AN2595 datasheet(7/20 Pages) STMICROELECTRONICS

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AN2595 - Application note

Oscillator

7/20

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 startup time.

The oscillation stability mainly depends on external parameters so only the

transconductance (gm) can be guaranteed and the startup time value is defined by

measurement at the application level.

1.2

Startup time

Ceramic resonators have a much shorter startup 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 startup time and frequency accuracy.

From an ST10 perspective, the worst case condition / environment for the oscillator startup

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

1.4

2.6

4.2

32 kHz

Startup

203150

µA/V

Normal run

min

×ω

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