# Selecting External Components for an Automotive Remote Antenna Regulator and Current-Sense Amplifier

Abstract: This application note helps system designers choose the correct external components for use with the MAX16946 remote antenna regulator and current-sense amplifier (CSA), ensuring that automobile antenna-detection subsystems meet their performance objectives. An electronic calculator is provided that helps specify the critical external components for the MAX16946. The calculator also determines the device's operational ranges and analog output voltage accuracy.

## Introduction

**Figure 1**shows a typical application for the MAX16946. The main external components and their functions are as follows:

- R
_{SENSE}is the resistor across which the load current is sensed. The CSA measures and amplifies the voltage across this resistor. For this reason, the value of the sense resistor is important in determining the overall system accuracy. - R
_{5}and R_{6}set the regulator output voltage. - The capacitor at COMP ensures the stability of the regulator under all operating conditions.
- R
_{1}and R_{2}set the current limit during a fault condition. If the current remains at current limit for a blanking time of 100ms (min), the output is turned off, the active-low SC output is asserted low, and a retry is attempted after 1100ms. - R
_{3}and R_{4}set the threshold for the open-load detection. Below this load current, the active-low OL output asserts low. - The Schottky diode D
_{OUT}protects the MAX16946 from negative voltage transients on its OUT pin when the output is turned off and L_{OUT}attempts to maintain current flow. Without this diode, OUT might go below its absolute maximum voltage of -0.3V, which should not be allowed.

*Figure 1. Typical operating circuit of the MAX16946 remote antenna CSA and switch.*

**Figure 2**). In addition, the accuracy of the CSA's analog output voltage must be verified.

*Figure 2. Operation ranges for the current-sense amplifiers.*

## Calculating the Sense Resistor Value

_{SENSE}(Figure 1). The upper limit is represented by the short-circuit current threshold of 1.7V compared to AOUT. The maximum load current in the application should never exceed the short-circuit current threshold, otherwise a short-circuit will be erroneously indicated. Calculate an initial value for R

_{SENSE}using:

(Eq. 1) |

_{SC}is the short-circuit threshold.

(Eq. 2) |

_{SENSE}is the value of the sense resistor calculated in Equation 1, and R

_{SENSE-TOLERANCE}is the tolerance of the sense resistor.

_{SENSE(NOM)}. Alternatively, serial or parallel combinations of standard resistors can be used to attain the optimal value for the sense resistor.

## Calculating the Short-Circuit Current-Detection Range

(Eq. 3) |

_{SENSE(NOM)}is the sense resistor selected above.

_{SC}will vary within a certain range. The limits of this range are:

(Eq. 4) |

(Eq. 5) |

_{SENSE(MAX)}is the maximum value of the sense resistor (including its tolerance) and R

_{SENSE(MIN)}is its minimum value. The active-low short-circuit flag (SC) will thus assert low when the current falls between I

_{SC(MIN)}and I

_{SC(MAX)}.

## Setting the Output Voltage

_{5}and R

_{6}in Figure 1 set the output voltage of the MAX16946. The equation governing their values is:

(Eq. 6) |

_{FB}is the voltage at the feedback pin in regulation (1V nominal). The minimum and maximum values of the output voltage are then:

(Eq. 7) |

(Eq. 8) |

_{FB(MIN)}is 0.97V and V

_{FB(MAX)}is 1.03V (over the current range of 5mA to 150mA). R

_{5(MAX)}, R

_{5(MIN)}, R

_{6(MAX)}, and R

_{6(MIN)}are the maximum and minimum values of R

_{5}and R

_{6}, respectively.

## Setting the Current-Limit Range

(Eq. 9) |

_{LIM}is the desired current-limit threshold. Choose the standard value of 100kΩ for R

_{1}. R

_{2}can then be calculated:

(Eq. 10) |

(Eq. 11) |

(Eq. 12) |

_{1(MAX)}, R

_{1(MIN)}, R

_{2(MAX)}, and R

_{2(MIN)}are the maximum and minimum values of R

_{1}and R

_{2}, respectively.

## Setting the Open-Load Detection Threshold

(Eq. 13) |

_{OL}is the desired open-load threshold. Choose the standard value of 100kΩ for R

_{3}. R

_{4}can then be calculated:

(Eq. 14) |

_{3}and R

_{4}, the following equations can calculate the range of the open-load detection threshold:

(Eq. 15) |

(Eq. 16) |

_{3(MAX)}, R

_{3(MIN)}, R

_{4(MAX)}, and R

_{4(MIN)}are the maximum and minimum values of R

_{3}and R

_{4}, respectively.

## Measuring the Output Current by Means of the AOUT Voltage

_{SENSE}, and a defined load current, I

_{LOAD}, the worst-case range of voltage values measured at the CSA's output, AOUT, can now be calculated. The general expression for the voltage on AOUT is:

(Eq. 17) |

(Eq. 18) |

(Eq. 19) |

_{AOUT(MIN)}and V

_{AOUT(MAX)}.

(Eq. 20) |

(Eq. 21) |

_{TOL}, is:

(Eq. 22) |

## A Sample Calculation

(Eq. 23) |

(Eq. 24) |

(Eq. 25) |

## Short-Circuit Threshold

(Eq. 26) |

(Eq. 27) |

## Output Voltage

_{6}(after first selecting a value of 22kΩ for R

_{5}) according to the following equation:

(Eq. 28) |

(Eq. 29) |

(Eq. 30) |

## Current Limit

_{1}:

(Eq. 31) |

(Eq. 32) |

(Eq. 33) |

## Open-Load Detection Threshold

_{4}using the following equation (having first selected a value of 100kΩ for R

_{3}):

(Eq. 34) |

_{4}of 20kΩ, calculate the minimum and maximum values of the open-load threshold:

(Eq. 35) |

(Eq. 36) |

## AOUT Accuracy

(Eq. 37) |

(Eq. 38) |

(Eq. 39) |

(Eq. 40) |