Micro-Module Regulator Counters Conventional Wisdom on Output Noise

This article appeared in Electronic Design and has been published here with permission.

The messages of conventional wisdom and “rules of thumb” that were perhaps once true can take a long time to change. Consider the widely held view on step-down (buck) switching dc-dc regulators compared to all-analog low-dropout (LDO) regulators. After all, “everyone” knows that the former is far more efficient but has much higher noise than the latter. Furthermore, the difference is due to inherent architectural limitations; thus, there’s little that can be done about it other than filtering and more filtering.

That may have been true up to a few years ago, but it isn’t always the case. First, in the range of 0.5- to 1-A output, the LDO may be far less efficient than the switcher in the general sense. However, in some scenarios, such as when the duty cycle is low, the actual energy-usage difference may be small and even negligible.

Even more noteworthy is the change in switching-regulator noise performance, going far beyond just adding more output filtering (which brings a new set of issues related to dynamic performance). The LTM8080 from Analog Devices continues the advances as an extremely low-noise, dual-output dc-dc μModule regulator. It uses a combination of patented silicon, layout, and packaging innovations to achieve its specified performance to integrate the best of both switcher and LDO worlds without their downsides.

The LTM8080’s front-end is a high-efficiency synchronous Silent Switcher step-down regulator that’s followed by two separate, low-noise, LDOs (Fig. 1). The package includes the controllers, power switches, inductors, and support components. As an integrated solution comprised of a switcher and LDOs, it offers advantages of both while maintaining a small size and significantly reduced PCB layout sensitivity. To further suppress switching noise, the LTM8080’s packaging incorporates an EMI barrier wall or shield.

The LTM8080 operates from inputs ranging from 3.5 to 40 V; the dual outputs can bet set from 0 to 8 V at 500 mA each or combined for a single 1-A maximum. It targets digital loads that are susceptible to switching-regulator noise such as data converters, RF transmitters, FPGA I/O and clock, op amps, transceivers, and medical scanners.

The result is exceptionally low noise values below1 µV_RMS (10 Hz to 100 kHz), 2 nV/√Hz spot noise (at 10 kHz), and 80-dB PSRR (100 kHz) (Fig. 2).

Compared to discrete solutions without an EMI shield, the LTM8080 reduces output ripple voltage by up to 70% for a simplified and quiet design. It minimizes EMI emissions and enables the device to pass CISPR22 Class B and CISPR25 Class 5 without an input filter. In addition, the adjustable switching frequency (200 kHz to 2 MHz) and selectable operation modes minimize the risk of frequency interference for very-low-noise instrumentation and high-speed/high-precision signal-chain applications.

Furthermore, the LTM8080 has a built-in voltage tracking function that automatically sets V_BUS to either 2.5 V nominal or 1 V higher than V_OUT1, whichever is greater, to achieve superior noise performance and minimize power dissipation. It also has provision for external synchronization and offers a programmable “power good” indication.

The LTM8080, with a 28-page datasheet replete with tables, curves, and applications insight, is packaged in a thermally enhanced, compact (9 × 6.25 × 3.32 mm) overmolded ball-grid-array (BGA) package. The −40 to 125°C version is priced at $12.83 (1,000 pieces), while the wider range −55 to 125°C version costs $14.11. The devices are supported by the DC3701A demonstration circuit board (Fig. 3), complete with its own 10-page User Manual.