In the quest to both shrink and integrate dc-dc converters for portable applications, chip makers are increasing the number of buck converters and low dropout regulators they can squeeze into a tiny surface-mount package. Two new ICs from Linear Technology and Maxim Integrated products demonstrate the current state of the art, packing four dc-dc channels into 3-mm x 3-mm footprints.
Linear Technology’s LTC3544 combines four independent synchronous buck regulators in a 3-mm x 3-mm QFN package (see the figure). These four buck converters are rated at 300 mA, 200 mA, 200 mA and 100 mA of continuous output. Using constant-frequency current-mode control, the LTC3544 steps down a 2.25-V to 5.5-V input to output voltages as low as 0.8 V. This input range accommodates the popular battery types, while the output range enables the part to power the latest generation of low voltage DSPs and microcontrollers. A high, 2.25-MHz switching frequency helps shrink the external passives, even permitting use of inductors less than 1 mm in height, yet still allows for good efficiency.
According to the vendor, the LTC3544 represents a significant advance in integration for buck converter ICs. “Nobody packs four synchronous buck converters in a 3-mm x 3-mm footprint,” says Tony Armstrong, product marketing manager of Linear’s Power Products Group, who notes that the most comparable devices until now have been duals such as Linear’s own LTC3547 in a 2-mm x 3-mm DFN.
Another point of reference is Texas Instruments’ TPS62400, a 400-mA/600-mA dual stepdown converter in a 3-mm x 3-mm QFN. Although this is a dual-channel converter, it produces higher output current levels than the Linear part. Also note that, like the ‘3544, it switches at a fixed 2.25 MHz. (For more on this part, see “Stepdown Converters Support Dynamic Voltage Scaling.”)
In terms of functionality alone, there are some highly integrated power management ICs (PMICs) developed specifically for portable applications and these devices can integrate many channels of dc-dc conversion. However, these are typically much larger chips than the LTC3544 and they have much higher pin counts. (For some examples, see “Portable Power Management Inspires Mega Integration.”) With the 16-lead LTC3544 and other devices, Linear takes the building block approach in addressing portable power requirements, offering modest levels of integration combined with high performance.
In the case of the ‘3544, that high performance includes high efficiency and low noise. For example, the converters achieve efficiency on the order of 92% to 93% when stepping down a 3.6 V supply to 1.8 V. In addition, efficiencies up to 95% are possible. In terms of standby operation, the LTC3544 offers Burst Mode operation where quiescent current is reduced to 70 µA total for all four channels under no load conditions.
In addition, quiescent current is less than 1 µA when the part is placed in shutdown mode. For applications that require lowest noise, the company offers a second version—the LTC3544B— that uses a pulse skipping mode in lieu of Burst Mode to minimize output ripple.
In addition, the LTC3544 features independent control of each channel with minimal crosstalk. According to the datasheet (see page 5), a 300-mA load step on the high-current output produces much less than 10 mV of cross talk on the other three channels.
Other features of the chip include internal soft-start, short-circuit protection and overtemperature protection. Pricing starts at $2.95 each in 1,000-piece quantities for both the LTC3544 and LTC3544B.
Meanwhile, Maxim Integrated Products has also targeted the 3-mm x 3-mm footprint for further integration with its introduction of the MAX8667/MAX8668 four-channel power-management ICs in a TQFN package. These parts integrate two synchronous stepdown dc-dc converters with two low-input, low-dropout regulators (LDOs), as well as all feedback networks. One of the dc-dc converters delivers up to 1.2-A output current, while the other delivers up to 600 mA.
Here, the buck converters use a 1.5-MHz switching frequency, which permits use of small passives such as 2.2-µH inductors in the 0805 case size. The buck converters achieve up to 93% efficiency, while drawing only 100 µA (typ) quiescent current with all regulators enabled. In addition, the LDOs operate down to 1.7-V input, saving 80% of total power loss in the system as compared with conventional linear-regulators.
The output voltages of the dc-dc converters are adjustable from 0.6 V to 3.3 V (MAX8668) or factory preset (MAX8667). The output voltages of the low-noise LDOs are all factory preset. Pricing starts at $2.95 each in quantities of 2500-up.