MCXA156VLH ADC INTERNAL DIODE RATINGS we require details of ratings of diodes mentioend in Pg:36 - Figure 12. ADC input impedance equivalency diagram. We want to know tolerable rating to design our application accordingly. Analog(ADC|CMP|DAC|OpAmps) Board Design Re: MCXA156VLH ADC INTERNAL DIODE RATINGS Hello @TEMCEFF ,
Thanks for your post. Please refer to our datasheet:
For MCXA156, the DC injection current limit is ±3 mA per I/O pin when VIN goes below VSS − 0.3 V or above VDD + 0.3 V. In addition, the regional injection-current limit for 16 contiguous pins is ±25 mA. If the input voltage can exceed these voltage boundaries, an external current-limiting resistor should be used.
Hope it helps.
BR
Celeste Re: MCXA156VLH ADC INTERNAL DIODE RATINGS Thank you for providing the ADC injection current limits (±3 mA per I/O pin and ±25 mA regional limit). We are designing an analog front-end for the MCXA156 ADC and would like to better understand the behavior and limitations of the internal protection structure shown in the ADC input equivalent circuit (Figure 12 / ADC input impedance equivalency diagram). Specifically, we would appreciate clarification on the following points: Clamp Behavior When an ADC input voltage exceeds VDD_ANA (for example, VDD_ANA = 3.3 V and VIN = 5 V), does the internal protection structure clamp the input voltage? If so, what is the typical voltage at the ADC pin during clamping (for example, approximately VDD_ANA + 0.3 V, VDD_ANA + 0.7 V, etc.)? Clamp Characteristics Is there any characterization data available for: Clamp voltage versus injection current Dynamic resistance of the protection structure Forward voltage of the internal clamp path Overvoltage / Withstand Capability If the injection current is externally limited to less than the specified ±3 mA limit, is there a maximum ADC input voltage that can be tolerated without device damage? For example, can the ADC input safely withstand temporary 5 V, 12 V, or higher transient voltages provided the injection current remains below the specified limit? Transient and Surge Capability Is there any available information regarding: Maximum transient pulse current Maximum transient duration Maximum energy that the internal protection structure can absorb System-Level Protection Recommendation Should the internal protection network be considered only as an ESD/injection-current protection structure, requiring external protection components for repetitive overvoltage conditions? Our goal is to determine the required external protection circuitry and resistor sizing for ADC channels that may be exposed to fault conditions or short-duration overvoltage spikes. Re: MCXA156VLH ADC INTERNAL DIODE RATINGS Hello @TEMCEFF ,
Thank you for the detailed questions.
I checked the available public documentation for the MCXA156. Some of your questions can be answered based on the datasheet, while the detailed characteristics of the internal protection structure will require further confirmation from our internal team.
Based on the datasheet:
The I/O pins have internal clamping to VSS and VDD through the ESD protection structure.
The maximum DC injection current is ±3 mA per pin, with a ±25 mA regional limit for 16 contiguous pins.
For positive injection current, the datasheet defines the injection condition as VIN > VDD + 0.3 V.
The datasheet provides the following equation for calculating the current-limiting resistor for positive injection:
R = (VIN - VDD_MAX) / IICIOmax
For transient voltages, the datasheet recommends using a resistor value approximately one order of magnitude higher than the value calculated from the basic equation.
However, VDD + 0.3 V should not be interpreted as a guaranteed or fixed clamp voltage. The public documentation does not provide detailed characterization of the clamp I-V behavior.
I also could not find public specifications for the following items:
Clamp voltage versus injection current
Dynamic resistance or detailed forward-voltage characteristics of the internal clamp
Maximum externally applied voltage, such as 5 V, 12 V, or higher, when the injection current is limited below 3 mA
Maximum transient pulse current and pulse duration
Maximum energy that the internal protection structure can absorb
Qualification of the internal clamp for repetitive system-level overvoltage conditions
Therefore, I do not want to make assumptions about these limits based only on the injection-current specification. These questions need to be escalated to our internal team for further confirmation.
According to our escalation process, could you please provide the following information?
End customer name
End application name
Once we have this information, we can proceed with the escalation.
BR
Celeste
Re: MCXA156VLH ADC INTERNAL DIODE RATINGS Consider below mentioned details as requested. End customer name - Indian Railways End application name - Intelligent Field device Application description - IoT device to monitor and control Railway assets like Lighting, Escalators, lift etc; Re: MCXA156VLH ADC INTERNAL DIODE RATINGS Hello @TEMCEFF ,
Thanks for your patience. Please see the reply from our internal:
"
Although the datasheet specifies ±3 mA per-pin and ±25 mA regional DC injection-current limits, these limits do not override the ADC/I/O input-voltage Absolute Maximum Rating. Therefore, keeping the injection current below 3 mA alone does not guarantee that temporary 5 V, 12 V, or higher inputs are safe.
The internal ESD clamp is not characterized in the datasheet with guaranteed clamp voltage, I-V curve, dynamic resistance, or transient pulse/energy capability. Therefore, we do not recommend relying on the internal clamp for repetitive or system-level fault protection.
For this application, we recommend external protection, such as a series resistor combined with a suitable low-forward-voltage Schottky clamp, to keep the ADC pin within the specified voltage limits.
"
If you need any further details, please let me know.
BR
Celeste
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