Using Antifuse 1T-OTP for Analog Trimming and Calibration Craig Downing and Jim Lipman Sidense

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The Need for Trimming Analog Circuits

Analog ICs, sensors and mixed-signal SoCs that include analog circuitry, such as Power Management ICs (PMICs), need to meet precise specifications for analog signal behavior. Variations in chip processing and effects of the packaging process result in unpredictable deviations of analog circuits and sensors from their target specifications. To compensate for these deviations, analog circuitry needs to be trimmed or calibrated by adjusting part of the circuitry, for example, resistance values that vary the behavior of an analog-to-digital converter. In addition, as IC developers continue to strive for reduced power consumption and cost by moving to smaller geometries, the problem of optimizing the analog circuitry becomes more important as process variations tend to have a larger effect on analog circuit behavior.

The traditional method of implementing a trimming circuit uses metal or poly fuses (eFuses). For example, the fuses may be connected to an array of resistors of varying values so they can be blown to adjust the resistance in a particular part of an analog circuit. This is similar to trimming a discrete circuit using a potentiometer. The problem with using fuses for this trimming function is that high currents are required to blow the fuse, which adds additional design challenges, and the blowing process could potentially affect chip yield. Using fuses also makes it harder to trim in the field after manufacturing.

Calibration and trimming may need to be performed at different test stages during or after chip packaging. This is done to subject the device to different environments in order to tune different components in the circuit for a range of operating parameters.

Certain devices such as silicon clocks can be adjusted to suit a range of customer requirements. For example, a manufacturer can produce a timing chip with a trimming circuit to set parameters to cover a wide range of frequency specifications. After fabrication, devices can then be configured to meet a range of product requirements from the same die.

Using 1T-OTP to Trim and Configure Devices.

A better solution is to use 1T-OTP memory bit settings for trimming and calibration functions. Embedded 1T antifuse one-time programmable (OTP) memory such as Sidense SLP and ULP, with its small size, low power consumption, high reliability, and field programmability, simplifies trim and calibrate operations, all without adding any processing cost to chip fabrication. 1T-OTP is used to store configuration data and to trim and configure analog circuits in many applications. Examples include automotive sensor calibration, display controller settings, ADCs and DACs, touch panel controllers, audio processors and amplifiers, PMICs, and implanted and wearable medical devices.

1T-OTP for Very Low Power Applications

Many devices such as silicon clocks and RFID tags are used in very low power applications, where the active power needs to be minimized and where power for any additional circuitry could be significant. In this case it is desirable to read trim information from OTP as fast as possible, so that the memory can be shut down to save active power.

For very low-power devices, Sidense ULP, 1T-OTP macros are an ideal solution for trimming and calibration. To enable low voltage operation, ULP macros use a differential-redundant read mode. This mode widens the operating margins so that read operations function reliably with a 1.5V supply. The ULP design also includes features to allow fast start-up read of data bits, then to power down the memory once the data has been latched in the macro. In this mode the macros use a very low supply voltage for standby operation. ULP macros support data bus widths up to 128 bits so that just one read operation at power-up can latch up to 16 bytes of data.

Silicon Clock Example

The following figure shows a Sidense 128-bit ULP macro used for setting the frequency of a silicon clock. The device can be trimmed during manufacture and test or after packaging to configure the device for a particular requirement. The power-up read operation supported by the ULP macro enables the device to quickly load settings and then put the OTP into a very low power standby mode, making it ideal for applications where low power consumption is critical.

Figure One

Multiple uses for 1T-OTP in an IC

Many devices such as those used to manage system power, including battery management ICs, PMICs and motor controllers, often have multiple uses for OTP, including configuring and trimming regulator and sensing circuitry in addition to storing code for a microcontroller.

A good example of PMIC use is for a smartphone to maximize battery life. Power management needs in a smartphone include battery charging, controlling power to the RF circuitry and applications processor, dimming a display after a pre-determined time period and putting the phone in sleep mode if there is no user activity after a fixed time period. 1T-OTP can be used for all these operations with both analog and digital circuitry: storing the charging circuit parameters for different battery types and capacities; adjusting RF and processor power parameters; and setting time constants for display dimming and reducing smartphone power when going to sleep.

Another advantage of 1T-OTP is that a single macro can be used for multiple purposes on a chip. In the following example, a single 1T-OTP macro in a PMIC is used for storing analog trim settings, microcontroller code and for configuring timing circuits. Additional operations 1T-OTP can also take on include secure key storage and chip IDs. Sidense 1T-OTP macros are currently available with up to 512 Kbits per instance, providing plenty of capacity for multiple uses.

Figure Two

Devices Fabricated in Power/HV processes

Sidense 1T-OTP macros are also available for high-voltage and power processes, expanding their use to cover devices manufactured using these processes. 1T-OTP is highly reliable and robust at high operating temperatures, due to the permanent antifuse oxide breakdown technology used in the bit-cell. While high-temperature endurance can be a problem for some types of NVM that use charge storage methods, Sidense 1T-OTP's programmed bit cells retain their contents for over 10 years at full duty cycle and an operating temperature of 125°C. This makes it an ideal storage solution for applications such as in automotive electronics where devices may be required to operate at high temperatures.

Field Programmability

A big advantage of 1T-OTP memory over eFuse or mask ROM technology is that it can easily be programmed in the field, using a programming voltage generated on-chip. This is important for calibrating devices that may go through different test stages before and after packaging, adjusting different parts of the circuitry accordingly. Field programmability is also important during system development, where devices will go through several testing phases and OTP can be used to set circuit parameters and configuration.

Field-programmable 1T-OTP is also used to calibrate analog components in a system, match sensor characteristics and configure chips for different feature sets. One example of this application is setting audio parameters for different automotive entertainment systems. 1T-OTP trimming and parameter setting is also used to match a system to individual preferences or, for medical applications, health requirements. Examples of preference matching include adjusting the frequency response of an audio amplifier and screen characteristics of an HDTV. Medical applications include shaping the audio response spectrum of a hearing aid to compensate for the wearer's hearing losses and adjusting the triggering threshold levels of an implanted pacemaker.

To further increase reliability in the field, 1T-OTP macros can be read in a redundant mode (two physical bits programmed identically to represent one logical bit). For such a wide range of field-programming use cases, 1T-OTP supports incremental programming of select bits in a word. A word does not have to be programmed fully but can be programmed bit by bit as required. For example some bits may be programmed at wafer test, some after packaging and later in system test or in the field. In addition to this flexibility, 1T-OTP can be used to emulate Multi-Time Programming.

eMTP for Trimming and Configuration in the Field

In an emulated Multi-Time Programmable (eMTP) mode, 1T-OTP can also be used in place of MTP memory such as flash for certain applications. This is done by leaving segments of the OTP memory macro initially un-programmed and available for future configuration.

An application that can benefit from 1T-OTP used in an eMTP mode would be an LCD monitor controller for an HDTV, where the 1T-OTP stores brightness and contrast settings. An HDTV may have multiple defaults for these settings depending on whether the viewer is watching a movie, playing a game, looking at still pictures or using the HDTV as a PC display. These settings, initially set by the manufacturer, could be changed by the user according to their preferences.

As the die area used by 1T-OTP is small, using a macro large enough to store enough copies of the data for many write cycles is feasible and cost-effective. For example 256 bits of configuration information can be stored and updated up to a thousand times using a 256 Kbit macro. In this case, each time the settings are changed, a new segment of memory can be programmed to replace the previous settings. A small amount of OTP is used to store a tag to keep track of which memory segment is currently being used and to address the current settings during a read operation.

1T-OTP - An Essential Component for Trimming and Calibration

For chips that need analog trimming, parameter setting and calibration, 1T-OTP macros are essential components. Based on Sidense's 1T-Fuse^TM bit-cell technology and requiring no additional process steps or masks, 1T-OTP macros offer the security, reliability and flexibility needed to cost-effectively implement analog trim and calibrate operations in consumer, automotive and mobile communication devices. Available in high bit-counts, many configurations and for a wide range of standard-logic and HV/power process nodes, 1T-OTP reduces the cost and risk of optimizing the analog circuitry for a broad variety of silicon-based devices.

Explore Sidense IP here

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• OTP/eMTP: Very low power 256 Kbit NVM in 180nm Standard Logic CMOS process
• OTP/eMTP: 128 Kbit NVM in 65nm Standard Logic CMOS process

About the Authors

Craig Downing
Sidense

Craig Downing is product marketing manager at Sidense. He received a B.Eng. in electrical engineering from McGill University in Montreal. Craig can be reached at cdowning@sidense.com.

Jim Lipman
Sidense

Jim Lipman is Sidense's marketing director. His work experience includes positions at TechOnLine, VLSI Technology, Hewlett-Packard and Texas Instruments. Jim has a D.Eng from SMU and an MBA from Golden Gate University. He can be reached at jim@sidense.com.