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lowpowerZONE Products for the week of June 16, 2008
National Semiconductor Says…
LM3691: 1-A, 4-MHz Synchronous Step-Down Dc-Dc Converter micro-SMD
Features ±1% Voltage Precision, 95% Peak Efficiency and High Transient Performance
National Semiconductor Corp. introduced the industry’s smallest 4 MHz synchronous step-down DC-DC converter that provides up to a 1A output current over an input voltage range of 2.3V to 5.5V. The LM3691 DC-DC converter is optimized for powering high-performance processors from a single Li-Ion cell battery in mobile phones, personal media players and other mobile devices. A member of National’s PowerWise energy-efficient product family, the LM3691 features peak efficiency of 95 percent, low quiescent current to maximize battery life and output voltage precision of ±1 percent to power baseband and next-generation applications processors.
National’s new LM3691 DC-DC converter is offered in a micro SMD package and features high switching frequency up to 4 MHz that enables use of miniature 1 uH multilayer inductors and tiny 4.7 uF capacitors for a solution size of less than 15 mm squared. Several fixed output voltage options are also available.
EN-Genius Says…
Size is the most important thing about this announcement. Squeezing a 1 A dc-dc converter into a 6 bump micro-SMD is quite an achievement, but performance is not compromised.
The input voltage range to the LM3691 is 2.3 V to 5.5 V, allowing for a single cell Li-Ion up to the latest 4.8 V versions, or for a 3-cell NiMH or NiCd input. The output can be as low as 0.75 V and goes up to 1.8 V making the product extremely suitable for low voltage portable applications. Output voltage accuracy is within ±1% and transient performance is within ±40 mV.
The switching frequency is 4 MHz, allowing for the use of small inductors and the only three external components are the inductor (at 1 µH), the input capacitor at 4.7 µF, and the output capacitor also at 4.7 µF. This gives a real estate solution of about 15 mm2.
An economy mode, being called ECO mode by National, can be automatically switched into operation at low loads – a quiescent current reduction mode to preserve efficiency – causing an interesting bump in the efficiency/current output transfer curve. The part can be forced into PWM with a mode pin. when it operates as a voltage-mode controller with feed forward input voltage.
Efficiency, as always, depends on how close the output voltage is to the input but, in most examples, the efficiency reached, and slightly exceeded, 90% above 100 mA loading. Quiescent current in the part in PWM mode is a typical 490 µA, and in ECO mode it reduces to 64 µA. An enable pin, taken to 0 V, shuts the part down and the current drops to 30 nA. The p-channel FET in the driver has an on-resistance of a typical 160 mΩ, while the n-channel FET has 115 mΩ. Both current overload and thermal shutdown protections are included.
Feedback is taken directly from the output voltage with the required resistive potential divider on-chip giving a single fixed output level. National is releasing parts with outputs of 0.75 V, 1.2 V, 1.5 V, and 1.8 V. As demand requires parts can also be supplied with outputs of 0.85 V, 0.90 V, 1.0 V, 1.1 V, 1.3 V, 1.375, and 1.6 V.
This part hits a neat niche in the portable market. With the efficiency, the low load quiescent savings, and the shutdown capability, the small packaging is the sweet extra. The LM3691 will do extremely well in the market, and I am happy to see the sensible pricing.
The LM3691 is in production in micro-SMD-6 priced at $2.25 in 1000-piece lots.
Data Sheet
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