AN-1380 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com Generating Secondary Fault Supplies for Fault Protected Switches by Paul O’Sullivan INTRODUCTION FAULT PROTECTION OVERVIEW The new range of fault protected switches from Analog Devices, Inc., (ADG5462F, ADG5243F, ADG5248F, and ADG5249F) allow for user defined fault protection levels. Two secondary power supplies on the device, POSFV and NEGFV, are the required secondary power supplies that set the level at which the overvoltage protection is engaged. POSFV can be supplied from 4.5 V up to VDD, and NEGFV can be supplied from VSS to 0 V. If a secondary supply is not available, these pins (POSFV and NEGFV) must be connected to VDD (POSFV) and VSS (NEGFV). In that case, the overvoltage protection then engages at the primary supply voltages. When the voltages at the source inputs exceed POSFV or NEGFV by a threshold voltage, VT, the channel turns off or, if the device is unpowered, the channel remains off. The source input remains high impedance while the channel is off. Internal circuitry enables the switch to detect overvoltage inputs by comparing the voltage on the source pins with POSFV and NEGFV. A signal is considered overvoltage if it exceeds the secondary supply voltages by the voltage threshold (VT). The threshold voltage is typically 0.7 V, but it ranges from 0.8 V at −40°C down to 0.6 V at +125°C. The secondary supplies (POSFV and NEGFV) provide the current required to operate the fault protection and, therefore, must be low impedance supplies. For that reason, they cannot be generated from a resistor divider off the main supply rails. This application note describes some of the options for generating the secondary supply rails depending on the system requirements, and the advantages and disadvantages of each of the supply configuration options. When an overvoltage condition is detected on a source pin (Sx), the switch automatically opens and the source pin becomes high impedance and ensures that no current flows through the switch. The drain pin is then either pulled to the supply that was exceeded or goes open circuit, depending on the device and the configuration of the DR pin, when available. POSFV ESD PROTECTION ESD Dx Sx ESD DR SWITCH DRIVER LOGIC BLOCK Figure 1. Switch Channel and Control Function VIN VPOSFV + VT VOUT VOUT VPOSFV + VT VPOSFV + VT OUTPUT SHOWN FOR DR = GND OUTPUT SHOWN FOR DR = FLOATING/HIGH Figure 2. ADG5462F Drain Output Response During Fault Condition Rev. 0 | Page 1 of 6 13636-002 OUTPUT DRAINS THROUGH LOAD OUTPUT CLAMPED AT VPOSFV NEGFV 13636-001 FAULT DETECTOR AN-1380 Application Note TABLE OF CONTENTS Introduction ...................................................................................... 1 Fault Protection Overview .............................................................. 1 Table of Contents .............................................................................. 2 Revision History ........................................................................... 2 Data Acquisition Signal Chain Using the ADG5462F Channel Protector ............................................................................................ 3 POSFV/NEGFV Configuration Options ...................................... 4 Using Separate Low Impedance Power Supplies for POSFV/NEGFV.............................................................................4 Adding a Diode from the Primary Supply Rail to the Secondary Supply Rail ..................................................................5 Using Reverse Breakdown Voltage of Zener Diode to Configure POSFV/NEGFV..........................................................5 Summary ............................................................................................6 Tying POSFV to VDD and NEGFV to VSS (or GND)................ 4 REVISION HISTORY 12/15—Revision 0: Initial Version Rev. 0 | Page 2 of 6 Application Note AN-1380 DATA ACQUISITION SIGNAL CHAIN USING THE ADG5462F CHANNEL PROTECTOR Figure 3 shows an example of a portion of a data acquisition signal chain where the ADG5462F channel protector is used. The PGA uses ±15 V supply rails to achieve optimal analog performance and the ADC downstream has an input signal range of 0 V to 5 V. VDD VSS +15V –15V POSFV NEGFV +5V GND VDD VSS POSFV NEGFV VIN PGA ADC 20 18 DATA 5.5V SINGLE SUPPLY 16 ON RESISTANCE (Ω) The channel protector sits between the programmable gain amplifier (PGA) and the analog-to-digital converter (ADC), allowing the signal to pass through in normal operation, but protecting the ADC by clamping any overvoltage outputs from the PGA to between 0 V and 5 V. Figure 4 highlights the advantage of using separate primary and secondary supply voltages on the ADG5462F channel protector. The ADC signal range in this example is 0 V to 5 V. Using a switch with a 5.5 V single supply results in a large variation in the on resistance (RON) of the switch across the signal range, causing a detrimental impact on system performance specifications such as THD + N. Utilizing the flat RON region of the switch with a ±15 V primary supply optimizes the system performance. The ADC is then protected by the thresholds set by the secondary supply rails. 14 12 10 8 ±15V DUAL SUPPLY 6 4 ADDITIONAL PROTECTED RANGE FLAT OPERATION REGION, MINIMIZED THD + N 0 –16 –14 –12 –10 –8 –6 –4 –2 0 2 4 6 8 10 12 14 16 +5V 13636-003 +5V SIGNAL VOLTAGE (V) Figure 4. Flat RON Range of Operation Figure 3. ADG5462F Channel Protector Application Example Rev. 0 | Page 3 of 6 13636-004 2 AN-1380 Application Note POSFV/NEGFV CONFIGURATION OPTIONS There are a number of ways in which the POSFV and NEGFV fault supplies can be configured. The main considerations are as follows: • • • Analog switch performance required: sets the requirements for primary supply rails Fault protection levels required by downstream components: sets voltage requirement for secondary rails Availability of other system supply rails: determines the requirement for generating POSFV/NEGFV supplies drain pin clamps to ~0.7 V above VDD/POSFV during a fault condition before the switch turns off; therefore, there is a small overshoot above VDD for a short duration (~500 ns) that is seen by downstream devices. This amount of energy, however, is more benign than a 1 kV ESD pulse and should not cause any concerns in a system, as shown in the scope plot in Figure 7. +29V SPIKE T +28V OV +22.7V CLAMP tRESPONSE ~500ns A number of different options are detailed in the following sections. +22V VDD Tying POSFV to VDD and NEGFV to VSS (or GND) is the simplest configuration and sets the fault thresholds at the same voltage as the primary supply rails. In the case of a fault, the drain pin clamps to VDD + 0.7 V or VSS − 0.7 V. CH1 5.00V CH3 5.00V VDD VSS 0.1µF 0.1µF CH2 5.00V M1.00µs A CH1 18.0V 13636-007 TYING POSFV TO VDD AND NEGFV TO VSS (OR GND) USING SEPARATE LOW IMPEDANCE POWER SUPPLIES FOR POSFV/NEGFV Using separate low impedance power supplies for POSFV and NEGFV is the default mode of operation in many applications. In the example described previously in the Data Acquisition Signal Chain Using the ADG5462F Channel Protector, there were already suitable supply rails available for the user, for example, ±15 V supplies for the PGA and +5 V/GND supplies for the ADC. In such a case, the wider supply range is used for optimal analog performance and the secondary supplies protect downstream components from overvoltage faults above the expected signal range. 13636-005 VDD VSS NEGFV POSFV Figure 7. Overshoot During Fault Condition Figure 5. Primary Supplies Shorted to Secondary Supplies T VDD/POSFV VDD VSS 0.1µF 0.1µF POSFV 0.1µF NEGFV 0.1µF DRAIN SOURCE A CH1 11.0V Figure 6. Drain Output Response to Positive Overvoltage (POSFV Tied to VDD) There are both advantages and disadvantages to consider when using this configuration. Advantages to Tying the Supplies This is the simplest configuration; no additional supply rails or discrete components are required. Disadvantages to Tying the Supplies If the VDD/VSS range is reduced to meet the protection voltage requirements, then RON performance is not optimized compared to setting a wider VDD/VSS range. In addition, the Rev. 0 | Page 4 of 6 VDD VSS 13636-008 M2.00µs NEGFV CH2 5.00V CH4 5.00V POSFV CH1 5.00V 13636-006 4 Figure 8. Separate Low Impedance Secondary Supply Rails Application Note AN-1380 T T VDD VDD POSFV POSFV DRAIN SOURCE DRAIN SOURCE CH1 5.00V CH3 5.00V CH2 5.00V CH4 5.00V M2.00µs A CH1 11.0V CH1 5.00V CH3 5.00V Figure 9. Drain Output Response to Positive Overvoltage (Dedicated POSFV Supply Rail) Advantages to Separate Supplies Using separate supplies provides optimum RON performance. In addition, the user sets fault thresholds according to specific protection requirements of downstream components. Disadvantages to Separate Supplies An important consideration is that separate low impedance power supply rails are required for this configuration. If these are not available in the system, they must be generated from a dc-to-dc converter or from a buffered resistor divider from the primary supplies. ADDING A DIODE FROM THE PRIMARY SUPPLY RAIL TO THE SECONDARY SUPPLY RAIL There may be cases where downstream components are very sensitive to overvoltage stresses above the primary supply rails. In these cases, the drain pin clamping to VDD + 0.7 V for 500 ns following a fault may not be acceptable. One option to reduce the clamp voltage to the approximate VDD voltage level is to add a diode between VDD and POSFV. With POSFV at a diode drop below VDD, the fault threshold and the clamp voltage are approximately equal to the VDD voltage. VDD VSS 0.1µF 13636-010 NEGFV VDD VSS A CH1 11.0V Because the internal drain clamp diodes are referenced to the secondary supplies and because the secondary supply is not driven by a low impedance source, this solution is only suitable for situations where the source pin goes into fault rapidly. If the source pin is brought into fault at a slow ramp rate, the POSFV or NEGFV pin can pull with the fault and remain a diode drop below the fault voltage. This can cause a scenario where the overvoltage event is not detected. If the ramp rate into a fault condition is slow, then a larger POSFV/NEGFV stabilization capacitor is helpful. There are both advantages and disadvantages to consider when using this configuration. Advantages to Adding a Diode Adding a diode from the primary supply rail to the secondary supply rail limits overshoot above VDD to sensitive downstream circuitry. In addition, this configuration sets a custom fault threshold without generating additional system rails. Disadvantages to Adding a Diode Additional discrete components (that is, two diodes) are required to generate the POSFV/NEGFV rails. There is also the possibility of a slightly reduced signal range (if internal and external diode drops do not match, the fault detector may trip inside the primary supply rails). This configuration is also not suitable for fault conditions with a slow ramp rate. USING REVERSE BREAKDOWN VOLTAGE OF ZENER DIODE TO CONFIGURE POSFV/NEGFV 0.1µF POSFV 0.1µF M2.00µs Figure 11. Drain Output Response to Positive Overvoltage (Diode from VDD to POSFV) There are both advantages and disadvantages to consider when using this configuration. 0.1µF CH2 5.00V CH4 5.00V 13636-011 4 13636-009 4 Figure 10. Diodes Configured Secondary Supply Rails In cases where additional custom supply rails are not available, the POSFV and NEGFV supplies need to be generated by the system designer. One option to achieve this is to use a Zener diode between the primary supply rail and the secondary supply rail, and then utilize the reverse breakdown voltage due to the POSFV/NEGFV supply current (Zener voltage) to configure the secondary rails. Zener diodes are readily available with breakdown voltages of 2 V and above, so any POSFV/NEGFV voltage can be generated using this method. Rev. 0 | Page 5 of 6 AN-1380 Application Note The drain response is similar to having a dedicated secondary supply rail, as shown in Figure 9. There may be some variation in Zener voltage across different devices, and the Zener voltage also varies over temperature. Therefore, the POSFV/NEGFV voltage (and, hence, the fault threshold voltage) is not as well regulated as a dedicated supply rail. However, it is a cheap and simple way to configure a secondary supply rail if the fault threshold tolerance is sufficient for the application. VDD VSS 0.1µF 0.1µF Using a Zener diode configuration sets the custom fault threshold without generating additional system rails. Disadvantages for Using a Zener Diode The Zener diode configuration requires additional discrete components to generate POSFV/NEGFV rails. Consequently, variations in Zener voltage across devices and temperature directly impact fault threshold accuracy. The Zener diode configuration is not suitable for fault conditions with a slow ramp rate (similar to the previous diode configuration). SUMMARY 13636-012 VDD VSS NEGFV 0.1µF POSFV 0.1µF Advantages for Using a Zener Diode Figure 12. Zener Diode Configured Secondary Supply Rails There are both advantages and disadvantages to consider when using this configuration. The fault protected switches allow the user to set a specific fault threshold at which the switch turns off. The ability to set a wide primary supply voltage enables the switch to achieve optimum analog performance (for example, flatter, lower RON). If a lower fault threshold voltage is required, then POSFV and NEGFV require separate low impedance supplies to generate those thresholds. ©2015 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN13636-0-12/15(0) Rev. 0 | Page 6 of 6