Maxim > Design Support > Technical Documents > Application Notes > Real-Time Clocks > APP 4247
Keywords: RTC, oscillator, crystal, battery current, low power APPLICATION NOTE 4247
Oscillator Design Considerations for Low-CurrentApplications
Jan 26, 2011
Abstract: This application note describes how low-frequency oscillator design and crystal selection affectoperating current. When running the oscillator from a limited-capacity supply, such as a lithium coin cellor super capacitor, minimizing the operating current increases the operating time.
Introduction
Maxim has a large portfolio of low-power, battery-backed real-time clocks (RTCs). A primary
consideration when Maxim designs an RTC is minimizing the power requirements of the oscillator whenthe RTC is running from the backup supply, so as to maximize the life of the backup source. Crystaloscillators provide reasonable accuracy, and development efforts for many years have focused onminimizing power consumption.
Oscillator design goals include the following:
Provide enough current and gain to start and maintain oscillationProvide a wide operating-voltage range
Minimize the influence of external noise on accuracy
Key Crystal and Oscillator Parameters
Figure 1 presents an equivalent circuit of a crystal resonator. A crystal has two frequencies of zerophase, as shown in Figure 2. The lower frequency is the series resonant frequency. At the seriesresonant frequency, L1 and C1 cancel, and the impedance is determined by R1.1 The second, higher,frequency is the parallel resonant frequency. At parallel resonance, resistance is at a maximum.A parallel resonant oscillator circuit (Figure 3) uses a crystal that is designed to operate with a specifiedload capacitance. This results in a circuit that operates at a frequency between the series resonant pointand the parallel resonant point. A change in the load capacitance causes a change in the oscillatorfrequency.
Crystal equivalent series resistance (ESR) tends to shift upwards after going through SMT reflow, so anSMT crystal that is otherwise similar to a through-hole package crystal may have a higher maximumESR specification. Likewise, smaller tuning-fork crystals will usually have higher ESR specifications thanlarger ones.
Figure 1. Equivalent circuit of a crystal resonator.
Figure 2. Crystal phase and impedance response.
Figure 3. Pierce-type (inverter variation) oscillator circuit.
Oscillator Current
As Eric Vittoz has observed, when both load capacitors in a crystal oscillator circuit (Figure 3) are of
equal value, the minimum current for oscillation (or critical transconductance) can be approximated by theequation below.2
gmcrit ≈ 4ω2 × CL2 × RESR
(Eq. 1)
Where ω is the frequency in radians, CL is the equivalent capacitive load, and RESR is the ESR of thecrystal. The oscillator is assumed to be a CMOS device operating in weak inversion.
Vittoz also determines that the oscillator amplitude and bias current are related by the following equation:
(Eq. 2)
Where
and IB0 and IB1 are zero- and first-order modified Bessel functions.
We can therefore show the relationship between oscillator current at a given oscillator voltage with
different values of ESR and CL, as shown in the following example. Assuming that V1 = 400mVP-P, nUT= 26mV, ESR = 35kΩ, and CL = 6pF, then
≈ 4, and
IO ≈ 4ω2 × CL2 × RESR × nUT × 4
≈ 4 × (32,768 × 6.283)2 × (6e-12)2 × 35e+3 × 0.026 × 4≈ 22.2nA.
Table 1 shows the relationship between both CL and ESR on oscillator current, using the values for V1and nUT above.
Table 1. Crystal ESR and CL versus Oscillator CurrentESR (Ω)35,00070,00035,00070,000
CL (pF)6612.512.5
IO (nA)22.244.488.9177.8
Since in the equation for gmcrit the CL term is squared, doubling the load capacitance has the effect ofincreasing oscillator current by four times. Increasing the crystal ESR by two times causes the requiredoscillator current to double. Note that this is the minimum estimated oscillator current, which does notinclude current consumed by additional circuits for amplifying the oscillator output, nor does it includecurrents in the devices used to divide the frequency down to 1Hz.
Oscillator Design Requirements
An oscillator should be designed so that it has sufficient gain to operate over the entire operating
temperature and voltage range. The amplitude must always be sufficient to drive the following gain andbuffer stages under operating conditions. To minimize oscillator current requirements, for a given
oscillation voltage, a low CL is desired. Lower CL, however, will increase an oscillator's susceptibility tothe influence of external noise. Poor availability of low CL crystals may make selection of a crystal with ahigher CL necessary, at the cost of increased oscillator current. Likewise, if a design requires a smallcrystal package, an oscillator design that will drive a high ESR crystal is needed, increasing thenecessary oscillator current.
Additionally, circuits used to add desirable functions, such as glitch filters to improve oscillator noiseimmunity, or circuits to detect when the oscillator has stopped, will add to the circuit's overall currentconsumption.3
Conclusion
There are a number of tradeoffs to be considered when designing an oscillator for a low-power RTC.Increasing CL will increase noise immunity and may provide a larger selection of crystal models to
choose from, at the expense of oscillator current. Likewise, designing an oscillator so that it will run withrelatively high ESR crystals requires higher oscillator currents. Adding a glitch filter or oscillator stopdetection circuit also adds beneficial functions, but draws additional current.
References
DS1342DS1343DS1344DS1371DS1374DS1388DS1390DS1391DS1392DS1393DS1500DS1501DS1558DS1670DS1672DS1673DS1677DS1678DS1678DS1685DS17285DS17485DS17885
Low-Current I2C RTCs for High-ESR CrystalsLow-Current SPI/3-Wire RTCsLow-Current SPI/3-Wire RTCs
I2C, 32-Bit Binary Counter Watchdog Clock
I2C, 32-Bit Binary Counter Watchdog RTC with TrickleCharger and Reset Input/Output
I2C RTC/Supervisor with Trickle Charger and 512 BytesEEPROM
Low-Voltage SPI/3-Wire RTCs with Trickle ChargerLow-Voltage SPI/3-Wire RTCs with Trickle ChargerLow-Voltage SPI/3-Wire RTCs with Trickle ChargerLow-Voltage SPI/3-Wire RTCs with Trickle ChargerY2K-Compliant Watchdog RTC with NV ControlY2K-Compliant Watchdog Real-Time ClocksWatchdog Clocks with NV RAM ControlPortable System ControllerI2C 32-Bit Binary Counter RTCPortable System ControllerPortable System ControllerReal-Time Event RecorderReal-Time Event Recorder3V/5V Real-Time Clock3V/5V Real-Time Clocks3V/5V Real-Time Clocks3V/5V Real-Time Clocks
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MEMORY存储芯片DS1307Z-1中文规格书 - 图文
