Protecting and Powering Automotive Electronics Systems with No Switching Noise and 99.9% Efficiency

Vol 54 No 1, February 2020

David Megaw, Senior Design Engineer

Introduction ISO 16750-2 Conditions for Automotive Powering automotive electronics systems can be challenging due to require- Electronics Systems ments of high reliability while contending with a relatively unstable battery Analog Devices has several publications covering the ISO 7367-2 and ISO 16750-2 voltage. The variety of electrical and mechanical systems that interface with a specifications in detail, along with how to simulate them using LTspice®.1,2,3,4 vehicle’s battery can cause wild voltage excursions in the nominal 12 V supply. In its latest iteration, the ISO 7367-2 electromagnetic compatibility specification In reality, 12 V can vary from –14 V to +35 V for extended periods of time and focuses on high amplitude (>100 V), short duration (150 ns to 2 ms) transients experience voltage spikes with extremes ranging from +150 V to –220 V. Some from relatively high impedance sources (2 Ω to 50 Ω). These voltage spikes can of these surges and transients arise from everyday use, others from fault condi- often be mitigated with passive components. Figure 1 shows ISO 7367-2 pulse 1 tions or human error. Regardless of cause, the damage they can produce in a as defined and with an added 330 µF bypass capacitor. The capacitance reduces vehicle’s electronics system can be difficult to diagnose and expensive to fix. the spike’s amplitude from –150 V to –16 V, well within the range of reverse The experience of automakers over the last century has led to a catalog of elec- battery protection circuitry. ISO 7367-2 pulses 2a, 3a, and 3b have significantly trical conditions and transients that are known to disrupt operation and cause lower energy than pulse 1 and require less capacitance to suppress. damage. The International Organization for Standardization (ISO) has compiled ISO 7367–2: Pulse 1 this industry knowledge into the ISO 16750-2 and ISO 7367-2 specifications for road vehicles. At a minimum, the power supply for an automotive electronic control unit (ECU) should survive these conditions without damage. For critical systems, functionality and tolerances must be maintained. This requires that the power supply regulates the output voltage through the transient to preserve

ECU operation. Ideally, a complete power solution avoids the need for fuses, –16 V minimizes power dissipation, and features low quiescent current to support always-on systems without draining the battery. 30 V/div

–150 V Nominal 330 µF Bypass Capacitor

2 ms/div Figure 1. ISO 7367-2: pulse 1 with and without a 330 µF bypass capacitor.

VISIT ANALOG.COM ISO 16750-2 focuses on longer duration pulses from low impedance sources. The concern with any linearly regulated power device such as a surge stopper is These transients cannot easily be filtered and frequently require active, regula- the potential for significant power dissipation in the N-channel MOSFET(s) when tor-based solutions. Some of the more challenging tests include the load dump limiting the output voltage during a load dump, or when limiting current with a (test 4.6.4), the reverse battery condition (test 4.7), the superimposed alternating shorted output. The safe operating area (SOA) constraints of the power MOSFET voltage test (test 4.4), and the engine starting profile (test 4.6.3). Figure 2 gives ultimately limit the maximum current possible with a surge stopper. It also puts a visual overview of these test pulses. The variety of conditions presented in ISO a time limit (typically set with a programmable timer pin) on how long regula- 16750-2, along with the voltage and current requirements of the ECU, frequently tion can be maintained before the N-channel MOSFET must be shut off to avoid require a combination of approaches in order to satisfy them all. damage. These SOA imposed limitations become more acute at higher operating voltages, making surge stopper use trickier for 24 V and 48 V systems. Load Dump A more scalable approach is to use a buck regulator capable of operating A load dump (ISO 16750-2: test 4.6.4) is a severe overvoltage transient that models with a 42 V input, such as the LT8640S. A switching regulator doesn’t have the a battery disconnect while the alternator is sourcing substantial current. The peak MOSFET SOA limitations of a linear regulator, but it is certainly more complex. voltage during a load dump is classified as either suppressed or unsuppressed, The efficiency of a buck regulator allows for very high current operation, and depending on whether avalanche diodes are used on the outputs of the 3-phase its top switch permits output disconnect and current limiting. Concerns about alternator. A suppressed load dump pulse is limited to 35 V, whereas an unsup- buck regulator quiescent current have been put to rest with the latest genera- pressed pulse peak ranges from 79 V to 101 V. In either case, it can take up to tion of parts that draw only a few microamps while in regulation under no load 400 ms to recover due to the large amount of magnetic energy stored in the conditions. Switching noise has also improved substantially with Silent Switcher® alternator’s stator windings. While most automakers utilize avalanche diodes, technology and spread spectrum frequency modulation techniques. increasing reliability demands are driving some manufacturers to require that ECUs satisfy peak load dump voltages approaching that of the unsuppressed case. Additionally, some buck regulators are able operate at 100% duty cycle such that the top switch is on continuously, passing the input voltage to the output One solution for facing load dumps is adding a transient voltage suppressor (TVS) through the inductor. Switching operation is triggered during overvoltage or diode to locally clamp the ECU supply. A more compact and tighter tolerance overcurrent conditions to limit output voltage or current, respectively. These approach is to use an active surge stopper, such as the LTC4364, which linearly buck regulators, such as the LTC7862, act as switching surge stoppers, achieving controls a series N-channel MOSFET to clamp the maximum output voltage to a low noise, low loss operation while still retaining the robustness of a switch- user programmed level (for example, 27 V). Surge stoppers add the ability to dis- mode power supply. connect the output, allowing programmable current limit and undervoltage lockout, and often provide reverse battery protection when back-to-back NFETs are used.

79 V to 101 V

35 V (Suppressed)

16 V

12 V 12 V 400 ms 5 V Sweep Frequency 50 Hz to 25 kHz 3 V Logarithmically over 60 s 15 ms

60 s

–14 V

Starting Proﬁle Load Dump Reversed Voltage Superimposed Alternating Voltage Figure 2. Overview of some of the tougher ISO 16750-2 tests.

2 Protecting and Powering Automotive Electronics Systems with No Switching Noise and 99.9% Efficiency Reverse Voltage of power MOSFETs. Polarized bypass capacitors such as aluminum electrolytics may also be damaged during the reverse battery condition. A reverse voltage condition (also known as a reverse battery condition) occurs when battery terminals or jumper cables are connected backward due to opera- A Schottky diode can prevent reverse currents, but this approach leads to tor error. The relevant ISO 16750-2 pulse (test 4.7) applies –14 V to the DUT for significant power loss at higher forward currents in normal operation. A simple 60 second durations, repeatedly. Some manufacturers add their own dynamic protection scheme based on series P-channel MOSFET shown in Figure 3 reduces versions of this test where the part is initially powered (for example, VIN = 10.8 V) this loss, but may not work well at low input voltages (for example, engine start) before a reverse bias (–4 V) is abruptly applied. due to the device threshold voltage. A more efficient approach is to use an ideal diode controller, such as the LTC4376, which drives a series N-channel MOSFET A quick survey of data sheets shows that few ICs are designed to tolerate nega- that cuts off input voltages below ground. In normal operation, an ideal diode tive biases, with IC absolute minimum pin voltages typically limited to –0.3 V. controller regulates the source to drain the voltage of the N-channel MOSFET to Voltages more than a diode below ground can cause excessive current flow 30 mV or less, reducing forward voltage drop and power dissipation by more than through internal junctions such as ESD protection devices as well as body diodes an order of magnitude compared to a Schottky diode.

Load Dump Engine Start Reverse Voltage Superimposed AC

VIN VOUT

VIN VOUT VIN VOUT VIN VOUT C C C Buck C Regulator

TVS Low Dropout Linear/Switching Regulator Schottky Diode Schottky Diode Pros: Simple and Inexpensive Pros: No Boost Regulator Required Pros: Simple and Inexpensive Pros: Simple and Inexpensive Cons: Bulky, High Power Dissipation Cons: Only Works for Low Output Voltage ECUs Cons: Bulky, High Power Dissipation, Cons: Bulky, High Power Dissipation, Examples: LT8610, LT8640S, LT8650S Large Forward Voltage Drop Large Forward Voltage Drop

L VIN VOUT V V IN OUT V V IN OUT High PSRR V V IN Regulator OUT Surge C C C Boost C Stopper Regulator

Surge Stopper Boost Regulator Series PMOS High PSRR Linear/Switching Regulator Pros: Compact, Adds Current Limiting Pros: Efficiently Step Up Input at Pros: Lower Power Dissipation/Voltage Drop Pros: Regulate Through Input AC and Output Disconnect High Currents Cons: High Resistance at Low Input Voltages Cons: Linear Regulator Current is

Cons: SOA of N-Channel MOSFET Limits Cons: Noise, Body Diode from VIN to VOUT Limited, Switching Regulator Requires Current and Transient Duration Prevents Current Limiting, Output Disconnect High Bandwidth Examples: LTC4356, LTC4366, LTC4380 Examples: LT8603, LT8364, LTC3859 Examples: LT3066

L VIN V L OUT VIN VOUT VIN VOUT

VIN VOUT C C Surge C

Higher Current, More Robust More Current, Higher Stopper Ideal Active C Simpler, Less Design Complexity Design Less Simpler, Buck and Boost Diode Rectiﬁer Regulator Regulator Controller

Boost with Surge Stopper Ideal Diode Controller Active Rectiﬁer Controller Buck Regulator Pros: Boosting Plus Output Disconnect Pros: Lower Power Dissipation/Voltage Drop Pros: Rectify up to 100 kHz Pros: Efficient, No NFET SOA Limitations, and Current Limit Cons: Too Slow to Rectify Superimposed AC AC Efficiently Can Disconnect Output and Limit Current Cons: NFET SOA Limits Curent and Examples: LTC4359, LTC4376 Cons: None Cons: Noise Increased Design Complexity Transient Duration Examples: LT8672 Examples: LT8640S, LT8650S, LTC3895 Examples: LTC3897, LTC3897-2

L L VIN VOUT VIN VOUT C C

4-Switch 4-Switch Buck-Boost Buck-Boost Regulator Regulator

4-Switch Buck-Boost Regulator 4-Switch Buck-Boost Regulator with Reverse Voltage Protection Pros: Efficiently Steps Up or Down at High Currents, Output Disconnect and Current Limit Pros: Solves Load Dump, Engine Start, Reverse Voltage, Superimposed AC at High Current Cons: 4× NFETs, Increased Design Complexity, No Reverse Voltage Protection Cons: 5× NFETs, Increased Design Complexity Examples: LT8390, LT8705A, LTC3779, LT8210 Examples: LT8210

Figure 3. Different approaches to solving difficult ISO 16750-2 tests.

VISIT ANALOG.COM 3 Superimposed Alternating Voltage controller, incorporate a surge-stopper front end to allow output disconnect and current limiting, as well as provide reverse voltage protection when back-to- The superimposed alternating voltage test (ISO 16750-2: test 4.4) models the back N-channel MOSFETs are used. This solution can address the load dump, impact of the ac output of the vehicle’s alternator. As the name implies, a sinu- engine start, and reverse battery conditions with a single integrated circuit, but soidal signal is superimposed on the battery rail with peak-to-peak amplitude available current is limited by the SOA of the surge-stopper MOSFET. of 1 V, 2 V, or 4 V, depending on severity level classification. For all severity levels, the maximum input voltage is 16 V. The frequency of the sinusoid is swept A 4-switch buck-boost regulator lifts this constraint by combining a synchronous logarithmically from 50 Hz to 25 kHz and then back to 50 Hz over 120 seconds buck regulator and a synchronous boost regulator through a shared inductor. and repeated a total of five times. This approach can satisfy both the load dump and engine starting profile tests without MOSFET SOA limitations on current level or pulse duration, while retain- This test causes large amplitude current and voltage swings in any connected ing the ability to disconnect the output and limit current. filter network with a resonance below 25 kHz. It can also cause problems for switching regulators where loop bandwidth limitations make it a struggle to The switching operation of a buck-boost regulator depends on the relationship regulate through high frequency input signals. One solution is an intermediate between the input and output voltages. If the input is significantly higher than rectifying element such as a power Schottky diode, but as with reverse voltage the output, the boost top switch turns on continuously while the buck power protection, this is a poor way to solve the problem. stage steps down the input. Similarly, when the input is significantly lower than the output, the buck top switch turns on continuously while the boost power An ideal diode controller will not work here like it did for reverse voltage protec- stage steps up the output. When the input and output are roughly equal (within tion because it cannot switch the N-channel MOSFET sufficiently fast to keep up 10% to 25%), the buck and boost power stages switch simultaneously in an with the input. The limiting factor is the gate pull-up strength, which is typically interleaved fashion. In this way, efficiency is maximized in the various switching limited to 20 µA or so by an internal charge pump. While the ideal diode control- regions (buck, buck-boost, boost) by limiting switching to only the MOSFETs ler can quickly turn the MOSFET off, turn on is painfully slow, unsuitable for required for regulation for input voltages above, roughly equal to, or below the rectification of anything beyond very low frequencies. output, respectively. A more elegant approach is to use the LT8672 active rectifier controller, which can toggle an N-channel MOSFET fast enough to rectify the input voltage at ISO 16750-2 Solution Summary frequencies up to 100 kHz. An active rectifier controller is an ideal diode controller Figure 3 summarizes the various solutions to addressing the load dump, with two important additions: a large reservoir of charge boosted up from the reverse input voltage, superimposed alternating voltage, and engine starting input voltage and a powerful gate driver to toggle the N-channel MOSFET both on profile tests along with the pros and cons of each approach. Several key take- and off quickly. This method can reduce power loss by more than 90% compared aways begin to emerge: to a Schottky. The LT8672 also protects downstream circuitry from a reverse battery condition just as an ideal diode controller would. X A series N-channel MOSFET with its drain facing the input is extremely desirable because it can be used to limit current and disconnect the output Starting Profile whether it’s used as a switch (for example, in a buck power stage) or linearly controlled (for example, in a surge stopper). The engine starting profile (ISO 16750-2: test 4.6.3) is an extreme undervoltage transient, sometimes referred to as the cold crank pulse because the worst-case X In the case of reverse input protection and superimposed alternating volt- battery droop occurs at lower temperatures. Specifically, when the starter turns age, using an N-channel MOSFET as the rectifying element (source facing over, the 12 V battery voltage can momentarily drop to 8 V, 6 V, 4.5 V, or 3 V, the input) significantly reduces power loss and voltage drop compared to a depending on severity level classification (I, IV, II, III, respectively). Schottky diode. X A switch-mode power supply is preferable to a linear regulator because it In some systems, a low dropout (LDO) linear regulator or switching buck regula- alleviates the reliability concerns and output current restrictions that come tor is sufficient to allow power rails to ride through this transient, provided that with operating within the SOA of the power device. It can regulate at input ECU voltages are less than the lowest input voltage. For example, if the highest voltage extremes indefinitely, whereas linear regulator and passive solutions ECU output voltage is 5 V, and it must satisfy severity level IV (minimum input have inherent time limitations that complicate design. voltage of 6 V), then a regulator with a dropout voltage less than 1 V is enough. The lowest voltage segment of an engine starting profile lasts for only 15 ms X A boost regulator may or may not be necessary depending on the severity to 20 ms, so a rectifying element (Schottky diode, ideal diode controller, active classification of the starting profile and the details of the ECU (what’s the rectifier controller) followed by a large bypass capacitor may be able to ride highest voltage it must provide). through this portion of the pulse if the voltage headroom briefly dips below the If boost regulation is called for, then a 4-switch buck-boost regulator com- regulator dropout. bines the above desirable traits into a single part. It can efficiently regulate through severe undervoltage and overvoltage transients at high current levels If, however, the ECU must support voltages higher than the lowest input voltage, for extended periods of time. This makes it the most robust and straightfor- then a boost regulator is required. Boost regulators can efficiently maintain a ward approach, from an application’s standpoint, despite the increased design 12 V output voltage from inputs less than 3 V at high current levels. There is one complexity. Nevertheless, a typical 4-switch buck-boost regulator has some problem with a boost regulator, though: the diode path from input to output pre- drawbacks. For one, reverse battery protection is not naturally provided and vents disconnect, so current is not naturally limited at startup or due to a short. must be addressed with additional circuitry. To prevent current runaway, specialized boost regulators, such as the LTC3897

4 Protecting and Powering Automotive Electronics Systems with No Switching Noise and 99.9% Efficiency The primary shortcoming of the 4-switch buck-boost regulator is that it spends 100 V LT8210 much of its operational life in the lower efficiency, noisier buck-boost switching Buck-Boost VOUT(BUCK) region. When the input voltage is nearly equal to the output (VIN ~ VOUT) all four Buck N-channel MOSFETs are actively switching to maintain regulation. Efficiency Pass-Thru Window drops as a result of increased switching losses and application of maximum gate VIN 99.9% Efficiency VOUT = VIN No EMI × I = 18 µA drive current. Radiated and conducted EMI performance suffer in this region Q as both the buck and boost power-stage hot loops are active and the regulator Boost V input and output currents are discontinuous. OUT(BOOST) 2.8 V A 4-switch buck-boost regulator can regulate through occasional large amplitude undervoltage and overvoltage transients, but at the price of operating with Figure 4. Buck-boost controller with Pass-Thru mode solves many of the problems imposed high quiescent current, lower efficiency, and increased noise in the far more by automotive standards tests. common, nominal conversion region. In Pass-Thru mode, the output voltage is regulated to the edges of a voltage window when the input voltage is outside the window. The top of the window Pass-Thru Operating Mode Brings High and the bottom are programmed via the FB2 and FB1 resistor dividers. When the Efficiency and EMI Performance Buck-Boost input voltage is within this window, the top switches (A and D) turn on continu- Region ously, passing the input voltage directly to the output. In this nonswitching state, The LT8210 is a 4-switch buck-boost dc-to-dc controller that can operate the total LT8210 quiescent current drops to tens of microamps. The absence of conventionally with a fixed output voltage and also features a new Pass-Thru™ switching means no EMI and no switching losses, enabling efficiencies of greater operating mode (Figure 4), which eliminates the switching losses and EMI than 99.9%. through a programmable input voltage window. It operates from 2.8 V to 100 V, For those who want the best of both worlds, the LT8210 allows transitioning allowing it to regulate through the most severe battery drop during engine start between its different operating modes on the fly by toggling the MODE1 and MODE2 to the peak amplitude of an unsuppressed load dump. It comes with built-in pins. In other words, the LT8210 can operate as a conventional buck-boost regula- reverse battery protection to –40 V, which is implemented with the addition of a tor with fixed output voltage( in CCM, DCM, or Burst Mode™) in some situations and single N-channel MOSFET (DG in Figure 5). then change to Pass-Thru mode as the conditions in the application change. This can be a useful feature for always-on systems and in start-stop applications.

DG A 2 mΩ 2.6 μH D 2 mΩ VIN 3 V to 100 V VOUT + 100 μF B C 10 μF 47 μF + 1 μF 470 μF ×2 0.1 μF 0.1 μF ×2 100 Ω

49.9 Ω 49.9 Ω

TG1 BST1 SW1 BG1 SNSP1 SNSN1 PGND BG2 SW2 BST2 TG2 10 nF V VINP OUT 3.3 μF DG SNSP2

VIN SNSN2 EN/UVLO EXTVCC DBST1 LT8210 1.58 MΩ BST1 GATEVCC FB1 VDD DBST2 2 MΩ 226 kΩ 2.2 μF MODE1 BST2 FB2 MODE2 100 kΩ PWGD V SYNC/ DD 124 kΩ 4.7 μF SPRD SSRT GND VC1 VC2 IMON 2.61 kΩ 23.2 kΩ 20 kΩ 140 kΩ 10 nF 820 pF 6.8 nF 220 pF 2.2 nF

(350 kHz) MOSFETS A, B: INFINEON BSZ097N10NS5 DBST1, DBST2: DIODES INC. US1GWF MOSFETS C, D: INFINEON BSZ034N04LS L1: 2.6 µH WÜRTH 7443556260 MOSFET DG: INFINEON BSZ042N06LS5 Figure 5. This 3 V to 100 V input buck-boost controller operates with 8 V to 17 V Pass-Thru outputs.

VISIT ANALOG.COM 5 Pass-Thru Performance VOUT 5 V/div V = 8 V The Pass-Thru solution in Figure 5 passes the input to output in the window VIN OUT 5 V/div between 8 V and 17 V. When the input voltage is above the Pass-Thru window, the

LT8210 bucks it down to a regulated 17 V output. If the input drops below 8 V, the VIN = 3 V LT8210 boosts the output to 8 V. As a protection feature, switching operation is triggered within the Pass-Thru window to control current if either the inductor current Pass-Thru Pass-Thru limit or programmed average current limit (via the IMON pin) are exceeded.

SW1 Figure 6, Figure 7, and Figure 8 show the response of the LT8210 circuit to the 10 V/div load dump, reverse voltage, and starting profile tests, respectively. Figure 9 and Figure 10 demonstrate the efficiency improvement and low current operation SW2 possible while in the Pass-Thru window (it’s amazing how efficient doing nothing 10 V/div can be). Figure 11 shows the dynamic transition between Pass-Thru mode and 20 ms/div CCM operation. An LTspice simulation of this circuit along with sped up versions Figure 8. Pass-Thru response to engine cold crank. of the toughest ISO 16750-2 test pulses is available at analog.com/media/en/ simulation-models/LTspice-demo-circuits/LT8210_AutomotivePassThru.asc. 100

90 99.8%

95 V 80 VIN 50 V/div 70

60 V = 16 V OUT 50 VOUT 20 V/div 40 Efﬁciency (%)

20 VIN = 12 V Pass-Thru Pass-Thru 10 CCM Pass-Thru SW1 50 V/div 0 0.0001 0.001 0.01 0.1 1 5

100 ms/div Load Current (A) Figure 6. Pass-Thru response to unsuppressed load dump. Figure 9. Efficiency for CCM and Pass-Thru operation.

160 Total I DG Q I Contribution from FB1, FB2 Dividers 5 V/div Q

VIN 5 V/div 120

VDG = VIN

80 Current (µA)

VIN = –14 V

40 VINP 1 V/div IIN 100 mA/div 0 2 ms/div –40 200–20 40 60 80 100 120 Figure 7. LT8210 response to reverse battery condition. Temperature (°C)

Figure 10. No load input current in Pass-Thru mode (VIN = 12 V).

6 Protecting and Powering Automotive Electronics Systems with No Switching Noise and 99.9% Efficiency Mode 1, 2 The novel LT8210 control scheme results in clean and fast transitions between the 5 V/div different switching regions (boost, buck-boost, buck, and nonswitching) allowing it to regulate through large signal, high frequency ac voltage on the input. The LT8210 can be toggled between Pass-Thru operation and conventional, fixed output voltage, buck-boost operating modes (CCM, DCM, or Burst Mode) while run- ning, and the fixed output can be set to any voltage within the Pass-Thru window VOUT CCM Pass-Thru CCM 5 V/div (for example, VOUT = 12 V for an 8 V-to-16 V window). This flexibility allows the user to alternate between Pass-Thru and normal buck-boost operation to trade the low SW1 10 V/div noise, low IQ, high efficiency operation of Pass-Thru mode for the tighter regulation and improved transient response in CCM, DCM, or Burst Mode. SW2 10 V/div References

1 400 µs/div Dan Eddleman. “Low Quiescent Current Surge Stopper: Robust Automotive Figure 11. Dynamic transition between Pass-Thru and CCM operation. Supply Protection for ISO 7637-2 and ISO 16750-2 Compliance.” LT Journal of Analog Innovation, January 2017.

Conclusion 2 Christian Kueck. “Distilled Automotive Electronics Design.” Analog Devices, Inc., When designing a power supply for an automotive electronic system, the LT8210 April 2013. 4-switch buck-boost dc-to-dc controller offers a superior solution with its 2.8 V 3 Bin Wu and Zhongming Yi. “Comprehensive Power Supply System Designs for to 100 V input operating range, built-in reverse battery protection, and its new Harsh Automotive Environments Consume Minimal Space, Preserve Battery Pass-Thru operating mode. Pass-Thru mode improves upon buck-boost operation, Charge, Feature Low EMI.” Analog Dialogue, Vol. 53, August 2019. achieving no switching noise, no switching losses, and ultralow quiescent current while regulating the output to a user-programmed window rather than a fixed 4 Dan Eddleman. “LTspice: Models of ISO 7367-2 and ISO 16750-2 Transients.” voltage. The output voltage minimum and maximum values are bounded during Analog Devices, Inc., 2019. large amplitude transients such as load dump and cold crank without MOSFET SOA ISO 7637-2:2011. International Organization of Standardization, March 2011. concerns, or the current or timing limitations that come from linear solutions. ISO 16750-2:2012. International Organization of Standardization, November 2012.

About the Author David Megaw is a senior design engineer with ADI. He can be reached at [email protected].

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