AN299 Datasheet, PDF (1/4 Pages) Silicon Laboratories – EXTERNAL 32.768 KHZ CRYSTAL OSCILLATOR

AN299
EXTERNAL 32.768 KHZ CRYSTAL OSCILLATOR
1. Introduction
This document describes an external 32.768 kHz oscillator circuit for use with the Si4700/01/02/03. It provides the
required components, the schematic, and recommended layout guidelines.
2. Oscillator Circuit Description
The circuit makes use of an unbuffered inverter, four external resistors, two capacitors, and a 32.768 kHz crystal.
3. Schematic and Bill of Materials
Figure 1 shows the external oscillator circuit schematic. Table 1 shows the bill of materials with component values,
vendors, and descriptions.
Figure 1. 32.768 kHz Crystal Oscillator Circuit Schematic
3.1. Schematic Description
Resistors R3 and R4 limit the current driving capability of the unbuffered inverter. This maintains current
consumption of the circuit at approximately 8 uA when VIO = 3.3 V. Resistor R2 biases the inverter to its linear
operating region to start and maintain oscillation. In order to not exceed the maximum crystal power dissipation, the
total power delivered to the crystal must be controlled. The resistor R1 provides this current limiting in the example
circuit shown in Figure 1 and has been tuned to ensure that the 1 uW power limit for the ECS-.327-12.5-13 is not
exceeded. For a different crystal check the data sheet to ensure that the maximum power dissipation is not
exceeded. Capacitors C1 and C2 provide the required load capacitance for crystal oscillation at the desired
32.768 kHz frequency.
Rev. 0.1 6/06
Copyright © 2006 by Silicon Laboratories
AN299