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Texas Instruments ESD Diode Current Specification (Rev. A) Application notes
Application Report
SLAA689A – December 2015 – Revised July 2019
ESD Diode Current Specification
Dietmar Walther ......................................................................................................... MSP430 Quality
ABSTRACT
This document explains the maximum ESD diode current specified for GPIOs on MSP microcontrollers.
Sometimes signals on specific pins exceed the supply of the MSP MCU. In such a case, the device can
handle this overvoltage condition through the ESD diodes, but the ESD diode specification must be
considered during application design. The items to be considered are described in this document.
NOTE: Many of the specifications and figures in this application report are from the MSP430FR5969
microcontroller data sheet as an example. However, the same theory and understanding
applies to all MSP430™ MCUs, which all have similar specifications in each device-specific
data sheet.
1
2
3
4
5
6
Contents
Important Side Effects of the ESD Diodes During Power Cycling..................................................... 2
Introduction and Clarification of the Specification ....................................................................... 2
Considerations During Application Design ............................................................................... 3
3.1
Consideration of the Correct Supply Circuit ..................................................................... 4
3.2
Alternative solutions ................................................................................................ 7
Clarification on ESD Diode Current and I/O Output Drive Capability ................................................. 9
Conclusion .................................................................................................................. 10
References .................................................................................................................. 10
List of Figures
........................................................................
1
Absolute Maximum Ratings of MSP430FR5969
2
ESD Ratings of MSP430FR5969 .......................................................................................... 3
3
Principle Schematic of an I/O .............................................................................................. 4
4
Typical Example for a Current-Sink LDO ................................................................................. 5
5
Typical Example for a LDO Circuit With Limited Current-Sinking Capability
6
7
8
9
10
11
2
........................................ 6
Selected Resistor Combinations for TPS7A16 .......................................................................... 6
Power-up and Power-Down Behavior of TPS715A ..................................................................... 7
Alternative Solution Using a Zener Diode ................................................................................ 7
Cathode Current vs Cathode Voltage ..................................................................................... 8
Schematic Example Using ATL431 ....................................................................................... 8
GPIO Port Output Specification of MSP430FR5969 .................................................................. 10
Trademarks
MSP430 is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
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ESD Diode Current Specification
1
Important Side Effects of the ESD Diodes During Power Cycling
1
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Important Side Effects of the ESD Diodes During Power Cycling
The methods described in this application report do not apply when the main supply (typically DVCC) is
unpowered and a voltage is applied on a GPIO pin. This condition can cause unintended power sourcing
to the MCU through the ESD structures on the GPIOs. This is a concern in both the power up and power
down scenarios. If the main supply and a GPIO are powered at the same time, ensure that delays in the
main supply (due, for example, to inline capacitance or LDO delays) are accounted for and that a GPIO is
not powered before the main supply.
Due to the physical architecture of the ESD diodes, which is explained in the following sections, other
secondary side effects can appear if the MSP microcontroller is not properly powered. Improper power
scenarios include sensors that are sourced by a different supply or communication lines that are driven by
other ICs in the application, if these signals are connected to the MCU while the main supply of the MCU
is off. During the power-down scenario, a side effect is that the device can be back powered through the
ESD diodes. However, this is not the intended powering scheme for the MSP microcontroller and can lead
to erroneous and unpredictable behavior, due to the current limitations of the ESD diodes and the
bypassing of the intended power path. In worst case scenarios, this can lead to physical damage to the
device, unexpected execution, or memory corruption causing a malfunction in the application.
2
Introduction and Clarification of the Specification
Most MSP microcontrollers specify the maximum ESD diode current in the Absolute Maximum Ratings
section (see Figure 1). The diode current specified as ±2 mA is a constant current that flows through the
ESD structure to the supply rails to protect the device. These ESD structures are triggered if a signal is
applied that exceeds the actual supply voltage of the device. To follow the specification, the application
must protect the device pin externally so that a signal does not exceed the ±2-mA specification.
In other words, voltages greater than the actual device supply (DVCC and AVCC) can be applied, but the
current that flows through the ESD diodes must be controlled.
Figure 1. Absolute Maximum Ratings of MSP430FR5969
While the current is specified as ±2 mA, the ESD structure can withstand much higher current levels, such
as those that appear during typical ESD events according HBM or CDM. The cells are made to pass
standard ESD tests like HBM or CDM as shown in Figure 2. During this high-voltage stress, current peaks
in the range of several amperes flow through the ESD structures.
2
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Figure 2. ESD Ratings of MSP430FR5969
However, the duration of the high-current pulse is quite short (in the range of several nanoseconds), which
leads to less thermal stress and much less heating compared to a long-term high-current event. This is the
main reason why the constant current through the ESD diodes is limited to ±2 mA for longer or constant
operation.
3
Considerations During Application Design
When considering the ESD diode current specification of the device data sheet during the development of
an application, first analyze if signals connected to the GPIO pins can exceed the actual supply. One
example would be the output of analog sensors, which might go above the MSP MCU supply for a longer
period of time (several milliseconds). In such a case, the maximum voltage levels that can be seen by the
MSP MCU must be defined or evaluated, and a current-protection mechanism must be developed. This
protection mechanism is important to prevent permanent damage on the ESD structure of the MSP
microcontroller caused by high current beyond the allowed level. This protection also prevents the
secondary effect of increasing the supply voltage due to energy introduced through the ESD diodes.
In most cases, the current limitation can be implemented as a simple series resistance that is sized based
on the maximum expected current. However, in some cases, this might not be enough to fulfill all aspects
of a reliable working application. Even if the current limitation specification of the ESD diodes is met, the
supply of the MSP microcontroller might be disturbed. This is because the ESD structures draw the
current to the supply rail of the MSP MCU, which boosts the supply as long as current is flowing. If the
supply connected to the MSP MCU cannot sink current, the maximum DVCC specification of the MSP
MCU might be violated over time. This high supply voltage can cause permanent damage that can lead to
malfunction of the device or high current consumption. The high supply voltage can also cause wear-out
effects that lead to functional and parametric failures over time.
The principle of supply voltage increase can be explained using a principle I/O schematic (see Figure 3).
The external serial resistors R1 and R2 limit the current that flows through the ESD diodes D1 and D2
when an overvoltage is applied. Protecting the CMOS devices against this kind of overvoltage is the
essential function of the internal ESD diode. Assuming that the rating for the continuous current through
the diode of ±2 mA is considered, no physical damage occurs. At the same time, the current of ±2 mA
maximum flows to the supply potential through the ESD diodes and raises the potential by providing
"extra" current. If this "extra" current is larger than the current that is consumed by a load connected to the
supply, the voltage increases. If more than one GPIO adds current to the supply, the sum of "extra"
currents added to the VCC potential flowing through all protection diodes must be considered. Figure 3
shows this case, when two GPIOs experience overvoltage at the same time. If the sum of currents
exceeds the maximum current consumption of the whole system connected to VCC, additional protection
mechanisms must be considered. The whole system is defined by the supply architecture itself which can
have current sink capability but also by the microcontroller and connected loads to the microcontroller. If
the microcontroller is running in active mode and driving some LEDs, it is probable that the energy
provided by the ESD diodes during overvoltage condition will be consumed. However, if the
microcontroller is in low-power mode and consuming only a few nanoamperes, the supply voltage will
increase due to the extra current from the ESD diodes.
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Figure 3. Principle Schematic of an I/O
To remove this voltage increase effect, TI recommends consideration of the best choice of the regulator to
fit the application requirements. In addition to form factor, performance parameters, cost, and power
consumption, the impact on the whole system must also be considered. Section 3.1 describes the
advantages and disadvantages of regulator circuits with good current sinking capabilities, and also lists
alternative solutions to prevent the supply increase effect.
A similar effect appears if the device is powered down and no voltage is applied to VCC. In this case, a
voltage greater than 0.3 V can result in backward supply through the ESD diodes. Section 1 provides
more details on this scenario.
WARNING
In battery-supplied applications, the supply voltage boosting effect
due to overvoltage on I/O pins can cause the battery to explode or
burn.
3.1
3.1.1
Consideration of the Correct Supply Circuit
Power Supplies With Good Current-Sinking Capability
Figure 4 shows an example of a power supply with good current-sink capability if the ESD diodes raise the
supply voltage through the 2-mA path. The main reason for the good sinking property of this circuit is the
relatively high feedback current drawn by the R1 and R2 combination for this specific regulator. As
described in the figure, the feedback current is approximately 10 mA, which is much higher than the 2 mA
that is allowed by the MSP microcontroller ESD diodes. Therefore, the energy can be dissipated by the
feedback path of the voltage regulator, and no increase of the supply will be seen. The significant
disadvantage of this power supply on the system level is the relatively high static power dissipation.
4
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Figure 4. Typical Example for a Current-Sink LDO
3.1.2
Power Supplies With Limited Current-Sinking Capability
Figure 5 shows an example of a power supply with limited current sinking capability. In comparison to the
regulator in Figure 4, the feedback divider R1 and R2 of the power supply with limited current sink
capability draws a static current of approximately 7 µA.
As soon as the ESD diodes of the MSP MCU connected to VO are operated with the maximum current of
2 mA, the output voltage of the power supply in Figure 5 increases. This increase in VO may lead to
overvoltage stress on the MSP microcontroller or the power supply itself.
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Figure 5. Typical Example for a LDO Circuit With Limited Current-Sinking Capability
An example for ultra-low-power supplies is the TPS7A16, which recommends large resistor values in the
feedback path (see Figure 6) to keep the overall quiescent current of the system low. This approach is
well suited for ultra-low-power applications where no overvoltage can be introduced through the MSP
MCU internal ESD diodes. In systems where the ESD diodes of the MSP MCU increase the supply rails,
this power supply can cause overvoltage on the MSP MCU, due to the limited current-sink capability.
Figure 6. Selected Resistor Combinations for TPS7A16
6
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3.1.3
Bidirectional Power Supplies
In addition to the unidirectional regulators previously introduced, there are also bidirectional LDOs with a
built-in reverse current operation. A typical bidirectional power supply is the TPS715A. The reverse current
operation is realized by a built-in back-gate diode of the LDO pass device. When the input voltage VIN
drops below the output voltage VOUT minus the forward voltage of the back-gate diode, current flows from
the output to the input. The bidirectional operation lets the power-up and power-down sequences enable a
certain SVS functionality of the LDO. Figure 7 shows the power-up and power-down behavior of the
TPS715A. This is only a recommended power cycle scenario for the MSP MCU, and this operation should
not be confused with the actual current-sink capability.
Figure 7. Power-up and Power-Down Behavior of TPS715A
3.2
Alternative solutions
Due to system requirements, a power supply with limited current-sink capability might be required by the
application. Hence, the risk of constant overvoltage at the I/O pins with respect to the ESD diode current
specification must be evaluated. In this special application, an alternative solution must be implemented to
protect the MCU from voltage stress.
3.2.1
Zener Diodes
A common solution is the use of a Zener diode connected to the supply (see Figure 8). This diode can
dissipate the additional energy introduced by the MSP MCU internal ESD diodes and clamps the voltage
to the desired domain. The Zener diode does not draw static current, so that the advantage of the lowpower LDO combined with the current-sink capability of the Zener diode offers a reasonable protection
and supply to the MSP MCU.
Figure 8. Alternative Solution Using a Zener Diode
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When using this alternative approach, the selection of the correct Zener diode depends on the application.
Consider characteristics like overall power consumption, power dissipation, and overall clamping behavior
to protect against overvoltage conditions caused by the MSP MCU internal ESD diodes during overvoltage
stress. One of the main items be considered is the constant current that is drawn by the Zener diode even
when it has not started to protect the MCU. The second parameter is the clamping voltage itself, which
should be above 4.1 V, which is the absolute maximum rating for MSP MCUs. Therefore, a Zener diode
that provides a good comprise between clamping voltage and current consumption in nonprotection
modes must be found.
3.2.2
Shunt Regulators
Shunt regulators like the ATL431 are offered as Zener diode replacements and are a good alternative to a
real Zener diode to address the use case described in this application report. The clamping voltage is
adjustable by using external resistors, and the regulators have a very low operating current when they are
not in clamping state. The most important function is the steep characteristic of the I-V curve at the
clamping point. Figure 9 is an extract from the ATL431 data sheet that clearly shows the fast increase in
current when the clamping voltage of 2.5 V is reached, while the regulator operates at low current below
this clamping threshold.
Figure 9. Cathode Current vs Cathode Voltage
Experiments using the TPS77033 in combination with the ATL431 prove the concept of protecting the
supply for continuous overvoltage caused by extra currents through the ESD diodes. In this example, the
supply voltage of 3.3 V for the MSP430 MCU is provided by the TPS77033 (see Figure 10).
Figure 10. Schematic Example Using ATL431
8
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Clarification on ESD Diode Current and I/O Output Drive Capability
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For test purposes, an MSP430F6638 MCU was programmed to stay in LPM3 with a timer active and
causing interrupts every 10 ms. At the same time, P1.4 was configured as an input pin and overvoltage
(4.4 V with limited current) was applied to this GPIO. This was done so that the current driven to the input
pin activates the current path through the ESD diodes to the supply rail. Without the ATL431, an increase
of the supply voltage to 3.8 V was observed on the DVCC pin and also on P1.0, which was driven in the
timer interrupt service routine.
To calculate the external resistor combination connected to the ATL431, use Equation 1.
1 + 41
8176 = l
p Û 8NAB F +NAB Û 41
42
(1)
To consider the reference current specification (Iref) of the ATL43x for output voltage (VOUT) variation, the
following equations can be used to calculate the minimum and maximum clamping voltages:
1 + 820G×
8176IEJ = l
p Û 2.58 F 150 J# Û 820G× = 3.528
1800G×
(2)
1 + 820G×
p Û 2.58 F 30 J# Û 820G× = 3.618
1800G×
1 + 820G×
8176I=T = l
p Û 2.58 F 0 J# Û 820G× = 3.648
1800G×
8176PUL = l
(3)
(4)
When the ATL431 was configured for clamping at approximately 3.6 V, no increase above this voltage
was seen. This simple experiment proves the principle concept of the proposed protection methodology.
However, if resistor combinations in the range of MΩ are used, the risk of leakage caused by external
contamination must be considered during application development.
4
Clarification on ESD Diode Current and I/O Output Drive Capability
This section clarifies the meaning of the GPIO output voltage specification to prevent any confusion with
the ESD diode current specification in the maximum ratings section.
The output voltage specification (see Figure 11) describes the driver capability of the I/Os. An I/O can
drive high output at a certain load with a certain loss in the voltage level. For example, an I/O can drive a
load of –0.6 mA, but the voltage level in this case may be as much as 0.6 V lower than the actual supply
voltage. The same is true if an I/O drives a zero level. In such a case, the low-level voltage might raised
by 0.6 V, maximum, if a current of 6 mA flows into the I/O.
However, these specifications are not related to the 2-mA ESD diode current specification, which
describes the maximum current that is allowed to flow through the ESD protection circuit when a voltage
higher than the MSP microcontroller supply is applied to an I/O.
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Figure 11. GPIO Port Output Specification of MSP430FR5969
5
Conclusion
This document describes how to interpret the ESD diode current specification of the MSP microcontroller
data sheets. In addition, it clarifies the differences between LDOs without and with current sinking
capability. The conditions and requirements of the whole application are important aspects to select the
correct supply circuit for the MSP microcontroller.
The best way to remove overvoltage due to ESD diode current is to prevent the overvoltage conditions at
the GPIO. If the system architecture can limit the overvoltage at the entry point, the considerations about
the correct supply circuit are less critical.
6
References
1.
2.
3.
4.
5.
6.
7.
10
MSP430FR59xx Mixed-Signal Microcontrollers
LMx37 3-Terminal Adjustable Regulators
TPS770xx Ultra Low-Power 50-mA Low-Dropout Linear Regulators
TPS7A16 60-V, 5-μA Iq, 100-mA, LDO Voltage Regulator With Enable and Power-Good
TPS715A High Input Voltage, MicroPower SON Packaged, 80mA LDO Linear Regulators
ATL43x 2.5-V Low Iq Adjustable Precision Shunt Regulator
Thermostat Implementation With FRAM Microcontroller Reference Design
ESD Diode Current Specification
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from December 8, 2015 to July 10, 2019 .......................................................................................................... Page
•
Moved and updated Section 1 Important Side Effects of the ESD Diodes During Power Cycling (formerly Section 4) ..... 2
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