AN2339

Power supply

5/20

2

Power supply

Figure 1.

Power Supply Overview

2.1

Main operating voltages

The STR91x devices are processed in 0.18 µm technology, ARM966E-S RISC core and I/O

peripherals need different power supplies. In fact, STR91x requires two separate operating

voltage supplies. The CPU and memories operate in a range from 1.65V to 2.0V on the

VDD pins, and the I/O ring operates in the 2.7V to 3.3V or 3.0V to 3.6V range on the VDDQ

pins.

All pins need to be properly connected to the power supplies. These connections, including

pads, tracks and vias should have an impedance as low as possible. This is typically

achieved with thick track widths and preferably dedicated power supply planes in multi-layer

PCBs.

In addition, each VDD/VSS and VDDQ/VSSQ pair should be decoupled with ceramic

capacitors which need to be placed as close as possible to the appropriate pins or below the

pins on the contrary side of the PCB. Typical values are 10nF to 100nF, but exact values

depend on the application needs. The following figure shows the typical layout on such a

VDD/VSS pair.

A/D converter

Core

(3V or 3.3V)

SRAM

RTC

I/O Ring

(1.8V)

Note 1

(from 1V

Core

SRAM

RTC

(1.8V)

Note 1

A/D converter

(3V or 3.3V)

I/O Ring

128-pin devices

80-pin devices

Note1: You can connect power to the SRAM with VBATT by firmware