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High Risk of Latch-up Caused By Improper TVS Device Selection for USB Type-C

2020/09/08

Abstract: With the development and improvement of the USB interface and protocol, more and more smart products adopt the USB Type-C as connection interface, which supports more diverse video, audio and data transmission with faster transfer speed. At the same time, the signal transmission is surely prone to be exposed to the interference of external power surges. For this reason, the use of TVS diode as surge protection should be more rigorous. However, if the user only pursues a TVS device with lower clamping voltage without taking into account other parameters such as its operating voltage, it may cause TVS the risk of latch-up issue resulting in the impact on the real-time communications of Type-C interface, or even the circuit damage due to EOS (electrical overstress) in severe cases. 


To deal with the proper and efficient transient voltage surge protection in the interface of electronic products with complex signal lines and power lines, the addition of a TVS diode at the interface shall be the most effective protection solution.


A. What is USB Type-C? 

As being a serial data connection interface, USB (universal serial bus) has the advantages of fast transfer speed, quick connection, easy-to-use contact form, support for hot-plugging, independent power supply, etc., and is widely used in various types of electronic devices. The USB interface has been defined to the latest USB 4 from the initial version of USB 1.0. Among them, the protocol of USB 3.1 still remains the most popular interface until today. Unlike USB 2.0 which defined only two connector types (Type A and Type B), the protocol of USB 3.1 has three interface standards: Type-A (Standard-A), Type-B (Micro-B) and Type-C, and two transfer rates: USB 3.1 Gen1 and USB 3.1 Gen2. The Type-C connector has been widely adopted by many electronic products, such as smartphones, tablets and laptops, etc.


USB official specification
Previous name
Theoretical maximum data rate
Marketing name
Release date
USB 2.0USB1.0
1.5Mbps
Low-Speed
January 1996
USB 2.0
USB1.1
12Mbps
Full-Speed
September 1998
USB 2.0
USB2.0
480Mbps
High-Speed
April 2000
USB 3.2USB3.1 Gen 1
5Gbps
Super-Speed USB
November 2008
USB 3.2USB3.1 Gen 2
10Gbps
Super-Speed USB 10Gbps
June 2013
USB 3.2
USB3.2 Gen 2x2
20Gbps
Super-Speed USB 20Gbps
September 2017
USB 4
USB4 Gen 3x2
40Gbps
USB4 40Gbps
September 2019

Table 1: USB versions


USB 3.2 Gen2x2 specification with Type-C connector can reach up to 20Gbps signal transfer rate and support high-speed charging with PD (power delivery) technology up to 20V/5A (100W) of power. Since Type-C increases not only the data transfer rate, but also power-transfer capability, to ensure that the data transmission will not be disturbed by power surges in practical applications and the stability of the power transmission, adding TVS protection at the interface becomes especially important.


Figure 1: USB Type-C Receptacle Pins


B. What is Latch-up 

Latch-up refers to a type of short circuit formed in an integrated circuit and is a parasitic effect generated in CMOS manufacturing process. The latch-up is produced by the N-P-N-P structure which is formed by the source of the NMOS, the p-substrate, the n-well and the source of the PMOS. When one of the bipolar junction transistors (BJNs) gets forward biased, it may form a positive feedback loop bringing up the latch-up effect.

Figure 2: N-P-N-P structure in CMOS


Figure 3: Equivalent circuit of CMOS latch-up

Figure 3: Equivalent circuit of CMOS latch-up


When the N-P-N-P structure of the SCR circuit is not triggered, both BJTs stay in cutoff state. The collector current is made up of the collector-to-base reverse leakage current with very small current gain, and latch-up does not occur at this moment. If some external disturbances appear causing the sudden increase of collector current in one of the BJTs to certain value, it will feed back to the other one and create the conduction of both BJTs by false triggering. A low impedance path will be formed between VDD and GND (VSS). Due to the positive feedback between these two transistors, an excessive current flow from VDD to GND continues to produce resulting in the creation of latch-up, even if the external disturbances disappear.


In the practical application of IC chips, the major reasons that create latch-up are as follows: 1. When the chip is operating, rapid change of the VDD voltage may cause latch-up. 2. When a transient surge event (such as ESD or power surge) occurs, the large current flowing in the CMOS circuit may lead to latch-up. 3. Unexpected change of power supply and GND voltage caused by circuit overload may trigger another BJT of SCR to create conditions for latch-up. 4. If a protection device with a snap-back voltage lower than 3.3V is applied on the signal line or power supply with operating voltage higher than 3.3V, the system may be seriously damaged by latch-up when above transient surge events occur.


There are many types of designs for TVS diodes in the market. The TVS diodes offered by Amazing Microelectronic Corp. are mainly designed for three specific areas: traditional semiconductor diode, N-P-N bipolar junction transistor (BJT) and SCR circuit. The characteristic curves of these three types of TVS diodes are as below:


Figure 4: TVS diode characteristic curves


In the USB Type-C protection circuit, if the TVS diode is not selected properly, such as applying a TVS diode for SCR circuit to the VBUS terminal or the CC pin line, the risk of latch-up events is more likely occur, as well as the potential latch-up hazard to the signal lines. The primary reason is that, when transient surges (such as ESD or power surges) are coupled onto USB interface, the voltage of external surge will be greater than the TVS breakdown voltage which results in the conduction of TVS diode. If wrong type of TVS diode is adopted on device, it makes the snapback voltage (Vsb or Vhold) lower than the normal operating voltage (Vbus) or the maximum voltage (VMAX) of data signal after current passing through the TVS diode, and the TVS diode will remain conducting even if the external disturbance goes away. In this case, the sudden increase of current leads to power supply voltage drop or interrupts normal signal transmission. Or even the TVS diode may be burned out and short-circuit, resulting in EOS damage to the device or entire circuit in the worse cases.


Normally USB Type-C interface is plugged and unplugged while the power is on, it is easy to be exposed to switching surges or lead to surge interference with ESD cable discharge. To reduce the possibility of latch-up in the practical application of TVS diode at USB Type-C interface, when choosing the TVS diode type, we must not only ensure that the VRWM (reverse working maximum voltage) applied to the TVS diode shall greater than circuit’s normal operating voltage or data signal’s VMAX, but also consider that, once the TVS diode is conducting, the lowest Vsb or Vhold of snapback TVS device should greater than the circuit’s maximum operating voltage after the snapback state.


C. Best TVS solution for USB Type-C interface 

Due to more power and data lines at a complete USB Type-C interface, the TVS diode protection solutions recommended by Amazing Microelectronic Corp. can be divided into three categories according to the signal type: VBUS power protection, Tx/Rx data lines protection and D+/D-/CC/SBU data line protection.


1. Since USB Type-C supports PD fast charging protocol, the VBUS transmission voltages commonly used are 5V and 12V, and the maximum can be 20V. Therefore, a TVS protection solution with proper operating voltage shall be taken for VBUS protection, and also ensure that no latch-up events will occur while selecting a TVS diode with lower clamping voltage. The following table shows the ESD and EOS protection solutions recommended by Amazing Microelectronic Corp. for applying on the VBUS power line of USB Type-C:


Part No
Application
VRWM (V)
VCL-ESD(V) @ 8kV
VCL-Surge (V) @ 5A
IPP(A)
Package
AZ3105-01F
Only for 5V VBUS
566.480DFN1610P2E
AZ4507-01F
Only for 5V VBUS
7108.5100DFN1610P2E
AZ4510-01F
For VBUS ≤ 10V
10132180DFN1610P2E
AZ4512-01F
For VBUS ≤ 12V
121514.538DFN1610P2E
AZ4520-01F
For VBUS ≤ 20V
2025.525.524DFN1610P2E
AZ4307-01F
For VBUS ≤ 7V
79.58.3180DFN2020P2E
AZ4310-01F
For VBUS ≤ 10V
101311.7150DFN2020P2E
AZ4712-01F
For VBUS ≤ 12V
121514.8225 DFN2020P2E
AZ4715-01F
For VBUS ≤ 15V
151817170DFN2020P3E
AZ4718-01F
For VBUS ≤ 18V
182222138DFN2020P3E
AZ4724-01F
For VBUS ≤ 20V
242828125DFN2020P3E

Table 2: TVS solution for Type-C VBUS


2.   As Tx/Rx lines are used for high-speed data transmission, we have to choose a low-capacitance ESD-protection TVS diode to prevent excessive capacitance that affects the signal integrity of USB Type-C interface. The followings are the protection solutions recommended by Amazing Microelectronic Corp. for Tx/Rx high-speed transmission lines:


Part No
Channel #
VRWM (V)
VCL-ESD(V) @ 8kV
Capacitance (pf)
Package
AZ1043-04F
4 lines
3.310.50.45DFN2510P10E
AZ1043-08F
8 lines
3.390.5DFN3810P9E
AZ1743-04F
4 lines
3.39.50.4DFN1308P5Z
AZ176S-04F
4 lines
1.54.30.29DFN2510P10E
AZ5B8S-01F
1 lines
1.55.50.18DFN0603P2Y

Table 3: TVS solution for Type-C TX/RX


3.   Recommended TVS solutions for the protection of USB D+/D- data cable and auxiliary signals (CC and SBU wires):

Part No
Channel #
VRWM (V)
VCL-ESD(V) @ 8kV
Capacitance (pf)
Package
AZ1045-04F
4 lines
512
0.5
DFN2510P10E
AZ1045-08F
8 lines
510.5
0.5DFN3810P9E
AZ5425-01F
1 lines
524
0.8DFN1006P2E
AZ5515-02F
2 lines
510
0.8DFN1006P3X
AZ5B85-01B
1 lines
5110.5CSP0603P2Y
AZC099-04S
4 lines
511.51.0SOT23-6L
AZC199-04S
4 lines
5111SOT23-6L

Table 4: TVS solution for Type-C D+/D-/CC/SBU



Reference: [1].  USB official website: https://www.usb.org/

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