Renesas Mobile Phone Solutions Power Supply IC for W-CDMA/CDMA2000 Mobile Phones

Quick-Response Step-Down DC-DC Converter for POL Applications Responds to Load Fluctuations in as Little as 5μsec

A quick-response step-down DC-DC converter IC for use close to a point of load (POL) aims primarily at W-CDMA/CDMA2000 mobile phone applications. The device reduces the component count and eases system design. Because it is implemented as an IC, the converter increases the stability of the power supply circuit.

Currently, GSM mobile phones obtain the drive voltage for the high-power amplifier (HPA) directly from the lithium ion battery. Also, they use low-saturation series regulators (also called LDOs, low-dropout regulators) to produce the local power supply voltage for other system elements. However, this power-distribution approach cannot be used for W-CDMA/CDMA2000 mobile phones for three reasons. It doesn't deliver the fast response-time required by the HPA voltage control specifications. It has difficulty obtaining the required immunity to voltage fluctuations and noise. And due to the series regulators, it consumes too much power to be practical for HPA-drive voltage control in such phones.

These problems are eliminated by alternate power-system designs that use a high-performance switching regulator. To achieve optimum power efficiency, such power systems use POL (point-of-load) power converter ICs that can be mounted right next to the loads they are driving, typically application processors, microcomputers, logic devices, memory chips, etc. The POL converters perform step-down conversion of the voltage from a battery or other DC source to the stable lower power supply voltage required by their loads. The close proximity between converter and load avoids voltage drops caused by wired parasitic resistance. This can be important for load LSI devices produced with a 90nm process that have a core voltage of only 1.2V. Moreover, such LSI chips are likely to suffer from misoperation if powered from slow DC-DC converters with load response times in the order of tens of microseconds.

The Renesas solution for W-CDMA/CDMA2000 mobile phones is the R2A20101 quick-response step-down DC-DC converter IC, which is ideal for POL applications. The synchronous-rectifier type device replaces multiple the components typically needed to build this function. It can deliver a maximum output current of 650mA and allows parallel operation for loads that require more current. The IC provides linear control of the output voltage, allows circuit simplification because it needs no external parts, and saves space with a compact mounting footprint.

The R2A20101 power converter uses CR feedback to achieve a fast response to load variations—as short as 5µsec, at least 50-percent quicker than previous devices. The IC incorporates innovative design techniques for which Renesas has received a patent in the USA. The proprietary CR feedback approach eliminates the need for the current-sense resistor used in previous circuits, for better power-conversion efficiency. Similarly, it doesn't need an error amplifier, eliminating the response delay associated with that circuit.

The DC-DC converter is simple to use, just requiring connections to the VIN, GND, EAO (Error Amp. Out) and output pins. It has a power supply voltage range of 2.5V to 5.5V and operates at PWM switching frequencies as high as 2MHz. The device achieves 92-percent power-conversion efficiency with a 2.5V input and 1.8V output. Also, the R2A20101 IC includes a voltage-tracking function and supports the execution of various kinds of sequence control. It is available in a small (4mm x 4mm x 0.8mm) QFN-24 low-thermal-resistance package, as well as an ultra-small (1.96mm x 1.26mm x 0.48mm) CSP-15 package

Plans are underway to introduce additional Renesas DC-DC step-down converter chips for POL applications. For instance, the R2A20102 IC is under development. This 600mA device will be able to maintain high efficiency under light loads, 10mA or less.