EMC & Thermal Management


Noise sensitive applications want ultralow noise LDO regulators


Linear regulators are integrated circuits that step a voltage down from a higher voltage to a lower voltage without the need of an inductor. The low dropout linear regulator (LDO) is a special type of linear regulator in which the dropout voltage is typically below 400mV. Early linear regulator designs offered dropouts on the order of ~1.3V, meaning for a 5V input, the maximum achievable output was only ~3.7V for the device to stay in regulation. More recently, “low” dropout is considered to be 500mV or less. Nevertheless, in these days of more sophisticated design techniques and wafer fabrication processes, “low” dropout is typically <100-300mV or so. Amit P. Patel, senior design engineer, and Steve Knoth, senior product marketing engineer, Power Products, Linear Technology tells us more


A


lthough the LDO is rarely the most expensive system component in any given system, it is often one of the


most valuable from a cost/benefit basis. One of the LDO’s tasks is to protect expensive downstream loads from harsh environmental conditions such as voltage transients, power supply noise, reverse voltage, current surges, etc. In short, its design must be robust and also contain all of the protection features needed to “absorb the punishment” from its environment while protecting the load.


LDOs vs other regulators Today’s generation of fast, high current, low voltage digital ICs such as FPGAs, DSPs, CPUs, GPUs and ASICs, have placed stringent demands on supplies that power the core and I/O channels. Traditionally, efficient switching regulators have been used to power these devices but they can have potential noise interference issues, transient response and layout limitations. As a result, LDOs are becoming an alternative


in these applications, as well as other low voltage systems. Thanks to recent product innovations and feature enhancements, LDOs offer some performance benefits that make them more desirable.


LDO design challenges Plenty of industry standard linear regulators perform the low dropout operation with a single voltage supply, yet most cannot achieve the combination of very low voltage conversion with low output noise, wide ranging input/output voltages and extensive protection features. PMOS LDOs achieve the dropout and run on a single supply but are limited at low input voltages by the pass transistor’s Vgs characteristics as well as lack the many protection features from high performance regulators. NMOS- based devices offer fast transient response but require two supplies to bias the device. NPN regulators offer wide input and output voltage range but either require two supply voltages or have higher dropout. By contrast, with the proper design


architecture, a PNP regulator can achieve low dropout, high input voltage, low noise, high PSRR and very low voltage conversion with bulletproof protection and all from a single supply rail.


New ultralow noise, ultrahigh PSRR LDO family It is clear that an LDO solution that solves the issues outlined herein should have the following attributes: very low output noise, low dropout operation, high PSRR across a broad range of frequencies, single supply operation (for ease of use and relaxed supply sequencing challenges), fast transient response time, operation over a wide input/output voltage range, moderate output current capability, excellent thermal


Figure 2: LT3045 PSRR performance


performance, and a compact footprint. To address these needs, Linear


Figure 1: LT3045 typical application schematic and features 34 June 2017 Components in Electronics


Technology introduced its LT304x family of ultrahigh PSRR, ultralow noise LDO regulators. The newest member is the LT3045, an ultralow noise, ultrahigh PSRR low dropout voltage linear regulator. It is a higher-output current version of the previously released 200mA LT3042 ultralow noise LDO. Its unique design features ultralow spot noise of only 2nV/√Hz at 10kHz and 0.8µVRMS integrated output noise across a wide 10Hz to 100kHz bandwidth. Low frequency PSRR exceeds 90dB out to 10kHz and high frequency PSRR exceeds 70dB out to 2.5MHz, thereby quieting noisy or high ripple input supplies. The LT3045 utilises Linear’s proprietary LDO architecture – a precision current source reference followed by a high performance unity gain buffer, resulting in virtually constant bandwidth, noise, PSRR and load regulation performance, independent of output voltage. In addition, this architecture permits paralleling of multiple LT3045s to further decrease noise, increase output current and spread heat across the circuit board for improved thermal performance. The LT3045 delivers up to 500mA output current with a 260mV dropout voltage at full load, across a wide 1.8V to 20V input voltage range. Output voltage range is 0V to 15V and output voltage tolerance is highly accurate at ±2 per cent over line, load and temperature. The device’s wide input and output voltage ranges, high bandwidth, high PSRR and ultralow noise performance are ideal for powering noise-sensitive applications such as PLLs / VCOs / mixers / LNAs, very low noise instrumentation, high speed/high precision data converters, medical applications such as imaging and diagnostics, precision power supplies and as a post regulator for switching supplies. It operates with a small, low cost, 10µF


ceramic output capacitor, optimising stability and transient response. A single resistor programs the external precision current limit (±10 per cent over temperature). A single SET pin capacitor lowers output noise and provides reference soft-start functionality, preventing output voltage overshoot at turn-on. Moreover, the device’s internal protection circuitry includes reverse battery protection, reverse current protection, internal current limit with foldback and thermal limit with hysteresis. The LT3045 requires an output capacitor for stability. Given its high bandwidth, the device requires low ESR and ESL ceramic capacitors. A minimum 10µF output capacitance with an ESR below 20mΩ and an ESL below 2nH is required for stability. Given the high PSRR and low noise performance attained using a single 10µF ceramic output capacitor, larger values of output capacitor only marginally improve the performance because the regulator bandwidth decreases with increasing output capacitance — hence, there is little to be gained by using larger than the minimum 10µF output capacitor.


Conclusion The LT3042 and LT3045’s breakthrough noise and PSRR performance, coupled with their wide voltage range, low dropout voltage, robustness and ease-of-use, make them ideal for powering noise-sensitive applications such as in test and measurement systems. With their current- reference based architecture, noise and PSRR performance remain independent of the output voltage. Additionally, multiple devices can be directly paralleled to further reduce output noise, increase output current and spread heat on the PCB.


www.linear.com Linear Technology, now part of Analog Devices Tel: 01628 477066


www.cieonline.co.uk


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52