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lowpowerZONE Products for the week of April 28, 2008
National Semiconductor Says…
LM34917A/919: Smallest Step-Down Switching Regulators for Handheld Devices and Automotive Power Adapters
PowerWise Buck Regulators Supply up to 1.2 A and Feature Intelligent Current Limiting and Overvoltage Protection
National Semiconductor Corp. announced two new step-down switching regulators well-suited for handheld devices and automotive power adapters. The highly integrated LM34917A and LM34919, members of National’s PowerWise energy-efficient product family, are the industry’s smallest non-synchronous buck regulators to feature a supply voltage rating up to 50V, 1.25A continuous output current, intelligent current limiting and over-voltage protection.
National’s new LM34917A and LM34919 feature programmable switching frequency up to 2 MHz. The products integrate MOSFET switches to reduce system component count, and a high switching frequency enables use of ultra-small inductors and capacitors for very small solution size.
The LM34917A and LM34919 utilize a constant on-time (COT) control scheme, which requires no loop compensation. The regulators are well-suited for use in applications that require high-input voltage regulations such as MP3 players utilizing Firewire connectivity (IEEE1394) as well as MP3 players or auxiliary power adapters with direct connection to a 12V (universal) automotive power outlet where transient protection requirements can exceed 40V.
The LM34917A and LM34919 feature peak efficiency equal to or greater than 90 percent, positioning it among National’s PowerWise family of energy-efficient products. The products feature ultra-small micro SMD packaging and extend National’s buck regulator product line which includes the LM2694, LM34910 and LM34914 buck regulators offered in LLP packaging.
EN-Genius Says…
The story here, of course, is size. Getting a 1.25 A regulator into MicroSMD is quite a feat.
The two parts are both synchronized buck switching regulators, but beyond that there are small but important differences between them. The LM34919 has a lower current limit (at 800 mA instead of 1.25 A in the LM34917A) but the former has a higher input voltage limit at 40 V (cf 33 V). The LM34917A, on the other hand, offers a higher maximum switching frequency of 2 MHz (cf 1.6 MHz).
Apart from those differences the parts are similar in architecture and operation and we’ll look at the higher current LM34917A as our example.
With an input voltage range of 8 V to 33 V the part has an integrated 7 V start-up regulator for the internal circuits. There is undervoltage lock-out, overvoltage protection at the input and at the output, together with current limit protection. There is also soft-start and thermal protection. To protect against excessive inductor current there is a variable valley current limit depending on the difference between the input and output voltages (fixed in the LM34919 to 0.64 A) with a range from about 1.35 A down to just over 1.1 A. There is also a remote shutdown capability.
The on-chip reference is 2.5 V and this is the lowest voltage the output can be set to (with the usual potential divider feedback loop). The n-channel switch has a Rds(on) of a typical 0.33 Ω.
Efficiencies depend on the difference between the input and output voltages (the closer they are, the higher the efficiency) and the current load being taken. In the curves given the highest efficiency is a little over 90% at mid-load with 8 V on the input, the lowest peaks in the low 80% range.
These are nice parts in great packaging. The range of applications is, of course, all over the spectrum and we should expect to see fixed-voltage versions as soon as National sees how market demand is looking. The news release headline suggests use of the parts in handhelds but with the 8 V minimum input voltage that is just not so. These will both be high volume products.
The LM34917A and LM34919 are both in production in 12-bump microSMD with the former priced at $1.35 and the latter at $1.05, both in 1000-piece lots. Evaluation boards are available.
Data Sheet LM34917A
Data Sheet LM34919
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