Circuit Designer's Companion (eBook)

Cover 1
Contents 6
Introduction 14
Introduction to the second edition 15
Chapter 1 Grounding and wiring 16
1.1 Grounding 16
When to consider grounding 17
1.1.1 Grounding within one unit 17
1.1.2 Chassis ground 17
1.1.3 The conductivity of aluminium 19
Other materials 19
1.1.4 Ground loops 20
1.1.5 Power supply returns 21
Varying loads 22
Power rail feed 23
Conductor impedance 24
1.1.6 Input signal ground 24
Connection to 0V elsewhere on the pcb 24
Connection to 0V within the unit 24
External ground connection 25
1.1.7 Output signal ground 26
Avoiding the common impedance 27
1.1.8 Inter-board interface signals 27
Partitioning the signal return 28
1.1.9 Star-point grounding 29
1.1.10 Ground connections between units 29
Breaking the ground link 31
1.1.11 Shielding 31
Which end to ground for LF shielding 32
Electrostatic screening 33
Surface transfer impedance 33
1.1.12 The safety earth 33
1.2 Wiring and cables 34
1.2.1 Wire types 34
Wire inductance 34
Equipment wire 36
1.2.2 Cable types 36
1.2.3 Power cables 36
1.2.4 Data and multicore cables 37
Data communication cables 38
Structured data cable 39
Shielding and microphony 40
1.2.5 RF cables 40
1.2.6 Twisted pair 41
1.2.7 Crosstalk 42
Digital crosstalk 43
1.3 Transmission lines 45
Transmission line effects 45
Critical lengths for pulses 47
1.3.1 Characteristic impedance 47
1.3.2 Time domain 47
Forward and reflected waves 48
Ringing 48
The Bergeron diagram 49
The uses of mismatching 50
1.3.3 Frequency domain 50
Standing wave distribution vs. frequency 50
Impedance transformation 51
Lossy lines 52
Chapter 2 Printed circuits 53
2.1 Board types 53
2.1.1 Materials 53
Epoxy-glass 53
2.1.2 Type of construction 54
2.1.3 Choice of type 55
2.1.4 Choice of size 57
Sub-division boundaries 57
Panelisation 57
2.1.5 How a multilayer board is made 58
2.2 Design rules 58
2.2.1 Track width and spacing 60
Conductor resistance 61
Voltage breakdown and crosstalk 61
Constant impedance 62
2.2.2 Hole and pad size 63
Vias 63
Through hole pads 63
Surface mount pads 63
2.2.3 Track routing 64
2.2.4 Ground and power distribution 65
Ground rail inductance 65
Gridded ground layout 66
The ground plane 66
Inside or outside layers 67
Multiple ground planes 68
2.2.5 Copper plating and finishing 68
2.2.6 Solder resist 68
Screen printed resists 69
Photo-imaged film 69
2.2.7 Terminations and connections 69
Two-part connectors 70
Edge connectors 71
2.3 Board assembly: surface mount and through hole 71
2.3.1 Surface mount design rules 73
Solder process 73
Printed circuit board quality 73
Thermal stresses 73
Cleaning and testing 75
2.3.2 Package placement 75
2.3.3 Component identification 76
Polarity indication 76
2.4 Surface protection 76
Variations in surface resistance 77
Circuit design vs. surface resistance 77
2.4.1 Guarding 77
2.4.2 Conformal coating 78
Coating vs. encapsulation 78
Steps to take before coating 79
Application 79
Test and rework 79
2.5 Sourcing boards and artwork 80
2.5.1 Artwork 80
Using a bureau 80
Disadvantages of a bureau 81
2.5.2 Boards 81
Chapter 3 Passive components 83
3.1 Resistors 83
3.1.1 Resistor types 83
Surface mount chip 83
Metal film 85
Carbon 85
Wirewound 85
Precision resistors 86
Resistor networks 86
3.1.2 Tolerancing 86
Tolerance variations 87
3.1.3 Temperature coefficient 87
3.1.4 Power 88
3.1.5 Inductance 89
3.1.6 Pulse handling 89
Limiting element voltage (LEV) 90
3.1.7 Extreme values 90
Very low values 90
Very high values 91
3.1.8 Fusible and safety resistors 92
3.1.9 Resistor networks 92
Production efficiency 92
Value tracking: thick film versus thin film 92
3.2 Potentiometers 93
3.2.1 Trimmer types 94
Carbon 94
Cermet 94
Wirewound 94
Multi-turn 94
3.2.2 Panel types 95
Carbon, cermet and wirewound 95
Conductive plastic 95
3.2.3 Pot applications 95
Use as a rheostat 96
Adjustability 96
Law accuracy 97
Manufacturing processes 97
3.3 Capacitors 98
3.3.1 Metallised film & paper
Polyester 100
Polycarbonate 100
Polypropylene and polystyrene 101
Metallised paper 101
3.3.2 Multilayer ceramics 102
COG 102
X5R and X7R 102
Y5V and Z5U 102
3.3.3 Single-layer ceramics 103
Barrier layer 103
Low-K and High-K dielectrics 103
3.3.4 Electrolytics 104
Construction 104
Leakage 104
Ripple current and ESR 105
Temperature and lifetime 105
3.3.5 Solid tantalum 106
Tantalum chip capacitors 106
3.3.6 Capacitor applications 107
Value shifts 107
3.3.7 Series capacitors and dc leakage 109
Adding bleed resistors 109
3.3.8 Dielectric absorption 110
3.3.9 Self resonance 110
Consequences of self-resonance 111
3.4 Inductors 112
3.4.1 Permeability 112
Ferrites 113
Iron powder 114
3.4.2 Self-capacitance 114
3.4.3 Inductor applications 114
Tuned circuits 114
Power circuits 115
Suppression 115
3.4.4 The danger of inductive transients 116
Relay coils 116
Transient protection 117
Protection against negative transients 117
AC circuits 118
3.5 Crystals and resonators 118
Angle of cut 118
3.5.1 Resonance 119
3.5.2 Oscillator circuits 120
Drive level resistance 120
Series circuit 120
Layout 121
3.5.3 Temperature 121
3.5.4 Ceramic resonators 121
Chapter 4 Active components 123
4.1 Diodes 123
4.1.1 Forward bias 123
Forward current 124
Temperature dependence of forward voltage 124
4.1.2 Reverse bias 126
Breakdown 126
4.1.3 Leakage 126
Leakage variability 126
4.1.4 High-frequency performance 127
4.1.5 Switching times 128
Reverse recovery 128
Interference due to fast recovery 129
4.1.6 Schottky diodes 129
General purpose 130
RF mixers 130
Rectifiers 130
4.1.7 Zener diodes 130
Slope resistance 131
Leakage 131
Temperature coefficient 132
Precision zeners 133
Zener noise 133
4.1.8 The Zener as a clamp 133
An application example 134
4.2 Thyristors and triacs 134
4.2.1 Thyristor versus triac 135
4.2.2 Triggering characteristics 135
4.2.3 False triggering 136
4.2.4 Conduction 137
4.2.5 Switching 137
Turn-off 138
4.2.6 Snubbing 138
Values for R and C 139
4.3 Bipolar transistors 140
4.3.1 Leakage 140
A simple leakage example 140
Adding a base-emitter resistor 141
4.3.2 Saturation 141
4.3.3 The Darlington 142
4.3.4 Safe operating area 142
Second breakdown 143
SOA curve 143
4.3.5 Gain 143
4.3.6 Switching and high frequency performance 145
Speeding up the turn-off 145
4.3.7 Grading 146
4.4 Junction Field Effect transistors 147
4.4.1 Pinch-off 148
4.4.2 Applications 149
Analogue switches 149
Current regulators 150
4.4.3 High impedance circuits 150
The gate current breakpoint 150
Depressed Z[sub(in)] 151
4.5 MOSFETs 152
4.5.1 Low-power MOSFETs 152
Gate breakdown 152
Protection for the gate 153
MOSFET tradeoffs 153
4.5.2 VMOS Power FETs 153
4.5.3 Gate drive impedance 154
Gate-source overvoltage 155
Source lead inductance 156
4.5.4 Switching speed 157
4.5.5 On-state resistance 157
P-channel VMOS 158
4.6 IGBTs 158
4.6.1 IGBT structure 158
4.6.2 Advantages over MOSFETs and bipolars 159
4.6.3 Disadvantages 160
Chapter 5 Analogue integrated circuits 161
5.1 The ideal op-amp 161
5.1.1 Applications categories 162
5.2 The practical op-amp 162
5.2.1 Offset voltage 162
Output saturation due to amplified offset 163
Reducing the effect of offset 163
Offset drift 164
Circuit techniques to remove the effect of drift 164
5.2.2 Bias and offset currents 165
Bias current levels 165
Output offsets due to bias and offset currents 166
5.2.3 Common mode effects 166
CMRR 166
PSRR 167
5.2.4 Input voltage range 167
Absolute maximum input 168
5.2.5 Output parameters 168
Power rail voltage 168
Load impedance 169
5.2.6 AC parameters 169
5.2.7 Slew rate and large signal bandwidth 170
Slew rate 170
Large-signal bandwidth 171
Slewing distortion 171
5.2.8 Small-signal bandwidth 171
5.2.9 Settling time 172
5.2.10 The oscillating amplifier 172
Ground coupling 173
Power supply coupling 173
Output stage instability 174
Stray capacitance at the input 174
Parasitic feedback 174
5.2.11 Open-loop gain 175
Sagging A[sub(OL)] 175
5.2.12 Noise 176
Thermal noise 176
Amplifier noise 176
Noise bandwidth 178
5.2.13 Supply current and voltage 179
Supply voltage 179
Supply current 179
I[sub(S)] vs. speed and dissipation 179
5.2.14 Temperature ratings 180
Specification validity 180
Package reliability 181
5.2.15 Cost and availability 181
When to use industry standards 181
When not to use industry standards 182
Quad or dual packages 182
5.2.16 Current feedback op-amps 182
5.3 Comparators 183
5.3.1 Output parameters 184
5.3.2 AC parameters 184
Overdrive 184
Load impedance 185
The advantages of the active low 185
Pulse timing error 186
5.3.3 Op-amps as comparators (and vice versa) 186
5.3.4 Hysteresis and oscillations 187
The subtle effects of edge oscillation 187
Minimise stray feedback 188
Hysteresis 188
5.3.5 Input voltage limits 189
Comparator parameters vs. input voltage 190
5.3.6 Comparator sourcing 190
5.4 Voltage references 190
5.4.1 Zener references 191
5.4.2 Band-gap references 191
Costs and interchangeability 191
5.4.3 Reference specifications 193
Line and load regulation 193
Output voltage tolerance 193
Ouput voltage temperature coefficient 193
Long-term stability 193
Settling time 193
Minimum supply current 194
5.5 Circuit modelling 194
Chapter 6 Digital circuits 196
6.1 Logic ICs 196
6.1.1 Noise immunity and thresholds 196
Susceptibility to noise 197
Current immunity 198
Use of a pull-up 198
Dynamic noise immunity 198
6.1.2 Fan-out and loading 199
Dynamic loading 200
6.1.3 Induced switching noise 201
Synchronous switching 201
6.1.4 Decoupling 202
Distance 202
6.1.5 Unused gate inputs 205
6.2 Interfacing 205
6.2.1 Mixing analogue and digital 205
Ground noise 205
Filtering 206
Segregation 206
Single-board systems 207
Multi-board systems 207
6.2.2 Generating digital levels from analogue inputs 208
De-bouncing switch inputs 209
6.2.3 Protection against externally-applied overvoltages 209
6.2.4 Isolation 210
Opto-coupler trade-offs 212
Coupling capacitance 212
Alternatives to opto-couplers 213
6.2.5 Classic data interface standards 213
EIA-232F 213
EIA-422 215
Interface design 216
6.2.6 High performance data interface standards 216
EIA-485 216
CAN 217
USB 218
Ethernet 219
6.3 Using microcontrollers 220
6.3.1 How a microcontroller does your job 221
Input processes 221
Instructions and internal processing 222
Output processes 223
6.3.2 Timing and quantisation constraints 223
Instruction cycle time 223
Real time interrupts and latency 223
Limits on A-D/D-A conversion 224
PWM-style outputs 225
Sleep and wake-up 226
6.3.3 Programming constraints 227
High-level language or assembler? 227
6.4 Microprocessor watchdogs and supervision 227
6.4.1 The threat of corruption 227
Power rail supervision 228
6.4.2 Watchdog design 228
Basic operation 229
Timeout period 229
Timer hardware 229
Connection to the microprocessor 230
Source of the re-trigger pulse 230
Generation of the re-trigger pulses in 231
Testing the watchdog 231
6.4.3 Supervisor design 232
Undervoltage and power fail monitor 233
Protecting non-volatile memory 233
V[sub(CC)] differential 234
6.5 Software protection techniques 235
6.5.1 Input data validation and averaging 235
Digital inputs 235
Interrupts 236
6.5.2 Data and memory protection 236
Data communication 236
Unused program memory 236
6.5.3 Re-initialisation 237
Chapter 7 Power supplies 238
7.1 General 238
7.1.1 The linear supply 238
7.1.2 The switch-mode supply 238
7.1.3 Specifications 239
7.1.4 Off the shelf versus roll your own 240
Costs 240
7.2 Input and output parameters 240
7.2.1 Voltage 240
7.2.2 Current 241
7.2.3 Fuses 242
7.2.4 Switch-on surge, or inrush current 243
Current limiting 244
PTC thermistor limiting 244
7.2.5 Waveform distortion and interference 245
Interference 245
Peak current summation 245
Power factor correction 246
7.2.6 Frequency 247
7.2.7 Efficiency 248
7.2.8 Deriving the input voltage from the output 248
Power losses at high input voltage 250
7.2.9 Low-load condition 250
Maximum regulator dissipation 251
Minimum load requirement 251
7.2.10 Rectifier and capacitor selection 251
Reservoir capacitor 251
Rectifiers 252
7.2.11 Load and line regulation 252
Thermal regulation 253
Load sensing 253
7.2.12 Ripple and noise 254
Switching noise 254
Layout to avoid ripple 254
Correct reservoir connection 255
7.2.13 Transient response 255
Switch-mode vs. linear 256
7.3 Abnormal conditions 256
7.3.1 Output overload 256
Constant current limiting 256
Foldback current limiting 257
7.3.2 Input transients 257
Interruptions 257
Spikes and surges 259
7.3.3 Transient suppressors 259
7.3.4 Overvoltage protection 260
Crowbar circuit requirements 261
7.3.5 Turn-on and turn-off 261
PSU supervisor circuits 261
7.4 Mechanical requirements 262
7.4.1 Case size and construction 262
Open frame 263
Enclosed 263
Encapsulated 263
Rack mounting modules or cassettes 263
7.4.2 Heatsinking 264
7.4.3 Safety approvals 264
7.5 Batteries 265
7.5.1 Initial considerations 265
Voltage and capacity ratings 265
Series and parallel connection 266
Mechanical design 266
Storage, shelf life and disposal 266
7.5.2 Primary cells 268
Alkaline manganese dioxide 268
Silver oxide 268
Zinc air 269
Lithium 269
7.5.3 Secondary cells 269
Lead-acid 269
Nickel-cadmium 270
Nickel Metal Hydride 271
Lithium-ion 272
7.5.4 Charging 273
Lead-acid 273
NiCad and NiMH 274
Lithium Ion 274
Chapter 8 Electromagnetic compatibility 275
8.1 The need for EMC 275
The importance of EMC 275
8.1.1 Immunity 276
Radio transmitters 276
Radars 277
Transients 277
ESD 278
Determining and specifying the effects of interference 278
8.1.2 Emissions 280
Emissions from digital equipment 280
8.2 EMC legislation and standards 280
United States 280
Australasia 281
8.2.1 The EMC Directive 281
Scope and coverage 281
Routes to compliance 281
Harmonised versus non-harmonised standards 282
8.2.2 Existing standards 282
Immunity 285
8.3 Interference coupling mechanisms 285
8.3.1 Conducted 285
8.3.2 Radiated 286
Electromagnetic induction 287
8.4 Circuit design and layout 288
8.4.1 Choice of logic 288
Noise margin, clock frequency and power supply noise 288
8.4.2 Analogue circuits 289
8.4.3 Software 290
8.5 Shielding 290
Shielding effectiveness 290
8.5.1 Apertures 292
8.5.2 Seams 294
8.6 Filtering 295
8.6.1 The low-pass filter 295
Impedances 296
Components and layout 297
8.6.2 Mains filters 298
Safety requirements 299
Insertion loss versus impedance and current 299
8.6.3 I/O filters 299
8.6.4 Feedthrough and 3-terminal capacitors 300
Feedthroughs 300
Circuit considerations 301
8.7 Cables and connectors 302
Properly terminating the cable shield 302
Screened backshells 303
8.8 EMC design checklist 304
Chapter 9 General product design 305
9.1 Safety 305
The hazards of electricity 305
9.1.1 Safety classes 306
9.1.2 Insulation types 307
Basic insulation 307
Double insulation 307
Reinforced insulation 307
9.1.3 Design considerations for safety protection 307
9.1.4 Fire hazard 309
9.2 Design for production 309
9.2.1 Checklist 310
Sourcing 310
Production 310
Testing and calibration 311
Installation 311
9.2.2 The dangers of ESD 311
Generation of ESD 311
Static protection 312
9.3 Testability 313
9.3.1 In-circuit testing 313
9.3.2 Functional testing 314
ATE 315
9.3.3 Boundary scan and JTAG 315
History 315
Description of the boundary scan method 316
Devices 317
Deciding whether or not to use boundary scan 317
9.3.4 Design techniques 318
Bed-of-nails probing 318
Test connections 318
Circuit design 319
9.4 Reliability 320
9.4.1 Definitions 320
Mean time between failures 320
Mean time to failure 321
Availability 321
9.4.2 The cost of reliability 321
9.4.3 Design for reliability 321
Temperature 322
De-rating 322
High reliability components 323
CECC 323
Stress screening and burn-in 324
Simplicity 324
Redundancy 324
9.4.4 The value of MTBF figures 325
9.4.5 Design faults 326
The design review 326
9.5 Thermal management 326
9.5.1 Using thermal resistance 327
Partitioning the heat path 328
Transient thermal characteristics of the power device 330
9.5.2 Heatsinks 330
Forced air cooling 332
Radiation 333
9.5.3 Power semiconductor mounting 334
Heatsink surface preparation 334
Lead bend 334
The insulating washer 335
Mounting hardware 336
9.5.4 Placement and layout 337
Appendix Standards 339
Bibliography 342
Index 346
A 346
B 346
C 346
D 348
E 348
F 348
G 349
H 349
I 349
J 349
K 350
L 350
M 350
N 351
O 351
P 351
Q 352
R 352
S 353
T 353
U 354
V 354
W 354
X 354
Z 354