# Selecting External Components for an Automotive Dual Remote Antenna Current-Sense LDO/Switch

Abstract: This application note helps system designers choose the correct external components for use with the MAX16948 dual remote antenna LDO/switch, thus ensuring that automobile-regulated phantom antenna supply and output-current-monitoring circuitry meet performance objectives. An electronic calculator is provided that helps specify the critical external components for the MAX16948, thus reducing design time. The calculator also determines the device's analog output voltage, output current-limit threshold, and output current-sensing accuracies. The calculator includes new automatic

**Step By Step**feature that assists designers with component choice. To use the new automatic feature, click on the

**Step By Step**button relative to the desired section.

## Introduction

## External Components for the MAX16948

**Figure 1**shows a typical application circuit for the MAX16948. The main external components and their functions are as follows:

- R1
_{_}and R2_{_}set the regulated output voltage when the device is in LDO mode with adjustable output voltage. - R
_{LIM_}sets the current-limit threshold. - R
_{SENSE}determines the ADC full-scale input voltage and the output current-sensing accuracy.

*Figure 1. The MAX16948 typical application circuit.*

## Input Capacitors

_{IN}may result in a temporary dip in the input voltage, which may trigger the reverse current detection fault. Occurrence of this erroneous fault detection is accentuated at low output current (under 80mA). To avoid this false trigger event, use an input electrolytic capacitor of at least 100µF.

## LDO Mode Output Voltage

*Figure 2. The MAX16948 regulated phantom supply.*

_{_}and R2

_{_}(

**Figure 2**) set the output voltage of the MAX16948. Choose an R2

_{_}standard resistor (R2

_{_(STD)}) that is less than or equal to 1kΩ. Calculate the optimum R1

_{_}value with the following equation:

_{FB_}is the voltage at the feedback pin in regulation (1V nominal).

_{_(STD)}) that is as close as possible to R1

_{_}has been selected, the typical output voltage is:

_{TOL}), the minimum and maximum values of the output voltage are:

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

_{FB_(MAX)}is 1.03V (over an output current range from 5mA to 150mA). R1

_{_(MIN)}, R1

_{_(MAX)}, R2

_{_(MIN)}, and R2

_{_(MAX)}are the minimum and maximum values of R1

_{_}and R2

_{_}, respectively:

_{_}by filling in the appropriate section of the MAX16948 Calculator with the desired output voltage (V

_{OUT_}), the standard value of R2

_{_}, and the tolerance on these resistors. Once the calculator has determined the optimum R1

_{_}value, insert a standard value in the correct cell in order to estimate the V

_{OUT_}range limits. Otherwise, click on the

**Step By Step**button to be guided through the calculator's LDO Mode Output Voltage section.

*Absolute Maximum Ratings*section of the data sheet. A Schottky diode is required as a clamp if transients are expected to go below ground, especially if an output inductance before the load is present on the PCB. The forward voltage of the selected Schottky diode must be under 0.3V with a forward current equal to the current-limit threshold.³

## Current-Limit Threshold

_{LIM}threshold (2.5V typ). The current sourced from LIM_ is proportional to the load current sourced from OUT_ by a factor called current-sense amplifier (CSA) gain. In this way, the current-limit threshold I

_{LIM_}is set with the R

_{LIM_}resistor.

_{LIM_}value using the following equation:

_{LIM_}is the desired current-limit threshold, V

_{LIM(TYP)}is the typical voltage threshold on the LIM_ pin where the output current is limited (2.5V), and A

_{(TYP)}is the typical value of the CSA gain (0.005mA/mA).

_{LIM_(STD)}) as close as possible to R

_{LIM_}. Then the typical current-limit threshold is:

_{LIM_(MIN)}is 2.375V and V

_{LIM_(MAX)}is 2.625V; A

_{(MIN)}is 0.00485mA/mA and A

_{(MAX)}is 0.00515mA/mA (for a typical output current of 100mA); and R

_{LIM_(MIN)}and R

_{LIM_(MAX)}are the minimum and maximum values of R

_{LIM_}based on the value of R

_{TOL}.

^{4}helps the user choose the R

_{LIM_}resistor by selecting the current-limit threshold range limit (I

_{LIM_(TYP)}, I

_{LIM_(MIN)}, or I

_{LIM_(MAX)}) to calculate the optimum value of R

_{LIM_}. This is done by selecting the I

_{LIM_(TYP)}, I

_{LIM_(MIN)}, or I

_{LIM_(MAX)}; the relative current-limit threshold value; and the tolerance on R

_{LIM_}. Once the calculator has determined the optimum R

_{LIM_}value, insert a standard resistor value in the appropriate cell in order to estimate the I

_{LIM_}range limits. Otherwise, click on the

**Step By Step**button to be guided through the calculator's Current-Limit Threshold section.

_{LIM_}) must be placed in parallel with R

_{LIM_}to establish a dominant pole in the current limiting loop. This maintains stability and prevents fast current transients from prematurely triggering the current limit (

**Figure 3**).

*Figure 3. The MAX16948 output-current limit.*

## ADC Input Range and Output Current-Sensing Accuracy

_{SENSE_}produces a voltage proportional to the output current. With this feature, it is possible to monitor the output current by sampling the voltage on the SENSE_ pin with an ADC.

_{ADCFS_}) is set with R

_{SENSE_}.

_{SENSE_}value using the following equation:

_{LIM_(TYP)}is the typical current-limit threshold calculated in the previous section and A

_{(TYP)}is the typical value of the CSA gain (0.005mA/mA).

_{SENSE_(STD)}) as close as possible to R

_{SENSE_}. Then the typical ADC full-scale input voltage is:

_{TOL}), the minimum and maximum values of the ADC full-scale input voltage are:

_{LIM_(MIN)}is 2.375V and V

_{LIM_(MAX)}is 2.625V; R

_{LIM_(MIN)}and R

_{LIM_(MAX)}are been calculated in the previous section; and R

_{SENSE_(MIN)}and R

_{SENSE_(MAX)}are the minimum and maximum values of R

_{SENSE_}based on the value of R

_{TOL}.

^{5}, the user can easily determine the R

_{SENSE_}value after inserting the desired ADC full-scale input voltage (V

_{ADCFS_}) and the tolerance on R

_{SENSE_}. Once the calculator has determined the optimum R

_{SENSE_}value, insert a standard value in the corresponding cell in order to estimate the V

_{ADCFS_}range limits. Otherwise, click on the

**Step By Step**button to be guided through the calculator's Output Current Sensing section.

_{LIM_}range limits in the calculator's Current-Limit Threshold section.

_{SENSE}) in parallel with R

_{SENSE_}to hold the voltage during ADC sampling cycles (

**Figure 4**).

*Figure 4. ADC input range.*

_{ADC_}), the output current can be estimated by the following formula:

_{SENSE_}tolerance and the parameter spread on the CSA gain, the estimated output current can vary between the following two equations:

## Detection of Open-Load and Overcurrent Conditions

_{SENSE_}into resistive dividers (R3

_{_}, R4

_{_}, and R5

_{_}) (

**Figure 5**).

*Figure 5. Open-load and overcurrent detection.*

Table 1. Operating State of the Circuit | ||

OC | OL | State |

0 | 0 | Normal Operation |

0 | 1 | Open-Load Condition |

1 | 0 | Overcurrent Condition |

1 | 1 | Invalid State |

_{_}value with the following equation:

_{OPEN-LOAD_}is the desired open-load threshold for the current flowing through the LDO/switch, V

_{OL_,TH}is the open-load voltage threshold for comparator U2

_{_}, and A

_{(TYP)}is the typical value of the CSA gain (0.005mA/mA). Once the R5

_{_}resistor value has been calculated, determine the R4

_{_}value using the following equation:

_{OVERCURRENT_}, which must be less than I

_{LIM_(MIN)}, is the desired overcurrent threshold for the current flowing through the LDO/switch; V

_{OC_,TH}is the overcurrent voltage threshold for comparator U1

_{_}; and A

_{(TYP)}is the typical value of the CSA gain (0.005mA/mA).

_{_}:

_{_}= R

_{SENSE_}- R4

_{_}- R5

_{_}

_{_}, R4

_{_}, and R5

_{_}standard resistors (R

_{TOL}), the minimum, typical, and maximum values of the open-load and overcurrent threshold are:

_{(MIN)}is 0.00485mA/mA and A

_{(MAX)}is 0.00515mA/mA (for a typical output current of 100mA); R4

_{_(MIN)}, R4

_{_(MAX)}, R5

_{_(MIN)}, and R5

_{_(MAX)}are the minimum and maximum values of R4

_{_}and R5

_{_}based on the tolerance of R

_{TOL}.

_{_}, R5

_{_}, and R6

_{_}using the Open-Load & Overcurrent Detection section of the calculator

^{6}. Insert I

_{OPEN-LOAD_}(I

_{OL_}), I

_{OVERCURRENT_}(I

_{OC_}), V

_{OL_,TH}, V

_{OC_,TH}, and the tolerance on the resistors. Once you have used the calculator to determine the optimum values for R4

_{_}, R5

_{_}, and R6

_{_}, insert standard resistor values in the appropriate cells in order to estimate the I

_{OPEN-LOAD_}and I

_{OVERCURRENT_}range limits. Click on the

**Step By Step**button to be guided through the calculator's Open-Load & Overcurrent Detection section.

_{SENSE_}value in the Output Current Sensing section.

## Example Calculations

_{_}and R2

_{_}must be dimensioned. Choose a standard value for R2

_{_}(R2

_{_(STD)}) less than or equal to 1kΩ and calculate R1

_{_}:

_{_(STD)}= 750Ω, R

_{TOL}= 1% (E96 Series)

_{_(STD)}= 3010Ω), the output voltage range is:

_{LIM_(MAX)}) equal to 120mA, R

_{LIM_}can be calculated after rearranging the I

_{LIM_(MAX)}equation:

_{LIM_}(STD) = 4530Ω), the current-limit threshold range is:

_{SENSE_}as follows:

_{SENSE_(STD)}= 6040O, R

_{TOL}= 1%), the ADC full-scale input voltage range is:

_{ADCFS_(TYP)}= R

_{SENSE_(STD)}× I

_{LIM_(TYP)}× A

_{(TYP)}= 6040Ω × 110.375mA × 0.005 = 3.333V

_{ADCFS_(MIN)}= R

_{SENSE_(MIN)}× I

_{LIM_(MIN)}× A

_{(MIN)}= 5979.6Ω × 100.794mA × 0.00485 = 3.104V

_{ADCFS_(MAX)}= R

_{SENSE_(MAX)}× I

_{LIM_(MAX)}× A

_{(MAX)}= 6100.4Ω × 120.685mA × 0.00515 = 3.571V

#### References

- An underscore after a pin name is used to indicate the possibility of multiple channels (e.g., SENSE_ means SENSE1/SENSE2).
- A Standard Resistor Table is available in the MAX16948 Calculator.
- See the following paragraph.
- Default A
_{(MIN)}and A_{(MAX)}are used by the MAX16948 Calculator for a typical output current of 100mA; it is possible to modify these values by changing them in the MAX16948 EC sheet. - Default A
_{(MIN)}and A_{(MAX)}are used by the MAX16948 Calculator for a typical output current of 100mA; it is possible to modify these values by changing them in the MAX16948 EC sheet. - Default A
_{(MIN)}and A_{(MAX)}are used by the MAX16948 Calculator for a typical output current of 100mA; it is possible to modify these values by changing them in the MAX16948 EC sheet.