Buck Regulator Exploits LDO to Lower Noise

Jul 27, 2005 12:17 PM

Linear Technology’s LTC3448 is a 2.25-MHz (or 1.5-MHz) synchronous buck regulator that delivers up to 600 mA of continuous output current. It operates in two modes. First, when output current is greater than 3 mA, it switches at a constant frequency of 2.25 MHz. Then, if output current drops to 3 mA or below, the device automatically transitions to a low-noise LDO mode consuming only 32 µA of quiescent current. Output ripple is less than 5 mV_P-P in switching mode and less than 300 µV rms (micro-volts rms) in LDO mode, making it well suited to applications such as handheld wireless devices.

Using a current-mode architecture, the LTC3448 operates from an input voltage range of 2.5 V to 5.5 V and generates output voltages as low as 0.6 V. The user can set the LTC3448's switching frequency to either 1.5 MHz or 2.25 MHz and can synchronize the device up to 4 MHz, allowing the use of tiny, low-cost ceramic capacitors and inductors less than 1.2 mm in height. Alternately, the part can be synchronized to an external clock.

The combination of tiny externals and a 3-mm × 3-mm DFN-8 (or MSOP-8E) provides a compact solution footprint. The LTC3448 uses internal synchronous switches with an R_DS(ON) of only 0.35 Ω and 0.40 Ω to deliver efficiencies as high as 96%. It also uses low dropout 100% duty cycle operation to allow output voltages approaching V_IN, further extending battery run-time. Light load quiescent current is only 32 µA and <1 µA in shutdown. The LTC3448's LDO mode can be enabled automatically or via an external input. Pricing starts at $2.05 each for both package options in 1000-piece quantities.

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