* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download AN10289 LPC900 external crystal start-up Rev. 01 — 27 April 2004 Application note
Survey
Document related concepts
Solar micro-inverter wikipedia , lookup
Electrical ballast wikipedia , lookup
Negative feedback wikipedia , lookup
Control system wikipedia , lookup
Power inverter wikipedia , lookup
Buck converter wikipedia , lookup
Time-to-digital converter wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Signal-flow graph wikipedia , lookup
Zobel network wikipedia , lookup
Rectiverter wikipedia , lookup
Semiconductor device wikipedia , lookup
Crystal oscillator wikipedia , lookup
Regenerative circuit wikipedia , lookup
Crystal radio wikipedia , lookup
Transcript
AN10289 LPC900 external crystal start-up Rev. 01 — 27 April 2004 Document information Info Content Keywords LPC900, external crystal Abstract LPC900 external crystal start-up Application note AN10289 Philips Semiconductors LPC900 external crystal start-up Revision history Rev Date Description 01 20040427 Initial version (9397 750 13134) Contact information For additional information, please visit: http://www.semiconductors.philips.com For sales office addresses, please send an email to: [email protected] 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 2 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up 1. Introduction The LPC900 family high frequency crystal oscillator consists of an inverting linear transconductance amplifier that is capable of amplifying signals from 4 MHz to 12 MHz. Integral to the amplifier is a feedback resistor that is connected from the input to the output of the amplifier. 1.1 Scope This document will describe how to bias the high frequency crystal oscillator of the LPC900 family by adding an external biasing resistor. Biasing the external crystal oscillator will result in a more robust start-up. This document takes a close look at the characteristics of the LPC900 high frequency oscillator such as the DC characteristics of the LPC900 family’s high frequency oscillator and the open circuit biasing point of the high frequency oscillator. Equations will be given to calculate the minimum gm required with a certain shunt capacitance. 2. LPC900 family high frequency oscillator characteristics 2.1 Oscillator feedback resistor The typical value of the feedback resistor between the in and output of the inverter range from 1.3 M: to 1.5 M:. The feedback resistor is shown in Figure 2. 2.2 DC transfer characteristics The buffer's DC transfer characteristics were measured for a group of parts. Figure 1 shows typical DC transfer characteristics of the high frequency oscillator on the LPC900 family. The two horizontal lines on the graph indicate the output voltage range of the buffer in the open state. The graph indicates that the DC operating point is not in the centre of the linear region. In fact, it is near the bottom where the gain is slightly smaller. 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 3 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up Crystal Inverter DC transfer curve part 1 part 2 3.5 part 3 Vout (Volts) 3 part 4 2.5 part 5 2 part 6 part 7 1.5 part 8 1 part 9 0.5 part 10 0 open circuit dc bias min 0.7 0.75 0.8 0.85 0.9 open circuit dc bias max Vin (Volts) Fig 1. Crystal inverter DC characteristics 2.3 Small signal output impedance Another measurement that was made on a group of parts was the small signal output impedance. This was done by AC coupling a signal into the output, and then measuring the ratio of the output voltage to the input voltage. Figure 2 shows the test circuit used to measure the output impedance and also the equation to calculate it based on the output to input voltage. 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 4 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up Vout Crystal Oscillator buffer 1uF Rout Rin = 110 Vin 1uF Vin Vout Vin Vout 1 Vin Rin output impedance Fig 2. Output impedance small signal test set up The data for this measurement ranged from 2 k: to 5 k: for the LPC900 family high frequency oscillator. 2.4 Small signal transconductance The small signal transconductance was measured by AC coupling a fixed load to the output, then measuring the ratio of the output current to the input current. Figure 2 shows the test circuit used to measure the transconductance, and the equation to calculate it based on the output to input voltage. The measurements ranged from 0.0022 to 0.0029 when biased in the ‘open’ configuration; that is, it is not biased with external components. 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 5 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up Crystal Oscillator buffer Vout 1uF Rout Vin 1uF Rin = 110 Vin Transcondu ctance Vout 110 Vin Fig 3. Transconductance measurement set-up 2.5 Bias point adjustment When looking at the DC transfer curves the natural bias point of the high frequency oscillator did not provide the maximum gain for start up. The open circuit output voltage of the buffer ranged from 0.77 V to 0.79 V. This puts the small signal gain on the shallow part of the curve. The DC operating point can be changed by putting a 1 M: resistor to ground on the input side of the oscillator buffer. This resistor is a external part and is labelled as Rbias in Figure 5. By adding this resistor, the DC operating point of the buffer is centred in the middle of the linear region of the buffer, by biasing the crystal oscillator to the maximum gain region it will make start-up more robust. 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 6 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up C rystal Inverter D C transfer C h aracteristics p a rt 2 3 p a rt 3 p a rt 4 2 .5 p a rt 5 p a rt 6 Vout (Volts) 2 p a rt 7 1 .5 p a rt 8 p a rt 9 1 p a rt 1 0 o p e n circu it b ia s m in o p e n circu it b ia s m ax 1 Mo h m b ia s lin e 0 .5 0 0 .7 0 .7 5 0 .8 0 .8 5 0 .9 V in (V olts) Fig 4. Measured inverter DC transfer characteristics with 1 M: bias line 2.6 Minimum required transconductance The minimum required transconductance for oscillation can be calculated from the formula shown in Equations (1) and (2). This equation is taken from [2], application note AN97090: (1) Z O2 CO2 C1 C2 1 C1 C2 1 C1 g m t RS C1C2 RP C1C2 RO C2 2 2 If C1 = C2, this equation reduces to: (2) g m t 4 RS Z O2 CO2 4 1 RP RO Equations (1) and (2) - Minimum requirements with no bias resistor. However, adding the bias resistor on the input side to ground increases the requirements. The equation for minimum requirements with the bias resistor is shown in Equations (3) and (4): 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 7 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up (3) Z O2 CO2 C1 C2 1 C1 C2 1 C1 1 C2 g m t RS C1C2 RP C1C2 RO C2 RBias C1 2 2 If C1 = C2, this equation reduces to: (4) g m t 4 RS Z O2 CO2 4 1 1 RP RO RBias Equations (3) and (4) - Minimum requirements with bias resistor. Crystal Model L1 Ca Rs CO Crystal Oscillator buffer Rp Ro Rbias C2 C1 Fig 5. Oscillator circuit 2.7 Calculated gm requirements Using the equations shown above, Table 1 shows the minimum gm required compared to what was measured. Table 1: gm calculations F0 = 12 MHz Rs = 3K Rp = 1.3 M: Co = 15 pF Rbias = none gm t 0.000387 gm measured min 0.0022 Rbias = 1M: gm t 0.000388 gm measured min 0.0031 The data in Table 1 shows that the part has significantly more gain than required with the 1 M: bias resistor. Rbias is the added external resistor to centre the oscillator buffer DC bias in the center of the linear region. 2.8 Additional gm verification test Figure 6 shows a graph of the minimum gm required as a function of C1 with a fixed C2 shunt capacitance. Also plotted on this graph are the min and max measured gm from a sample of parts. It shows that the part should stop oscillating with a shunt capacitance 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 8 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up between 65 pF and 85 pF. A test circuit showed that oscillations stopped with approximately 80 pF for C1. This was another verification that the measured gm was within the correct range. Required transconductance as a function of input shunt capacitance (output shunt C2 capacitace fixed at 12pF) Min Gm requirement 0.0040000 Min Measured Gm Required Transconductance 0.0035000 Max Measured Gm 0.0030000 0.0025000 0.0020000 0.0015000 0.0010000 0.0005000 0.0000000 0.0E+00 2.0E-11 4.0E-11 6.0E-11 8.0E-11 1.0E-10 C1 Shunt Capacitance (F) Fig 6. Graph of min gm required as a function of shunt capacitance C1 with C2 fixed 2.9 Bias resistor testing Besides the calculations that were presented in this paper, the oscillator start up was also verified over temperature using the external bias 1 M: bias resistor. The temperature was adjusted from 55 qC to +125 qC and started over the full temperature range. 3. Conclusions The oscillator DC characteristics shows that the dc bias point was toward the bottom side of the transfer curve. This indicated that the start up gain of the oscillator could be slightly lower.Using an additional external resistor to raise the bias point to the centre of the inverter’s linear region improves the start-up gain. This resistor increases the gm requirements slightly, but the part has adequate margin to overcome this increase. 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 9 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up 3.1 Important note Every crystal circuit will vary with the main variables being the selected crystal the shunt capacitors the microcontroller’s oscillator characteristics and the PCB’s board capacitance and noise immunity on the crystal pins. All these variables will influence the crystal start-up of an application, care has to be taken to qualify the crystal start-up of each application. 4. References [1] X-tal oscillator on 8 bit microcontrollers AN96103 — Audre Pauptit, Erik Norberg, Application note, 20 Sept 1996, Draft, Philips System laboratory group, Eindhoven the Netherlands, Telecom products group, Zuerich. [2] X-tal oscillators on 8 bit microcontrollers (2) AN97090 — Audre Pauptit, Wim Rosink, , Application note, 11 Dec 1997, Draft, Philips System laboratory group, Eindhoven the Netherlands. 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 10 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up 5. Disclaimers Life support — These products are not designed for use in life support appliances, devices, or systems where malfunction of these products can reasonably be expected to result in personal injury. Philips Semiconductors customers using or selling these products for use in such applications do so at their own risk and agree to fully indemnify Philips Semiconductors for any damages resulting from such application. Right to make changes — Philips Semiconductors reserves the right to make changes in the products - including circuits, standard cells, and/or software - described or contained herein in order to improve design and/or performance. When the product is in full production (status ‘Production’), relevant changes will be communicated via a Customer Product/Process Change Notification (CPCN). Philips Semiconductors assumes no responsibility or liability for the use of any of these products, conveys no licence or title under any patent, copyright, or mask work right to these products, and makes no representations or warranties that these products are free from patent, copyright, or mask work right infringement, unless otherwise specified. 9397 750 13134 Application note © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Rev. 01 — 27 April 2004 11 of 12 AN10289 Philips Semiconductors LPC900 external crystal start-up 6. Contents 1 1.1 1.2 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 4 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 LPC900 family high frequency oscillator characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Oscillator feedback resistor . . . . . . . . . . . . . . . 3 DC transfer characteristics . . . . . . . . . . . . . . . . 3 Small signal output impedance. . . . . . . . . . . . . 4 Small signal transconductance . . . . . . . . . . . . . 5 Bias point adjustment . . . . . . . . . . . . . . . . . . . . 6 Minimum required transconductance . . . . . . . . 7 Calculated gm requirements . . . . . . . . . . . . . . . 8 Additional gm verification test . . . . . . . . . . . . . . 8 Bias resistor testing . . . . . . . . . . . . . . . . . . . . . 9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Important note. . . . . . . . . . . . . . . . . . . . . . . . . 10 Disclaimers. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 © Koninklijke Philips Electronics N.V. 2004 All rights are reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. The information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. No liability will be accepted by the publisher for any consequence of its use. Publication thereof does not convey nor imply any license under patent- or other industrial or intellectual property rights. Date of release: 27 April 2004 Document order number: 9397 750 13134 Published in U.S.A.