In my recent project, I decided to swap out an older device with a newer and more energy-efficient model. To my surprise, this change led to unexpected issues—my equipment started malfunctioning, and the new device even got damaged. I was puzzled by this outcome. How could something that seemed like an improvement end up causing such a problem?
As someone who deals with linear regulators regularly, I know they’re relatively straightforward components. Yet, there are still moments when things go awry. Recently, I reached out to Abhinay Patil, the Field Application Manager at ADI, to help shed light on this issue. His insights were both enlightening and educational.
Abhinay recounted a similar scenario he encountered when he worked as a field application engineer. Often, customers would approach him requesting device replacements from different vendors. These decisions were typically driven by procurement teams, with little involvement from the original circuit designers. The logic behind the swap was simple: the new device needed to match or exceed the functionality and performance of the old one, while ideally consuming less power. With all these criteria met, the new part would be added as an alternative supplier option. At first glance, it sounds foolproof, but complications can arise post-replacement.
This exact situation unfolded in one of his cases. A client had decided to replace an RS-485 transceiver from another vendor with one of theirs. Both devices appeared identical in terms of shape, size, and function, with the new device boasting superior electrical specifications. Confident in their choice, the client placed a significant order. However, upon testing, the new transceiver began failing on the production line. Since nothing else in the design had changed, the issue clearly lay with the new device.
Further investigation revealed that the linear regulator supplying the bus side of the transceiver wasn’t holding steady at 5V, as anticipated, but was creeping up to a higher voltage. This was perplexing because everything seemed correct in theory. It was only after scrutinizing the datasheets of both the old and new transceivers alongside the regulator’s that the root cause became clear.
The term "better" is subjective and context-dependent. For instance, higher speeds, common-mode rejection ratios (CMRR), and power supply rejection ratios (PSRR) are generally desirable. Conversely, lower offset voltages and drift are preferable. However, power consumption isn’t always better when reduced too far. The original transceiver consumed about 15mA under idle conditions, whereas the new one drew just 2mA. On paper, this made the new device look superior, but it overlooked a critical aspect of linear regulators—they require a minimum load current to function correctly. Without this, the regulator can't maintain regulation, leading to voltage fluctuations.
Many contemporary linear regulators address this limitation, but some older models don’t. This oversight necessitates extra caution during system design. Sometimes, the feedback resistor network of an adjustable linear regulator (LDO) contributes to this minimum load current requirement. If you drastically increase the resistance to keep the same ratio, you might inadvertently create problems. Another pitfall arises when the load connected to the LDO meets requirements during normal operation but falls short in standby modes.
To avoid such mishaps, always review the LDO datasheet thoroughly. Minimum load current requirements are often explicitly stated. Here are a few examples:
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Figure 1 shows an illustration of the minimum load current requirement in the datasheet.
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Figure 2 illustrates the difference between stable (left) and unstable (right) regulator behavior due to insufficient load current.
Returning to our case study—once we understood the underlying issue, resolving it was straightforward. Adding a bleeder resistor to the regulator output ensured the minimum load current was met. While some clients might have been quick to blame our product, the customer in this instance appreciated learning about this technical nuance.
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Figure 3 demonstrates how adding a bleeder resistor resolved the problem.
It’s a bit like the conclusion of a fairy tale—there’s a sacrifice, but ultimately, everyone learns and moves forward happily.
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