Powering car electronics with no switching noise and 99.9% efficiency: Page 2 of 7

August 20, 2020 //By David Megaw, ADI
Powering car electronics with no switching noise and 99.9% efficiency
Powering automotive electronics systems can be challenging due to requirements of high reliability while contending with a relatively unstable battery voltage. This article describes a solution that keeps voltage stable – at lowest power dissipation and without the need for fuses.

One solution for facing load dumps is adding a transient voltage suppressor (TVS) diode to locally clamp the ECU supply. A more compact and tighter tolerance approach is to use an active surge stopper, such as the LTC4364, which linearly controls a series N-channel MOSFET to clamp the maximum output voltage to a user programmed level (for example, 27 V). Surge stoppers add the ability to disconnect the output, allowing programmable current limit and undervoltage lockout, and often provide reverse battery protection when back-to-back NFETs are used.

The concern with any linearly regulated power device such as a surge stopper is the potential for significant power dissipation in the N-channel MOSFET(s) when limiting the output voltage during a load dump, or when limiting current with a shorted output. The safe operating area (SOA) constraints of the power MOSFET ultimately limit the maximum current possible with a surge stopper. It also puts a time limit (typically set with a programmable timer pin) on how long regulation can be maintained before the N-channel MOSFET must be shut off to avoid damage. These SOA imposed limitations become more acute at higher operating voltages, making surge stopper use trickier for 24 V and 48 V systems.

A more scalable approach is to use a buck regulator capable of operating with a 42 V input, such as the LT8640S. A switching regulator doesn’t have the MOSFET SOA limitations of a linear regulator, but it is certainly more complex. The efficiency of a buck regulator allows for very high current operation, and its top switch permits output disconnect and current limiting. Concerns about buck regulator quiescent current have been put to rest with the latest generation of parts that draw only a few microamps while in regulation under no load conditions. Switching noise has also improved substantially with Silent Switcher® technology and spread spectrum frequency modulation techniques.

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 through the inductor. Switching operation is triggered during overvoltage or overcurrent conditions to limit output voltage or current, respectively. These buck regulators, such as the LTC7862, act as switching surge stoppers, achieving low noise, low loss operation while still retaining the robustness of a switch-mode power supply.

Reverse Voltage

A reverse voltage condition (also known as a reverse battery condition) occurs when battery terminals or jumper cables are connected backward due to operator error. The relevant ISO 16750-2 pulse (test 4.7) applies –14 V to the DUT for 60 second durations, repeatedly. Some manufacturers add their own dynamic versions of this test where the part is initially powered (for example, VIN = 10.8 V) before a reverse bias (–4 V) is abruptly applied.

A quick survey of data sheets shows that few ICs are designed to tolerate negative biases, with IC absolute minimum pin voltages typically limited to –0.3 V. Voltages more than a diode below ground can cause excessive current flow through internal junctions such as ESD protection devices as well as body diodes of power MOSFETs. Polarized bypass capacitors such as aluminum electrolytics may also be damaged during the reverse battery condition.

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