AN2353 datasheet(5/18 Pages) STMICROELECTRONICS

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AN2353

Oscillator

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The load capacitors C1 and C2 transform the gain of the amplifier (gm) into a negative series

resistance RL to compensate for the losses of the crystal.

The best frequency stability is obtained when C1 = C2. The oscillation occurs when the sum

of RL and Rs (the series resistance of the crystal) is negative.

By choosing C1 = C2 = C, the minimal gain of the amplifier (gm) is expressed as follows:

The minimal gain of the amplifier also implicitly sets the on-chip oscillator start-up time.

The oscillation stability mainly depends on external parameters so only the

transconductance (gm) can be guaranteed and the start-up time value is defined by

measurement at the application level.

1.2

Start-up time

Ceramic resonators have a much shorter start-up time than crystals (about 100 times faster)

but have a lower accuracy on the frequency (initial tolerance, temperature variations and

drift).

Depending on applications requirements and possibilities, users can choose between short

oscillator start-up time and frequency accuracy.

From an ST10 perspective, the worst case condition / environment for the oscillator start-up

time is high temperature and low voltage.

Table 1.

gm values for different types of oscillators

Symbol

Parameter

Version

Conditions

Value

Unit

Min

Typ

Max

Oscillator transconductance

Wide-swing

mA/V

Low-power

0.7

2.5

32 kHz

Start-up

A/V

Normal run

min

×ω

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