Redundant dual-channel safe torque off (STO) reference design for AC inverters and servo drives


Design files


This reference design implements safe torque off functionality in variable speed drives (per IEC61800-5-2) by using dual channel isolated STO signals to control the inverter that supplies power to the motor. This is accomplished by independently enabling/disabling power to the isolated gate driver IC through load switches on VCC1 and VCC2. This reference design also provides diagnostic coverage for detecting various faults which helps in increasing safe failure fraction. 

This design uses ISO1212 as an isolated 24-V digital input receiver, compliant to IEC 61131-2 Type 1 characteristics to accept the two STO signals. Unlike traditional optocoupler solutions with discrete components having imprecise current limiting circuitry, the ISO1212 devices provide a simple, low-power solution with accurate current limits and a longer lifespan. The adjustable current-limit function of the load switch improves the reliability of the system. The ready pin of the gate driver, ISO5852S, helps in monitoring the supply rails.

  • Implements safe torque off (STO) to accomplish the Stop Category 0 specified in IEC 61800-5-2
  • Dual channel STO signals with multiple options for preventing generation of torque in the motor:
    • Disconnect power from the gate driver IC (both on low voltage and high voltage domains)
    • Generation of hardware trip signals to the terminate the PWM generation inside the MCU
  • Self-diagnostics coverage which includes monitoring safety pulses and detecting various faults within the circuit
  • ±60-V input tolerance with reverse polarity protection helps ensure STO pins are protected in case of faults
  • Accurate output current monitoring by the high side smart switch
  • Detection of input and output under-voltage lockout (UVLO) of the gate driver with ready (RDY) pin indication

A fully assembled board has been developed for testing and performance validation only, and is not available for sale.

Design files & products

Design files

Download ready-to-use system files to speed your design process.

TIDUDS9.PDF (6569 K)

Reference design overview and verified performance test data


Detailed schematic diagram for design layout and components


Complete listing of design components, reference designators, and manufacturers/part numbers


Detailed overview of design layout for component placement

TIDRVA6.ZIP (2483 K)

Files used for 3D models or 2D drawings of IC components


Design file that contains information on physical board layer of design PCB


PCB layer plot file used for generating PCB design layout


Includes TI products in the design and potential alternatives.

AND gates

SN74HC70014-ch, 4-input, 2-V to 6-V AND gates with Schmitt-Trigger inputs

Data sheet document-pdfAcrobat PDF open-in-new HTML
Buck modules (integrated inductor)

LMZ14201SIMPLE SWITCHER® 6V to 42V, 1A Power Module in Leaded Surface Mount TO Package

Data sheet document-pdfAcrobat PDF open-in-new HTML
ESD & surge protection ICs

TVS330033V flat-Clamp surge protection device

Data sheet document-pdfAcrobat PDF open-in-new HTML
High-side switches

TPS27S10040-V, 80-mΩ, 4-A, 1-ch, Industrial high-side switch with adjustable current limit

Data sheet document-pdfAcrobat PDF open-in-new HTML
Isolated digital inputs

ISO1212Dual-channel Isolated 24-V to 60-V digital input receiver for digital input modules

Data sheet document-pdfAcrobat PDF open-in-new HTML
Isolated gate drivers

ISO5852S2.5-A / 5-A, 5.7-kV RMS single channel isolated gate driver with split output and protection

Data sheet document-pdfAcrobat PDF open-in-new HTML
Load switches

TPS228600.73-Ω, 5-V, ultra-low leakage load switch

Data sheet document-pdfAcrobat PDF open-in-new HTML

Technical documentation

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Type Title Date
* Design guide Redundant Dual-Channel Reference Design for Safe Torque Off in Variable Speed Nov. 25, 2017
E-book Ein Techniker-Leitfaden für Industrieroboter-Designs Mar. 25, 2020
E-book E-book: An engineer’s guide to industrial robot designs Feb. 12, 2020

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