A synchronous buck-boost converter IC from Linear Technology builds on the success of the company’s LTC3440 by extending that device’s input voltage range and by making burst-mode operation programmable. Like its predecessor, the new LTC3530 is a high efficiency, low-noise monolithic converter that delivers 600 mA continuous of regulated output, while operating from a battery voltage that is above, below, or equal to the output voltage (Fig 1.). However, the input voltage range of the LTC3530 has been expanded on the low end down to 1.8 V. Meanwhile, users can now program the current threshold for Automatic Burst-Mode operation, which was invoked at a fixed value in the ‘3440.
With its 1.8-V to 5.5-V input range, the LTC3530 accommodates newly developed Li-ion cells with their reduced discharge voltages. When the ‘3440 was introduced in 2001, single-cell Li-ion batteries would discharge to approximately 3 V. The 2.5-V lower limit on input voltage range on the ‘3440 was a reflection of that common operating condition. However, the recent adoption of new electrode materials developed to increase battery capacity is reducing this voltage. For example, Sony’s Nexelion cell discharges down to 2.5 V.* The expanded input range of the ‘3530 allows for operation at this lower voltage, while also enabling the buck-boost converter to operate from two-cell alkaline or NiMH cells.
The LTC3530 operates with up to 96% efficiency as depicted in the Fig. 2 graph, which charts efficiency versus load and input voltage at 3.3-V output. The chart includes a plot of efficiency when the part is operating in burst-mode operation. The load current threshold at which the converter shifts into burst-mode is programmable, which is an advantage versus the ‘3440 as well as competing parts that have a fixed threshold.
Tony Armstrong, product marketing manager for power products at Linear Technology points out the LTC3530’s quiet operation in burst mode also offers a significant competitive advantage. Fig. 3 depicts typical burst-mode waveforms for the application circuit shown in Fig.1. As shown in Fig. 3, the burst mode ripple is approximately 50 mVp-p . This low-noise performance is said to limit filtering requirements in the application.
The LTC3530’s ability to deliver its rated 600 mA current across the full 1.8 to 2.5-V input range provides another advantage versus existing parts according to Armstrong. Although, like other buck-boost converters the LTC3530 is capable of delivering higher current levels in the stepdown mode, Linear conservatively rates its part to reflect the converter’s output capability in the boost mode.
Other features of the LTC3530 include a programmmable 300-kHz to 2-MHz switching frequency, output disconnect in shutdown, and less than 1 µA of shutdown current. The device is offered in a thermally enhanced, 10-lead, 3-mm x 3-mm DFN as well as in 10-lead MS packages. Pin compatible with the LTC3440, the LTC3530 will be priced at or below the price of the earlier part, which costs $2.74 each in quantities of 1000.
*For more on recent battery developments, see “New Materials Extend Li-Ion Performance” available online at http://powerelectronics.com/power_new_materials_extend/
For a look at another recently introduced buck-boost converter, Texas Instruments’ TPS63000, see “Efficient Buck-Boost Converter Extends Battery Life” online at http://powerelectronics.com/ti_tps63000_converter/ .