Oscillator ultra low power

For battery-powered circuits, it is easy to build an ultralow-current crystal oscillator designed around a 32.768-kHz crystal. This crystal is common for real-timeclock circuits. Because these circuits must operate at all times, achieving the lowest current draw possible is mandatory. Traditional gate-oscillator circuits – the 74HC04, for example – can draw several milliamps; the circuit shown in Figure 1 normally draws only about 5 µA. This circuit uses the Texas Instruments LPV7215MF comparator, which is housed in a five-pin SOT-23 package. The operating current for this comparator is 580 nA; the entire circuit shown in Figure 1 draws only 5 µA when running from a 3.3V supply.

Vezi și

1. Schema de tratament pentru cazurile ușoare de Covid-19

2. Romania traiește , încă ,  din inertia bogățiilor create in Epoca Comunistă

3. Scara de valori a societății romanești 

4. Europa privită din viitor

5. Hrana vie

6. Planurile in derulare sunt o munca in progres,  veche de sute de ani  

7. Destinatii uimitoare pe glob

8. Miracolul japonez- Drum reconstruit în patru zile

9. Primarul care nu frură

10. Duda a pus mâna pe Casa Regală

11. Nu poti multiplica bogatia divizand-o !  

12. Evolutia Laptop - Cântărea 5,44 kg

13. O Nouă Republică

14.    A fi patriot nu e un merit, e o datorie.! 

15. În vremea monarhiei, taranii romani reprezentau 90% din populatie si nu aveau drept de vot.

16. Miracolul din Noua Zeelandă - LYPRINOL

17. Cea mai frumoasă scrisoare de dragoste

18. Locul unde Cerul se uneste cu Pamantul

19. Fii propriul tău nutriționist

20. Maya ramane o civilizatie misterioasa

21. Slăbești daca esti motivat

22. Serbet de ciocolata

23. Set medical Covid necesar acasă

24. Medicament retras - folosit în diabet

25. Brexit-ul - Spaima Europei

26. Virusul Misterios

27. Inamicul numărul unu al acumulatorilor 

28. Sistemele solare - apă caldă

29. Economisirea energiei electrice

30.  Hoțul de cărți

31. Aparitia starii de insolventa

32. TRUMP ESTE PRESEDINTE

33. Microbii din organismul uman

34. Despre islamizarea Europei. O publicăm integral.  Și fără comentarii. 

35. „Naţiunea este mai importantă ca Libertatea !”

36. Masca ce omoară virusul     O veste de Covid  

37. Primul an de viaţă - Alocatia pentru copil  

38. Tavalugul Marelui Razboi - Globaliyarea - Asasinii Economici

Figure 1.	Traditional gate-oscillator circuits can draw several milliamps; the circuit shown normally draws only about 5 μA.

Multiple copies of this circuit have been built and tested to confirm the 5-µA current draw. The largest portion of the 5 µA goes to the largely unavoidable operations of charging and discharging the output load capacitance. The circuit was tested using a standard 10-MΩ oscilloscope probe with about 10 pF of shunt capacitance; operating current into more capacitive loads will be higher. Table 1 breaks down the power consumption piece by piece.

Capacitive loads must be charged by the upper transistor in the active output stage of the LPV7215. To charge a capacitor to 3.3V, note the capacitor equation

Q=C×V.

This charge is transferred into the capacitance during the first half of each cycle of the 32.768-kHz oscillation. During the second half, the charge is transferred to ground. As a result, the output-stage current, i, will be

i=f×Q=f×C×V.

For 20 pF (a 10-pF scope probe plus a PCB parasitic),

i=(32.768 kHz) (20 pF) (3.3V)=2.163 µA.

From the equation above, it can be seen that additional output loading or higher operating frequencies will draw more output current. Anything that can be done to reduce the capacitive load will reduce the total current draw. Figure 2 shows an example of the test boards used to create the crystal-oscillator circuit.

Figure 2.	Test boards of a crystal-oscillator circuit are based on the Texas Instruments LPV7215MF comparator.

Typical LR44 alkaline button-cell batteries (Figure 3) have a capacity of 150 mAhr. With 5 µA of current draw, this clock circuit could run for about 30,000 hours, or 3.4 years.

Figure 3.	Typical LR44 alkaline button-cell batteries supply 150 mAhr of capacity.

Table 1. Loading estimates

Source	Supply current (μA)
LPV7215MF	0.58
R1 and R2 bias	0.33
R3	0.083
Crystal network (estimated)	2
10-MΩ probe load	0.165
20-pF load capacitance	2.163
Estimated total	5.321
Actual total	5

Voltage-controlled oscillators

A typical voltage-to-frequency converter – also known as a VCO (voltage-controlled oscillator) – IC has a simple linear tuning characteristic relating input voltage to output frequency. It is usually of the form F=kV/RC, where RC is the time constant of the associated timing resistor and capacitor. The output frequency range of these parts varies, but few, if any, tune over the entire range with one set of RC timing components. If you change the timing ratio as the input voltage changes, however, you can magnify the tuning range to take advantage of nearly the entire frequency range in one implementation.
One way to achieve this goal is to replace the timing capacitor with a variable capacitor whose capacitance inversely changes with bias voltage: a varactor diode (Reference 1). For this design, theAnalog Devices AD654 voltage-to-frequency converter was considered because of its simplicity and bandwidth of at least 1 MHz (Reference 2).

Figure 1.	The usual usage of this VCO IC is with a fixed timing capacitor, CT, at pins 6 and 7.

Figure 1 shows a typical implementation using a fixed resistor and capacitor. For the values shown, this setup gives a frequency range of approximately 10 Hz to 30 kHz over 0 to 10V on the input. After replacing the timing capacitor with an NTE618 hyperabrupt varactor, as shown in Figure 2, the same input voltage range of 0 to 10V produced a tuning range of approximately 10 Hz to more than 1 MHz (Reference 3).

Figure 2.	A voltage-variable capacitor (varactor) and an ac-coupled dc-bias network replace the fixed timing capacitor.

The plot in Figure 3 compares the tuning curves of each converter configuration. Note the dramatic increase in range, but at the expense of linearity. Temperature stability will also be affected. Overall, precision is traded for tuning range, which should be acceptable in basic applications that do not require the specified precision.

Figure 3.	Replacing the fixed timing capacitor with a voltage-variable capacitor results in a much wider tuning range.

The hyperabrupt varactor allows large changes in frequency for small changes in bias voltage because of the large capacitance ratio. For some hyperabrupt varactors, the ratio can be as high as 15, as in the case of the NTE618, a varactor that AM receivers use. As the frequency of the converter increases with higher voltage, the capacitance decreases, which in turn increases the frequency. This combination of responses generates the wide tuning range. The 0.01-μF coupling capacitors separate the varactor bias voltage from the operation of the converter core. Light varactor biasing with high-value, 1M resistors prevents additional loading of the oscillator.
This behavior is calculable and predictable to some degree, even from the data sheets. The tuning curve for the varactor diode can be generated in Microsoft Excel. This information then can be used in the voltage-to-frequency conversion equation for the converter. For the NTE618, the approximate relationship of capacitance to voltage is expressed as

C = 800×10⁻¹⁰×e^−0.46V.

Figure 4 shows the similarity between the calculated and measured results. The higher frequencies differ more as the varactor capacitance reduces to the order of the stray capacitance in the circuit and parts. Careful layout can minimize this issue and increase the range.

Figure 4.	The calculated and measured responses are in close agreement.

Note that at low input voltages, the varactor-based response and the fixed-capacitor converter response are nearly identical because of the varactor’s inverse exponential relationship to the voltage. One useful result of achieving this range is eliminating the need to switch between converters to extend the tuning range. You can explore other useful and interesting applications using this approach with phase-locked loops, modulators, or function generators.
Editor’s note:
Figure 13 of the Analog Devices AD654 data sheet contains errors (Reference 2). The 74LS86 andLM360 cannot be subjected to 15V, and R7 is most likely 8.2k, not 8.2Ω.
References

1. Williams, Jim, and David Beebe, “Switching-regulator supply provides low-noise biasing for varactor diodes,”EDN, Nov 9, 2000, pg 117.
2. A2“AD654: Low Cost MonolithicVoltage-to-Frequency Converter,” Analog Devices.
3. “NTE618 Varactor Silicon Tuning Diode for AMRadio,” NTE Electronics.