1、How to Read a Datasheet Prepared for the WIMS outreach program 5/6/02, D. Grover In order to use a PIC microcontroller, a flip-flop, a photodetector, or practically any electronic device, you need to consult a datasheet. This is the document that the manufacturer provides telling you the typical dev
2、ice performance minimum and maximum requirements and characteristics what you can do to the device without harming it suggested uses and hints Manufacturers want you, the designer, to have a successful experience with their device. They are trying to be helpful. They dont always succeed. The datashe
3、et on the following pages is a relatively good datasheet. It tries to concisely tell you everything you need to know about the device, a common 555 timer chip (the duct-tape of the electronics hobbyist). Most datasheets for ICs follow the same general layout. You dont have to understand everything i
4、n a datasheet. Theres a lot of information that might not be of any use to you. The annotations that follow try to point out parts of the datasheet that you should pay particular attention to. Where do you find datasheets? Nowadays you can find almost any datasheet on the internet, often in PDF (Acr
5、obat) form. For example, the LM555 datasheet from National Semiconductor is on their website at . What is the LM555? The LM555 is a timer chip that uses external resistors and capacitors to generate either a single pulse of a certain duration, or a continuous sequence of pulses with a variety of pul
6、se widths possible. Because it is a very general purpose collection of functional blocks such as comparators, a flip-flop, internal voltage divider, high power output stage, and so on, a number of different timing-related functions are possible. Entire books have been written about the 555, though i
7、t is often used when another IC would work better. (See for example the CD4538 timer chip.) LM555TimerGeneral DescriptionThe LM555 is a highly stable device for generating accuratetime delays or oscillation. Additional terminals are providedfor triggering or resetting if desired. In the time delay m
8、ode ofoperation, the time is precisely controlled by one external re-sistor and capacitor. For astable operation as an oscillator,the free running frequency and duty cycle are accuratelycontrolled with two external resistors and one capacitor. Thecircuit may be triggered and reset on falling wavefor
9、ms, andthe output circuit can source or sink up to 200mA or driveTTL circuits.Featuresn Direct replacement for SE555/NE555n Timing from microseconds through hoursn Operates in both astable and monostable modesn Adjustable duty cyclen Output can source or sink 200 mAn Output and supply TTL compatible
10、n Temperature stability better than 0.005% per Cn Normally on and normally off outputn Available in 8-pin MSOP packageApplicationsn Precision timingn Pulse generationn Sequential timingn Time delay generationn Pulse width modulationn Pulse position modulationn Linear ramp generatorSchematic DiagramD
11、S007851-1February 2000LM555Timer 2000 National Semiconductor Corporation DS007851 Connection DiagramOrdering InformationPackage Part Number Package Marking Media Transport NSC Drawing8-Pin SOIC LM555CM LM555CM RailsM08ALM555CMX LM555CM 2.5k Units Tape and Reel8-Pin MSOP LM555CMM Z55 1k Units Tape an
12、d ReelMUA08ALM555CMMX Z55 3.5k Units Tape and Reel8-Pin MDIP LM555CN LM555CN Rails N08EDual-In-Line, Small Outlineand Molded Mini Small Outline PackagesDS007851-3Top ViewLM 2Absolute Maximum Ratings (Note 2)If Military/Aerospace specified devices are required,please contact the National Semiconducto
13、r Sales Office/Distributors for availability and specifications.Supply Voltage +18VPower Dissipation (Note 3)LM555CM, LM555CN 1180 mWLM555CMM 613 mWOperating Temperature RangesLM555C 0C to +70CStorage Temperature Range 65C to +150CSoldering InformationDual-In-Line PackageSoldering (10 Seconds) 260CS
14、mall Outline Packages(SOIC and MSOP)Vapor Phase (60 Seconds) 215CInfrared (15 Seconds) 220CSee AN-450 “Surface Mounting Methods and Their Effecton Product Reliability” for other methods of solderingsurface mount devices.Electrical Characteristics (Notes 1, 2)(TA= 25C, VCC= +5V to +15V, unless othewi
15、se specified)Parameter Conditions Limits UnitsLM555CMin Typ MaxSupply Voltage 4.5 16 VSupply Current VCC= 5V, RL= VCC= 15V, RL= (Low State) (Note 4)310615 mATiming Error, MonostableInitial Accuracy 1%Drift with Temperature RA= 1k to 100k, 50 ppm/CC = 0.1F, (Note 5)Accuracy over Temperature 1.5 %Drif
16、t with Supply 0.1 %/VTiming Error, AstableInitial Accuracy 2.25 %Drift with Temperature RA,RB= 1k to 100k, 150 ppm/CC = 0.1F, (Note 5)Accuracy over Temperature 3.0 %Drift with Supply 0.30 %/VThreshold Voltage 0.667 x VCCTrigger Voltage VCC= 15V 5 VVCC= 5V 1.67 VTrigger Current 0.5 0.9 AReset Voltage
17、 0.4 0.5 1 VReset Current 0.1 0.4 mAThreshold Current (Note 6) 0.1 0.25 AControl Voltage Level VCC= 15VVCC=5V92.6103.33114VPin 7 Leakage Output High 1 100 nAPin 7 Sat (Note 7)Output Low VCC= 15V, I7= 15mA 180 mVOutput Low VCC= 4.5V, I7= 4.5mA 80 200 mVLM3Electrical Characteristics (Notes 1, 2) (Cont
18、inued)(TA= 25C, VCC= +5V to +15V, unless othewise specified)Parameter Conditions Limits UnitsLM555CMin Typ MaxOutput Voltage Drop (Low) VCC= 15VISINK= 10mA 0.1 0.25 VISINK= 50mA 0.4 0.75 VISINK= 100mA 2 2.5 VISINK= 200mA 2.5 VVCC=5VISINK= 8mA VISINK= 5mA 0.25 0.35 VOutput Voltage Drop (High) ISOURCE
19、= 200mA, VCC= 15V 12.5 VISOURCE= 100mA, VCC= 15V 12.75 13.3 VVCC= 5V 2.75 3.3 VRise Time of Output 100 nsFall Time of Output 100 nsNote 1: All voltages are measured with respect to the ground pin, unless otherwise specified.Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the
20、device may occur. Operating Ratings indicate conditions for which the device is func-tional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which guar-antee specific performance limits. This assu
21、mes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit isgiven, however, the typical value is a good indication of device performance.Note 3: For operating at elevated temperatures the device must be derated above 25C based on a +150C max
22、imum junction temperature and a thermal resistanceof 106C/W (DIP), 170C/W (S0-8), and 204C/W (MSOP) junction to ambient.Note 4: Supply current when output high typically 1 mA less at VCC=5V.Note 5: Tested at VCC= 5V and VCC= 15V.Note 6: This will determine the maximum value of RA+RBfor 15V operation
23、. The maximum total (RA+RB)is20M.Note 7: No protection against excessive pin 7 current is necessary providing the package dissipation rating will not be exceeded.Note 8: Refer to RETS555X drawing of military LM555H and LM555J versions for specifications.LM 4Typical Performance CharacteristicsMinimui
24、m Pulse WidthRequired for TriggeringDS007851-4Supply Current vs.Supply VoltageDS007851-19High Output Voltage vs.Output Source CurrentDS007851-20Low Output Voltage vs.Output Sink CurrentDS007851-21Low Output Voltage vs.Output Sink CurrentDS007851-22Low Output Voltage vs.Output Sink CurrentDS007851-23
25、LM5Applications InformationMONOSTABLE OPERATIONIn this mode of operation, the timer functions as a one-shot(Figure 1). The external capacitor is initially held dischargedby a transistor inside the timer. Upon application of a nega-tive trigger pulse of less than 1/3 VCCto pin 2, the flip-flop isset
26、which both releases the short circuit across the capacitorand drives the output high.The voltage across the capacitor then increases exponen-tially for a period of t = 1.1 RAC, at the end of which time thevoltage equals 2/3 VCC. The comparator then resets theflip-flop which in turn discharges the ca
27、pacitor and drives theoutput to its low state. Figure 2 shows the waveforms gener-ated in this mode of operation. Since the charge and thethreshold level of the comparator are both directly propor-tional to supply voltage, the timing internal is independent ofsupply.During the timing cycle when the
28、output is high, the furtherapplication of a trigger pulse will not effect the circuit so longas the trigger input is returned high at least 10s before theend of the timing interval. However the circuit can be resetduring this time by the application of a negative pulse to thereset terminal (pin 4).
29、The output will then remain in the lowstate until a trigger pulse is again applied.When the reset function is not in use, it is recommended thatit be connected to VCCto avoid any possibility of false trig-gering.Figure 3 is a nomograph for easy determination of R, C val-ues for various time delays.N
30、OTE: In monostable operation, the trigger should be drivenhigh before the end of timing cycle.ASTABLE OPERATIONIf the circuit is connected as shown in Figure 4 (pins 2 and 6connected) it will trigger itself and free run as a multivibrator.The external capacitor charges through RA+RBand dis-charges t
31、hrough RB. Thus the duty cycle may be preciselyset by the ratio of these two resistors.In this mode of operation, the capacitor charges and dis-charges between 1/3 VCCand 2/3 VCC. As in the triggeredmode, the charge and discharge times, and therefore the fre-quency are independent of the supply volt
32、age.DS007851-5FIGURE 1. MonostableDS007851-6VCC= 5V Top Trace: Input 5V/Div.TIME = 0.1 ms/DIV. Middle Trace: Output 5V/Div.RA= 9.1k Bottom Trace: Capacitor Voltage 2V/Div.C = 0.01FFIGURE 2. Monostable WaveformsDS007851-7FIGURE 3. Time DelayDS007851-8FIGURE 4. AstableLM7Physical Dimensions inches (mi
33、llimeters) unless otherwise noted (Continued)LIFE SUPPORT POLICYNATIONALS PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORTDEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERALCOUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:1. Lif
34、e support devices or systems are devices orsystems which, (a) are intended for surgical implantinto the body, or (b) support or sustain life, andwhose failure to perform when properly used inaccordance with instructions for use provided in thelabeling, can be reasonably expected to result in asignif
35、icant injury to the user.2. A critical component is any component of a lifesupport device or system whose failure to performcan be reasonably expected to cause the failure ofthe life support device or system, or to affect itssafety or effectiveness.National SemiconductorCorporationAmericasTel: 1-800
36、-272-9959Fax: 1-800-737-7018Email: National SemiconductorEuropeFax: +49 (0) 180-530 85 86Email: Deutsch Tel: +49 (0) 69 9508 6208English Tel: +44 (0) 870 24 0 2171Franais Tel: +33 (0) 1 41 91 8790National SemiconductorAsia Pacific CustomerResponse GroupTel: 65-2544466Fax: 65-2504466Email: National S
37、emiconductorJapan Ltd.Tel: 81-3-5639-7560Fax: 81-3-5639-Molded Dual-In-Line Package (N)NS Package Number N08ELM555TimerNational does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
