
NXP Semiconductors |
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| UM10301 | |
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| User Manual PCF85x3, PCA8565 and PCF2123, PCA2125 | ||||||||||||||
Taking the numbers from Table 4 yields for L1 and Q: |
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L1 = | 1 |
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| 1 |
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| = 11234 H | |||||||||
(2π ⋅ f | 0 | )2 ⋅ C |
| (2π ⋅ 32768)2 ⋅ 2.1⋅10−15 | ||||||||||||||||||||
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| 1 |
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Q = |
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| 1 |
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| 1 |
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| = 42053 |
(2π ⋅ f |
| )⋅ C |
| ⋅ R | ( π ⋅ | 32768 | )⋅ | 2.1 | ⋅ | 10 | −15 | ⋅ | 55 | ⋅ | 10 | 3 | ||||||||
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| 0 |
| 1 | 1 | 2 |
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This L of around 11000 H resulting in a Q of around 42000 explains why starting up the oscillator as well as stopping it can easily take more than a second. An oscillating quartz crystal is actually a mechanical oscillation and starting or stopping this takes time.
Calculations of start up time and more
The use of AGC’s improve start up by high drive initially to get it going and then reduce drive for low power.
Table 4. Typical values for crystal and surrounding capacitors
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Parameter | Value | Unit |
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f0 | 32768 | Hz | [2] |
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∆f / f0 | ±100 | ppm | [2] |
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Aging; ∆f / f0 | ±3…±5 | ppm | [2] |
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B, freq(T) | ppm / °C2 | [2] |
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C1 | 2.1 | fF | [2] |
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C0 | 1.2…1.5 | pF | [2] |
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CIN | 25 ± 10 | pF | [1] |
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CIN, temp co. | +47 | ppm/°C | [1] |
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R1 | 50…80 | kΩ | [2] |
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CT variable | 4…25 | pF | [3] |
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CT, temp co. | 300 | ppm/°C | [3] |
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CT fixed 0603 | Any | pF | [4] |
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CT fixed, tc | ±30 for C0G | ppm/°C | [4] |
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Sources for values in table 4:
[1]NXP, Datasheet PCF8563, February 2008.
[2]Product Data Sheets, MicroCrystal.
[3]Murata TZB04 trim capacitor
[4]Vishay Beyschlag, datasheet ceramic multilayer capacitor, C0G
UM10301_1 |
| © NXP B.V. 2008. All rights reserved. |
User manual | Rev. 01 — 23 December 2008 | 14 of 52 |