ROHM’s New LDOs Are Stable At Nanoscale Output Capacitances
Aimed at 12 V automotive applications, the tiny LDO (low-dropout) regulators are AEC-Q100 compliant, feature low magnetic field radiation, and sport high current ratings.
Released last Tuesday, ROHM’s BD9xxN1 series of LDOs operate from input voltages ranging from 4.5 to 42 V. Fixed outputs of 3.3 and 5.0 V are available, as are units whose outputs are adjustable over a 1-18 V range. Maximum out current for series members is 150 mA.
ROHM's new BD9xxN1 series. Image used courtesy of ROHM
The six new devices employ the company’s trademarked Nano Cap control technology to operate in a stable manner even with output capacitances in the 100 nF range; ROHM claims that this figure is a ten-fold improvement over the 1 µF required by standard industrial devices. At the other end of the spectrum, the company’s fresh series can also accommodate output capacitances as high as 470 µF.
Nano Cap-enabled LDOs are stable across a wide spectrum of output capacitances. Image [modified] used courtesy of ROHM
The key difference enabling such a range is that Nano Cap control technology exploits current feedback as well as voltage feedback, while standard devices utilize only the latter.
Below, we’ll first provide an educational overview for those unfamiliar with LDOs and the terms that define them, before describing additional LDO-specific technology developed by ROHM. We’ll then round out our discussion of the company’s new series with a quick discussion of applications and physical specifications.
For full details regarding these new products, comprehensive device specifications can be found in the series datasheet, while all six models of the BD9xxN1 series are described on the series product page.
What is an LDO?
An LDO is a simple, inexpensive tool to provide a stable, lower output voltage from a higher input voltage source. Its basic operation is illustrated below.
An internal circuit outline. Screenshot used courtesy of ROHM
The output voltage is sampled by the resistors and compared to reference. When the MOSFET is turned on, VIN – VOUT transverses it, as does all of the LDO’s output current. If the output current is high and the difference between VIN and VOUT is large, the MOSFET heats up, and power is wasted. That’s why LDOs work best when VOUT isn’t much less than VIN, and the supplied output current is low.
The LDO’s great advantage is that there is no switching, so no bulky magnetics are needed, and there is no possibility of EMI or switching noise. There are also fewer ancillary components, and LDOs tend to be cheaper and much smaller than switching power supplies.
Why is Low Output Capacitance Important to Engineers?
The name of the game in modern electronics is to pack more function into less space, and nowhere is that more true than in automotive electronics. The space that a smaller output capacitor saves as compared to a larger one can readily be reassigned to other functionality on the PCB.
The topology of any LDO requires that the output voltage be lower than the input voltage, and the minimal separation between the two is described as the dropout voltage. Depending on circumstances described in detail in the series datasheet, typical dropout voltage for members of the BD9xxN1 series ranges from 420 to 780 mV.
For any voltage conversion device, it is vital that the output voltage remains stable even in response to abrupt changes in the output current load. But, if the circuit corrects itself too quickly, it is subject to unstable oscillations.
QuiCur, employed by members of ROHM’s new LDO series, is a set of techniques developed by ROHM to achieve that end. Surprisingly, ROHM provides little information on QuiCur in this latest notice, but EE Power covered the technology in response to an earlier announcement.
Essentially, QuiCur employs a second-stage error amplifier in tandem with the first.
A QuiCur circuit. Image [modified] used courtesy of ROHM
This second error amp is purposed to prevent the feedback circuit from entering an unstable feedback frequency area. This second stage, as illustrated above, allows for gain adjustment via current drive. Though the critical zero-cross frequency can vary as a function of the connected output capacitance, that gain adjustment enables engineers to set the device to operate within stable parameters.
Critically, this is achieved without external ancillary components.
Aimed primarily at automotive applications, the new LDOs can be employed in automotive electronic control units (ECU), powertrains, advanced driver assistance systems (ADAS), and infotainment systems.
The fresh regulators operate from -40 to +150℃, and are available in 2.9 x 2.8 x 1.25 mm SSOP5 packages.
Moving forward through this year and into the next, ROHM plans to aggressively expand its existing lineup of Nano Cap-enabled LDO regulators. Per its release, the company will sport 22 such models by the end of fiscal year 2022, and will look to add 24 additional devices supporting 500 mA output current by the end of FY2023.