.

Output Capacitor

In an analogous fashion to the input capacitor, an out-

put capacitor protects the device against transients

due to any series inductance in the output. Under no

conditions should the OUT pin voltage go below -0.3V

as specified in the

Absolute Maximum Ratings. If a

capacitor alone is not sufficient to avoid large negative

transients on OUT, then a Schottky diode should be

used to clamp transients which go below ground. With

a 100µH output series inductor, a 220µF output capaci-

tor is needed to eliminate potential problems. With larg-

er inductor values or smaller capacitors, a Schottky

clamp diode will be necessary.

Layout and Thermal Dissipation

To optimize the switch response time to output short-

circuit condition, it is very important to keep all traces

as short as possible to reduce the effect of undesirable

parasitic inductance. Place input and output capacitors

as close as possible to the device (no more than 5mm).

R(k )

(R +R )(k )

(1

(V/V))+ 0.4V

3(V)

0.177

2

12

Ω

Ω

Ω

=

××

=

()

.

( )

001

13

A

R(k )

(R +R )(k )

(R

I

(V/V))+ 0.4V

V

(V)

2

12

SENSE

OL

V

REF

Ω

Ω

Ω

=

××

()

( )

AA

R

V

(V)

I

(A)

SENSE

DIFF(MIN)

LOAD(FULL-SCALE)

()

Ω=

MAX16913

REF

OLT

Figure 3. Open-Load Threshold Selection