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发表于 2010-12-24 07:14:48
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本帖最后由 fhchen2002 于 2010-12-24 14:45 编辑
1. Build an I_CTAT (current source, Constant to Absolute Temperature) from Bandgap Vref. The bias current of the CTAT current source is subject to resistor's Rsh (sheet resistance) change over process corner.
2. Build an oscillator, use this I_CTAT to charge or discharge the capacitor associated with the oscillator.
V = I_CTAT * T / C
f = 1 / T = I_CTAT / (C * V)
V has to be derived from Bandgap Vref.
For instance, Bandgap Vref = 1.210 V.
V = 90/121 * Vref = 0.9 V
This V is almost constant over temperature, process and power supply.
3. Use switches to decode (i.e. select) the needed capacitance to get the desired oscillation frequency. On-chip MiM or PiP capacitor is subject to +/-10% capacitance variation over process corner, but will have excellent (i.e., extremely small) temperature and voltage coefficient.
Once the desired capacitance (based on frequency measurement) is determined, the oscillation frequency will have low temperature, process and power supply drift.
Step 3 is called "trimming."
This design approach can achieve < +/-2% frequency variation over [0 degrees C, 85 degrees] temperature range. |
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