<div class=WordSection1> <h1>RT1985/RT1986/RT1987/RT1988 Idea Diode Protection Switch: Design Considerations and Applications</h1> <br> <p class=bady>Shihchun Liu | AN089 March 2026</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image001_en.jpg" ><img width=680 height=271 src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image001_en.jpg" alt="Technical Document Image Preview"></a> </p> <p class=bady>In modern electronic systems, efficient power management is crucial to ensuring reliable operation and extending battery life. Protection switches have become essential components in simplifying power control and protection for various loads. Compared to discrete MOSFET solutions, integrated protection switches offer benefits such as smaller size, enhanced protection features, and simplified design processes. </p> <p class=bady>The RT1985/RT1986/RT1987/RT1988 family is the high-performance Ideal Diode Protection Switch, specifically designed to enhance the power management of modern electronic systems such as USB-C/Thunderbolt power delivery, docking stations, and Power ORing configurations. This application note provides a detailed overview of the product’s features, applications, and key design considerations.</p> </div> <div class=WordSection2> <br><br> <h2 id="__Bookmark__0">1 Key Features of Protection Switches</h2> <p class=MsoNormal>Featuring low forward voltage, programmable soft-start, start-up short-circuit protection, and robust fault protection mechanisms, the RT1985/RT1986/RT1987/RT1988 family simplifies power path management while increasing system reliability. Compared to discrete MOSFET implementations, this family offers a smaller footprint, easier design integration, and superior protection features.</p> <div> <table class=TableGrid1 border=1 cellspacing=0 cellpadding=0 width=680 style='width:509.7pt;border-collapse:collapse;border:none'> <tr> <td width=90 valign=top style='width:67.15pt;border:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpFirst>Product</p> </td> <td width=126 valign=top style='width:94.85pt;border:solid windowtext 1.0pt; border-left:none;padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>Operation Vin</p> </td> <td width=161 valign=top style='width:120.8pt;border:solid windowtext 1.0pt; border-left:none;padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>Maximum Continuous Current</p> </td> <td width=146 valign=top style='width:109.35pt;border:solid windowtext 1.0pt; border-left:none;padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>OVP Threshold</p> </td> <td width=157 valign=top style='width:117.55pt;border:solid windowtext 1.0pt; border-left:none;padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpLast>Package</p> </td> </tr> <tr> <td width=90 valign=top style='width:67.15pt;border:solid windowtext 1.0pt; border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpFirst>RT1985</p> </td> <td width=126 valign=top style='width:94.85pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>3.4V to 23V</p> </td> <td width=161 valign=top style='width:120.8pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>8A</p> </td> <td width=146 valign=top style='width:109.35pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>Fixed</p> </td> <td width=157 valign=top style='width:117.55pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpLast>VDFN-12TL 3x3</p> </td> </tr> <tr> <td width=90 valign=top style='width:67.15pt;border:solid windowtext 1.0pt; border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpFirst>RT1986</p> </td> <td width=126 valign=top style='width:94.85pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>3.4V to 23V</p> </td> <td width=161 valign=top style='width:120.8pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>5.5A</p> </td> <td width=146 valign=top style='width:109.35pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>Fixed</p> </td> <td width=157 valign=top style='width:117.55pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpLast>VDFN-12TL 3x3</p> </td> </tr> <tr> <td width=90 valign=top style='width:67.15pt;border:solid windowtext 1.0pt; border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpFirst>RT1987</p> </td> <td width=126 valign=top style='width:94.85pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>3.4V to 32V</p> </td> <td width=161 valign=top style='width:120.8pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>8A</p> </td> <td width=146 valign=top style='width:109.35pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>Programmable</p> </td> <td width=157 valign=top style='width:117.55pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpLast>VDFN-12T1L 3x3</p> </td> </tr> <tr> <td width=90 valign=top style='width:67.15pt;border:solid windowtext 1.0pt; border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpFirst>RT1988</p> </td> <td width=126 valign=top style='width:94.85pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>3.4V to 53V</p> </td> <td width=161 valign=top style='width:120.8pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>8A</p> </td> <td width=146 valign=top style='width:109.35pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpMiddle>Programmable</p> </td> <td width=157 valign=top style='width:117.55pt;border-top:none;border-left: none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=badyCxSpLast>VDFN-20TL 5.2x4</p> </td> </tr> </table> </div> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image002_en.png" ><img width=672 height=115 src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image002_en.png" alt="A screenshot of a computer&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=bady>Figure 1. Protection Switch Features</p> <br> <h3>1.1 Power Control (On/Off Switching)</h3> <p class=MsoNormal>Protection switches provide a straightforward method to control the connection and disconnection of power to specific loads without mechanical relays. This allows for better power sequencing and dynamic power management. The RT1985, RT1986, RT1987 or RT1988 provides precise power path control through an active-high EN pin. When EN is asserted high and the input voltage exceeds the UVLO threshold, the power path is enabled with a defined delay and controlled ramp-up behavior.</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image003_en.png" ><img width=518 height=296 src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image003_en.png" alt="A screenshot of a computer&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=bady><a name="_Ref197509561">Figure</a> 2. Enable Control of the Protection Switch</p> <br> <h3>1.2 Programmable Soft-Start and Inrush Current Control</h3> <p class=MsoNormal>Many protection switches incorporate built-in soft-start features by controlling the gate drive voltage, allowing a gradual increase in output voltage and current. The RT1985, RT1986, RT1987 or RT1988 includes an adjustable soft-start feature. By adjusting an external soft-start capacitor, designers can set the VOUT slew rate and limit the inrush current. This prevents voltage glitches on the main bus, protecting sensitive devices that share the same supply. This feature is critical for systems with large capacitance. Refer to the datasheet for detailed soft-start capacitor design guidelines.</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image004_en.png" ><img width=381 height=293 src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image004_en.png" alt="A diagram of a device&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=MsoNormal><a name="_Ref193903408">Figure </a>3. Startup without the Inrush Current Control</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image005_en.png" ><img width=370 height=293 src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image005_en.png" alt="A screenshot of a computer&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=MsoNormal>Figure 4. Startup with the Inrush Current Control</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image006_en.png" ><img width=660 height=359 src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image006_en.png" alt="A screen shot of a computer&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=MsoNormal>Figure 5. RT1985 Soft-Start Test Results</p> <br> <h3>1.3 Start-Up Short-Circuit Protection (SCP)</h3> <p class=MsoNormal>During startup, the RT1985, RT1986, RT1987 or RT1988 monitors the output current. If an excessive inrush current or a hard short is detected, the device safely shuts down and retries after a defined delay, ensuring robust protection even under fault conditions.</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image007_en.png" ><img width=405 height=157 id="Picture 19" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image007_en.png" alt="A black background with red and blue text&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=MsoNormal>Figure 6. A MOSFET is used as a protection switch</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image008_en.png" ><img width=444 height=174 id="Picture 10" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image008_en.png" alt="Technical Document Image Preview"></a></p> <p class=MsoNormal>Figure 7. The RT1985/RT1986/RT1987/RT1988 is used as a protection switch</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image009_en.png" ><img width=665 height=366 id="Picture 17" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image009_en.png" alt="A screen shot of a computer&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=MsoNormal>Figure 8. RT1985 Startup-into-Short and Retry Test Results</p> <br> <h3>1.4 Reverse Current Blocking</h3> <p class=MsoNormal>Some protection switches offer reverse current blocking to prevent current from flowing backward from the output to the input, which is especially important in redundant power source systems. The RT1985, RT1986, RT1987 or RT1988 offers Ideal Diode True Reverse Current Blocking (TRCB) capability, regulating the forward voltage drop and preventing reverse current flow from VOUT to VIN. This is essential in ORing applications where multiple power sources share a common load.</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image010_en.png" ><img width=535 height=248 id="Picture 20" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image010_en.png" alt="A diagram of electrical wiring&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=MsoNormal>Figure 9. Reverse Blocking from the Output to the Input</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image011_en.png" ><img width=655 height=365 id="Picture 15" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image011_en.png" alt="A screen shot of a computer&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=MsoNormal>Figure 10. RT1985 Reverse Blocking Protection Test Results</p> <br> <h3>1.5 Overvoltage Protection (OVP)</h3> <p class=MsoNormal>Some protection switches integrate overvoltage protection to safeguard downstream circuitry from input voltage spikes or abnormal conditions. The RT1985, RT1986, RT1987 or RT1988 includes OVP functionality that disconnects the load when the input voltage exceeds a defined threshold, preventing damage to sensitive components. This feature is particularly valuable in USB-C and power ORing systems, where unpredictable input sources may expose the system to voltage surges.</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image012_en.png" ><img width=537 height=231 id="Picture 11" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image012_en.png" alt="Technical Document Image Preview"></a></p> <p class=MsoNormal>Figure 11. Overvoltage Protection from Excessive Input Voltage</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image013_en.png" ><img width=658 height=349 id="Picture 37" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image013_en.png" alt="A screenshot of a computer&#10;&#10;AI-generated content may be incorrect."></a></p> <p class=MsoNormal>Figure 12. RT1985 Overvoltage Protection Test Results</p> <br> <h3>1.6 Over-Temperature Protection</h3> <p class=MsoNormal>Advanced protection switches integrate thermal protection mechanisms. If the device's junction temperature exceeds a critical threshold, it automatically turns off to prevent permanent damage. The RT1985, RT1986, RT1987 or RT1988 actively monitors the die temperature. If an excessive temperature (~140°C) is detected, the device disconnects the load and asserts a fault indication, enhancing system safety.</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image014_en.png" ><img width=554 height=208 id="Picture 12" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image014_en.png" alt="Technical Document Image Preview"></a></p> <p class=MsoNormal>Figure 13. Over-Temperature Protection from Abnormal Thermal Conditions</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image015_en.png" ><img width=579 height=313 id="Picture 13" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image015_en.png" alt="Technical Document Image Preview"></a></p> <p class=MsoNormal><a>Figure </a>14. RT1985 Over-Temperature Protection Test Results</p> <br> <h3>1.7 Low On-Resistance (20mΩ)</h3> <p class=MsoNormal>The RT1985, RT1986, RT1987 or RT1988 adopts advanced process technology to reduce the on-resistance (R_ON) of the internal MOSFET to as low as 20mΩ, effectively minimizing power loss during high current transmission (P = I&sup2;R). This not only reduces device heating but also improves overall thermal efficiency.</p> <br> <h3>1.8 Ultra-Low Quiescent Current</h3> <p class=MsoNormal>The RT1985, RT1986, RT1987 or RT1988 features an ultra-low quiescent current design, meaning that the IC itself consumes minimal power under no-load or light-load conditions. This further optimizes overall system power consumption and extends battery life, making it well-suited for portable devices such as laptops and tablets.</p> <br><br> <h2 id="__Bookmark__1">2 Application Examples</h2> <h3>2.1 USB-C/Thunderbolt Power Delivery</h3> <p class=MsoNormal>Protection switches are critical in USB-C and Thunderbolt applications, where they control and protect power delivery to devices. They ensure safe connection and disconnection during dynamic power negotiation processes, managing inrush currents, and preventing backflow. <u>Figure 15</u> illustrates the RT1985 used for sink and source control in a dual-port USB PD application.</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image016_en.png" ><img width=684 height=418 id="Picture 26" src="/content/dam/richtek-eds/resources/application-notes/an089/html-images-en/image016_en.png" alt="Technical Document Image Preview"></a></p> <p class=MsoNormal><a name="_Ref199341822">Figure 15</a>. RT1985 for Sink and Source Control in a Dual-Port USB PD Application</p> <p class=MsoCaption>Note 1. The above system diagram is for reference only. Actual products should be evaluated and adjusted based on your specific application requirements. For further assistance, please contact our regional office.</p> <br> <h3>2.2 Docking Stations</h3> <p class=MsoNormal>In docking stations for laptops and tablets, protection switches are used to selectively enable or disable power to various ports and peripherals, helping optimize overall system efficiency, minimize standby power losses, and provide fault isolation.</p> <br> <h3>2.3 Power ORing Applications</h3> <p class=MsoNormal>Protection switches with true reverse current blocking capability enable reliable power ORing. This allows seamless switching between multiple power sources (such as adapters, batteries, or auxiliary supplies) without backfeeding, ensuring system continuity and reliability.</p> <h2 id="__Bookmark__2">3 Conclusion</h2> <p class=MsoNormal>Proper integration of protection switches enhances system reliability, reduces design complexity, and optimizes energy efficiency. As systems become increasingly complex, protection switches will continue to play a vital role in intelligent power distribution strategies. The RT1985, RT1986, RT1987 or RT1988 offers a highly integrated and robust solution for power path control, featuring Ideal Diode reverse current blocking, programmable soft-start, and advanced fault protection. Its compact design and versatile protection features make it ideal for demanding applications such as USB-C sink power delivery, docking stations, and power ORing systems. Leveraging the RT1985, RT1986, RT1987 or RT1988 simplifies power management design, increases system reliability, and enhances the end-user experience in next-generation electronic products.</p> </div> <br><br><br> <table border="1" cellspacing="0" cellpadding="0" style="width: 637.5pt; border-collapse: collapse; border: medium none;" class="3-51"> <tbody> <tr style="height: 17pt;"> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #009eb8; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;" colspan="2"> <span style="color: #ffffff;">Next Steps</span> </td> </tr> <tr> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #ffffff; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> Richtek Newsletter </td> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #ffffff; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> <a class="link" href="http://www.richtek.com/My%20Richtek/Newsletters" target="_blank">Subscribe Richtek Newsletter</a> </td> </tr> <tr style="height: 17pt;"> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #ffffff; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> Download </td> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #ffffff; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> <a class="link" href="~/media/AECB55B0464748CEA750215DD6386EC1.ashx">Download PDF</a> </td> </tr> </tbody> </table>

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