A synchronous step-down dc-dc converter in chip-scale packaging reduces the board space required to build a 500-mA buck regulator, yet still achieves efficiencies up to 93% and a high degree of dc accuracy. Operating at a fixed switching frequency of 3 MHz, the TPS623xx dc-dc converter from Texas Instruments (TI) converts a 2.7-V to 6-V input to a fixed or adjustable output at voltages as low as 0.6 V. The combination of small size, good efficiency and a battery-friendly input-voltage range makes the TPS623xx suited to such portable applications as cell phones, PDAs, and WLAN and Bluetooth devices.
Housed in an 8-pin, 2-mm × 1-mm chip-scale package, the IC requires just three external passives to complete a 500-mA step-down converter. When a 1-µH output inductor, a 4.7-µF output capacitor and a 4.7-µF input capacitor are used, the complete converter design measures just 5 mm × 5 mm and stands less than 1-mm tall.
To put these space requirements in perspective, consider an existing device with comparable performance. TI's TPS6222x is a 1.5-MHz, 400-mA dc-dc converter in a 3-mm × 3-mm ThinSOT-23 package. With its external components, this chip requires about three times the board area of the TPS623xx. As a tradeoff for this size reduction, there is some drop in efficiency. Nevertheless, the efficiency of the two devices is still comparable in a typical application (see the figure).
To achieve high-efficiency over load current, the TPS623xx operates in a 3-MHz (typical) fixed-frequency PWM mode at moderate to heavy currents but then switches to a power-saving PFM mode at light load currents. For low-noise applications, the converter can be forced into the fixed-frequency PWM mode by pulling the MODE/SYNC pin high. In addition, the 3-MHz switching frequency may be synchronized “on the fly” to an external oscillator. The chip draws a quiescent current of 105 µA max under no load, which reduces to 1 µA max in the shutdown mode.
The TPS623xx also features a dc output voltage accuracy of -0.5% to +1.3% over the -40∞C to +85∞C temperature range, assuming a 3.6-V input, PWM mode operation and no load. When test conditions are expanded to cover inputs over the specified 2.7-V to 6-V range, loads over the 0-mA to 500-mA range, and dual-mode PFM/PWM operation, dc voltage accuracy becomes ±2% for most of the TPS623xx models.
Load and line transient response are also noteworthy. When faced with a 10-mA to 400-mA load step slewing at about 2 A/µs, the converter output exhibits a 15-mV deviation and a response time of about 1 µs. These measurements were taken with an input of 3.6 V and a nominal output of 1.6 V (see Figs. 9 and 11 on datasheet). Line transient response was tested under similar input and output voltages. In this case, a 1-V change on the input produced about 7 mV of change on the output (see Fig. 8 on datasheet). In both cases, converters were configured with an output inductance of 0.9 µH and an output capacitance of 10 µF.
In addition to two models with adjustable output (TPS62300 and TPS62320), this converter family currently includes seven models with fixed outputs at 1.2 V, 1.5 V, 1.6 V, 1.8 V, 1.875 V or 1.9 V. These models are offered in either the lead-free, chip-scale package or in a 3-mm × 3-mm 10-pin QFN. In quantities of 1000, unit pricing is $1.85.
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