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

The MAX16946 is a high-voltage regulator with a precision current-sense amplifier (CSA), designed to provide phantom power to remote radio antennas in automotive applications. The device provides short-circuit protection, current-limit protection, and open-load detection. The current levels for current-limit protection and open-load detection are programmable with external resistors. To ensure that antenna detection circuitry meets performance objectives, the design engineer must choose the correct external components for the design.

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.
• R₅ and R₆ set the regulator output voltage.
• The capacitor at COMP ensures the stability of the regulator under all operating conditions.
• R₁ and R₂ 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.
• R₃ and R₄ 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.

When working with CSAs and switches for antenna applications, the designer must often determine the ranges for open load, normal operation, short circuit, and current limiting (Figure 2). In addition, the accuracy of the CSA's analog output voltage must be verified.

Use the MAX16946 Calculator to determine the correct values for the sense resistor and the resistive dividers, which set the current-limit and the open-load thresholds. It takes into account the tolerances of both the external components and the MAX16946. By calculating the tolerance ranges for each of the design parameters, the designer ensures that the design parameters are within the limits imposed by the system specification.

Calculating the Sense Resistor Value

Ideally, the maximum load current develops the full-scale sense voltage across the current-sense resistor, R_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:

Where 1.7V is the short-circuit current threshold, 0.4V is the zero-current offset of AOUT, 26V/V is the gain of the current-sense amplifier, and I_SC is the short-circuit threshold.

Because the sense resistor has some tolerance, the nominal value needed will be lower than the value calculated in Equation 1. To calculate for the tolerance:

Where R_SENSE is the value of the sense resistor calculated in Equation 1, and R_{SENSE-TOLERANCE} is the tolerance of the sense resistor.

Normally one would choose the closest smaller standard value for R_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

After choosing the nominal value of the sense resistor, next the typical current through the sense resistor needs to be determined. This current, which allows a short circuit to be detected, can be calculated as follows:

Where R_SENSE(NOM) is the sense resistor selected above.

However, since the short-circuit current threshold (1.7V), the CSA gain (26V/V), and the AOUT zero-current offset voltage (0.4V) have uncorrelated tolerances (i.e., have minimum and maximum values that vary independently of each other), the value of I_SC will vary within a certain range. The limits of this range are:

And

Where R_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

The resistors R₅ and R₆ in Figure 1 set the output voltage of the MAX16946. The equation governing their values is:

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

And

Where V_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 R₅ and R₆, respectively.

Note that the output voltage can be set to 8.5V by connecting the FB pin to REG. In this mode, higher output voltage accuracy is achieved because the tolerance of external resistors does not need to be taken into account.

Setting the Current-Limit Range

The MAX16946 limits its output current when the AOUT voltage reaches the voltage on the LIM pin, which is set using a resistor-divider between REF, LIM, and GND. The nominal REF voltage is 3V. Thus, the following equation applies:

Where I_LIM is the desired current-limit threshold. Choose the standard value of 100kΩ for R₁. R₂ can then be calculated:

Considering uncorrelated tolerances, the worst-case current-limit range is:

And

Where R_1(MAX), R_1(MIN), R_2(MAX), and R_2(MIN) are the maximum and minimum values of R₁ and R₂, respectively.

Setting the Open-Load Detection Threshold

The open-load threshold for the MAX16946 can be adjusted externally with a resistor-divider placed between REF, OLT, and GND. The following equation applies:

Where I_OL is the desired open-load threshold. Choose the standard value of 100kΩ for R₃. R₄ can then be calculated:

After defining the values of R₃ and R₄, the following equations can calculate the range of the open-load detection threshold:

And

Where R_3(MAX), R_3(MIN), R_4(MAX), and R_4(MIN) are the maximum and minimum values of R₃ and R₄, respectively.

Measuring the Output Current by Means of the AOUT Voltage

With a given sense resistor, R_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:

If we again take into consideration all uncorrelated tolerances, the AOUT voltage will lie between the following equations:

And

In other words, the sensed current produces a worst-case AOUT voltage variation between V_AOUT(MIN) and V_AOUT(MAX).

Normally the AOUT voltage is measured using the ADC of a microcontroller and the load current is then calculated based on the nominal values of all parameters. With the above worst-case AOUT voltages, the microcontroller would then conclude that the current is within range of the following two values:

And

The tolerance of the current measurement made by the ADC, I_TOL, is:

A Sample Calculation

For the following example, we assume an antenna phantom supply application where the upper end of the normal operation range is set at 100mA and the antenna requires a regulated voltage of 5V. If we set the short-circuit threshold 10% higher at 110mA, then the initial value of the sense resistor is:

When using a resistor with a 1% tolerance, the maximum nominal value of the sense resistor is:

If the next-lowest E12 series value of 0.39Ω is selected, the typical value for short-circuit detection with this resistor can be calculated:

Short-Circuit Threshold

The minimum and maximum values of the short-circuit detection threshold can then be determined using the minimum and maximum values of the sense resistor (0.386Ω and 0.394Ω, assuming a 1% type is used):

And

Output Voltage

The output voltage of 5V is set by selecting resistor R₆ (after first selecting a value of 22kΩ for R₅) according to the following equation:

If the nearest E12 series resistor of 5600Ω is selected, the nominal output voltage will be 4.93V and the variation of the output voltage will be between:

And

Current Limit

Next, the resistors can be selected to set the output current limit. Assuming a current limit of approximately 200mA, use a 100kΩ resistor for R₁:

Choosing the nearest E12 standard value of 390kΩ gives an actual current limit of 0.196A. Considering all tolerances and assuming 1% resistors are used, the minimum and maximum values for the current-limit range are:

And

Open-Load Detection Threshold

To set a nominal open-load detection current of 10mA, select R₄ using the following equation (having first selected a value of 100kΩ for R₃):

Using a standard resistor for R₄ of 20kΩ, calculate the minimum and maximum values of the open-load threshold:

And

AOUT Accuracy

To evaluate the analog output (AOUT) accuracy, we assume the same sense resistor selected above (0.39Ω) and evaluate the accuracy at a load current of 100mA. At this current, the minimum and maximum values of the AOUT voltage are:

And

Taking these voltages and using the microcontroller's software to calculate these voltages back to current (i.e., using the typical values from the data sheet), a range of an evaluated current can be derived between:

And

Thus, the range of error in the measurement made by the microcontroller is ±6.7% at 100mA. This range does not take into account other errors in the ADC measurement such as reference error, quantization error, etc.