
Keywords: automotive, automotive antenna protector, phantom power, active antenna, active antenna power protector, phantom power antenna protector, automotive antenna, LDO, LNA, low dropout linear regulator, currentsense, remote antenna diagnostic
Related Parts


Selecting External Components for an Automotive Dual Remote Antenna CurrentSense 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 automobileregulated phantom antenna supply and outputcurrentmonitoring 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 currentlimit threshold, and output currentsensing 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
The
MAX16948 is a dual highvoltage low dropout linear regulator (LDO)/switch with outputcurrent sensing. The device provides phantom power over a coax cable to remote radio frequency (RF) lownoise amplifiers (LNAs) in automotive systems with a maximum current of 300mA per channel. The MAX16948 is designed to operate with an input voltage range from 4.5V to 28V (45V load dump tolerant).
The device provides a fixed regulated output voltage of 8.5V or an adjustable 1V to 12V regulated output voltage (LDO mode). The device can also be configured as a switch (SW mode).
The MAX16948 monitors the load current for each channel and provides two analog output currents proportional to the sensed output current (sourced from the SENSE_¹ pin). Accurate internal adjustable current limits protect the input supply against both overcurrent and shortcircuit conditions.
The device features shorttobattery protection, reverse current detection, input overvoltage and thermal overload shutdown to latch off the internal LDO/switch during these fault conditions. The MAX16948 includes two independent activelow, highvoltagecompatible shutdown inputs (activelow SHDN_) to place each channel in lowpower shutdown mode and two activelow opendrain error outputs (activelow ERR_).
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 currentlimit threshold.
 R_{SENSE} determines the ADC fullscale input voltage and the output currentsensing accuracy.
Figure 1. The MAX16948 typical application circuit.
With these external components, the user can easily configure the MAX16948 to work properly for an application. Other external components are necessary. They will be discussed below.
Input Capacitors
Connect a parallel electrolytic capacitor and a lowESR ceramic capacitor from IN and GND to limit the inputvoltage drop during momentary output shortcircuit conditions and to protect the device against transients due to inductance on the IN line. For example, use at least a 0.1µF ceramic capacitor in parallel with at least a 10µF electrolytic capacitor if the input inductance (including any stray inductance) is estimated to be 20µH.
When the MAX16948 is operated with input voltage close to the output voltage, as in SW mode operation or LDO mode in dropout, care must be taken to avoid a false reversecurrent detection in the presence of a short circuittoground fault. If both channels are enabled and one channel is shorted to ground after startup, the current drawn from C_{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
When used in LDO mode, the device is configurable to provide a fixed 8.5V output or an adjustable 1V to 12V output voltage for each of the two channels. 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 as the tolerance of external resistors does not need to be taken into account.
In case a different output voltage is necessary, connect a resistive divider between OUT_, FB_, and GND. Equations to determine the resistive divider value are shown below.
Figure 2. The MAX16948 regulated phantom supply.
The resistors R1_{_} 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:
Where V_{FB_} is the voltage at the feedback pin in regulation (1V nominal).
Once a standard resistor² R1_{_(STD)}) that is as close as possible to R1_{_} has been selected, the typical output voltage is:
Considering the tolerance on the resistors (R_{TOL}), the minimum and maximum values of the output voltage are:
And
Where V_{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:
If the nominal output voltage is too far removed from the desired output voltage, serial or parallel combinations of standard resistors can be used to obtain the optimal resistor divider.
The user can easily determine the value of R1
_{_} 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.
Connect > 1µF in parallel with 0.1µF lowESR (< 900mΩ) capacitors between OUT_ and GND for regulator stability. These capacitors should be placed as close to the device as possible. Utilize capacitors with an X7R dielectric to ensure stability over the operating temperature range of the device.
In analogous fashion to the input capacitor, an output capacitor protects the device against transients due to any series inductance in the output. Under no conditions should the voltage on OUT_ go below 0.3V, as indicated in the 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 currentlimit threshold.³
CurrentLimit Threshold
The MAX16948 limits the output current of OUT_ when the voltage on the LIM_ pin reaches the V_{LIM} threshold (2.5V typ). The current sourced from LIM_ is proportional to the load current sourced from OUT_ by a factor called currentsense amplifier (CSA) gain. In this way, the currentlimit threshold I_{LIM_} is set with the R_{LIM_} resistor.
Determine the optimum R_{LIM_} value using the following equation:
Where I_{LIM_} is the desired currentlimit 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).
Select a standard resistor value (R_{LIM_(STD)}) as close as possible to R_{LIM_}. Then the typical currentlimit threshold is:
Considering the uncorrelated tolerances, the worstcase currentlimit threshold range is between the following two equations:
And
Where V_{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}.
If the currentlimit threshold range is not suitable, a serial or parallel combination of standard resistors can be used to obtain the optimal range.
The
MAX16948 Calculator^{4} helps the user choose the R
_{LIM_} resistor by selecting the currentlimit 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 currentlimit 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 CurrentLimit Threshold section.
A 0.1µF compensation capacitor (C_{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 outputcurrent limit.
ADC Input Range and Output CurrentSensing Accuracy
The MAX16948 sources a current proportional to the load current from the OUT_ pin by the CSA gain. This current flowing across R_{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.
The ADC fullscale input voltage (V_{ADCFS_}) is set with R_{SENSE_}.
Determine the optimum R_{SENSE_} value using the following equation:
Where I_{LIM_(TYP)} is the typical currentlimit threshold calculated in the previous section and A_{(TYP)} is the typical value of the CSA gain (0.005mA/mA).
Select a standard resistor value (R_{SENSE_(STD)}) as close as possible to R_{SENSE_}. Then the typical ADC fullscale input voltage is:
Considering the tolerance on the resistors (R_{TOL}), the minimum and maximum values of the ADC fullscale input voltage are:
And
Where V_{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}.
Using the Output Current Sensing section of the electronic calculator^{5}, the user can easily determine the R_{SENSE_} value after inserting the desired ADC fullscale 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.
However, before using this section, it is necessary to determine the I_{LIM_} range limits in the calculator's CurrentLimit Threshold section.
Place a 0.1µF capacitor (C_{SENSE}) in parallel with R_{SENSE_} to hold the voltage during ADC sampling cycles (Figure 4).
Figure 4. ADC input range.
Once the voltage on the SENSE_ pin has been measured with the ADC (V_{ADC_}), the output current can be estimated by the following formula:
Due to the R_{SENSE_} tolerance and the parameter spread on the CSA gain, the estimated output current can vary between the following two equations:
And
Replace the value of the CSA gain for a typical output current of 100mA:
Finally, the accuracy of the current measurement made by the ADC is:
This parameter can also be determined in the O.C.S. Accuracy row of the calculator's Output Current Sensing section.
Detection of OpenLoad and Overcurrent Conditions
In addition to ADC sampling, openload or overcurrent conditions can be detected by utilizing external comparators and splitting R_{SENSE_} into resistive dividers (R3_{_}, R4_{_}, and R5_{_}) (Figure 5).
Figure 5. Openload and overcurrent detection.
The output of the comparators (OC_ and OL_) indicates the operating state of the circuit as shown Table 1.
Table 1. Operating State of the Circuit 
OC 
OL 
State 
0 
0 
Normal Operation 
0 
1 
OpenLoad Condition 
1 
0 
Overcurrent Condition 
1 
1 
Invalid State 
Determine the R5_{_} value with the following equation:
Where I_{OPENLOAD_} is the desired openload threshold for the current flowing through the LDO/switch, V_{OL_,TH} is the openload 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:
Where I_{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).
Finally, calculate R3_{_}:
R3_{_} = R_{SENSE_}  R4_{_}  R5_{_}
Considering the tolerance of the R3_{_}, R4_{_}, and R5_{_} standard resistors (R_{TOL}), the minimum, typical, and maximum values of the openload and overcurrent threshold are:
Where A_{(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}.
The user can determine the values of R4_{_}, R5_{_}, and R6_{_} using the OpenLoad & Overcurrent Detection section of the calculator^{6}. Insert I_{OPENLOAD_} (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_{OPENLOAD_} and I_{OVERCURRENT_} range limits. Click on the Step By Step button to be guided through the calculator's OpenLoad & Overcurrent Detection section.
Before using the calculator's OpenLoad and Overcurrent Detection section, it is necessary to determine the R_{SENSE_} value in the Output Current Sensing section.
Example Calculations
For these example calculations, we assume an antenna phantom supply application where the supply input voltage is 5V, the typical load current is 100mA, the maximum currentlimit threshold value is 120mA, and the output current is monitored with a 3.3V input range ADC.
Assuming that the IN pin of the MAX16948 is directly connected to the battery, the device must be configured in LDO mode to obtain an OUT_ voltage of 5V. For this purpose, the external resistor divider composed of R1_{_} and R2_{_} must be dimensioned. Choose a standard value for R2_{_} (R2_{_(STD)}) less than or equal to 1kΩ and calculate R1_{_}:
R2_{_(STD)} = 750Ω, R_{TOL} = 1% (E96 Series)
Selecting a standard resistor from the E96 Series R1_{_(STD)} = 3010Ω), the output voltage range is:
To obtain a maximum currentlimit threshold (I_{LIM_(MAX)}) equal to 120mA, R_{LIM_} can be calculated after rearranging the I_{LIM_(MAX)} equation:
Selecting a standard resistor from the E96 Series (R_{LIM_}(STD) = 4530Ω), the currentlimit threshold range is:
With an ADC input range of 3.3V, calculate the optimum R_{SENSE_} as follows:
Selecting a standard resistor from the E96 Series (R_{SENSE_(STD)} = 6040O, R_{TOL} = 1%), the ADC fullscale input voltage range is:
V_{ADCFS_(TYP)} = R_{SENSE_(STD)} × I_{LIM_(TYP)} × A_{(TYP)} = 6040Ω × 110.375mA × 0.005 = 3.333V
V_{ADCFS_(MIN)} = R_{SENSE_(MIN)} × I_{LIM_(MIN)} × A_{(MIN)} = 5979.6Ω × 100.794mA × 0.00485 = 3.104V
V_{ADCFS_(MAX)} = R_{SENSE_(MAX)} × I_{LIM_(MAX)} × A_{(MAX)} = 6100.4Ω × 120.685mA × 0.00515 = 3.571V
The accuracy of the output current monitoring is then as follows:
*For more details on these calculations, refer to the
MAX16948 data sheet.
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.
Related Parts 
MAX16948 
Automotive Dual Remote Antenna CurrentSense LDO/Switch 

© Jan 03, 2012, Maxim Integrated Products, Inc.

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