MC3423 Overvoltage Crowbar Sensing Circuit This overvoltage prote
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MC3423 Overvoltage Crowbar Sensing Circuit
This overvoltage protection circuit (OVP) protects sensitive electronic circuitry from overvoltage transients regulator failures when used conjunction with external "crowbar" SCR. device senses overvoltage condition quickly "crowbars" short circuits supply, forcing supply into current limiting opening fuse circuit breaker. protection voltage threshold adjustable MC3423 programmed minimum duration overvoltage condition before tripping, thus supplying noise immunity. MC3423 essentially "two terminal" system, therefore used with either positive negative supplies.
OVERVOLTAGE SENSING CIRCUIT
SEMICONDUCTOR TECHNICAL DATA
SUFFIX PLASTIC PACKAGE CASE
MAXIMUM RATINGS
Rating Differential Power Supply Voltage Sense Voltage Sense Voltage Remote Activation Input Voltage Output Current Operating Ambient Temperature Range Operating Junction Temperature Storage Temperature Range Symbol VCC-VEE VSense1 VSense2 Vact Tstg Value +150 Unit
SUFFIX PLASTIC PACKAGE CASE (SOP-8)
CONNECTIONS
(Top View)
Drive Output Indicator Output Remote Activation
Simplified Application
Sense Sense Current Source
Current Limited Power Supply Cout
Vout
MC3423
ORDERING INFORMATION
Device MC3423D MC3423P1 Operating Temperature Range +70°C Package SO-8 Plastic
Motorola, Inc. 1996
MOTOROLA ANALOG DEVICE DATA
MC3423
ELECTRICAL CHARACTERISTICS (5.0 Tlow Thigh, unless otherwise noted.)
Characteristics Supply Voltage Range Output Voltage Indicator Output Voltage (IO(Ind) Sense Trip Voltage 25°C) Temperature Coefficient VSense1 (Figure Remote Activation Input Current (VIH (VIL Source Current Output Current Risetime 25°C) Propagation Delay Time 25°C) Supply Current
NOTES: Tlow Thigh +70°C
Symbol VCC-VEE VOL(Ind) VSense1, VSense2 TCVS1
VCC-2.2 2.45
VCC-1.8 0.06
2.75
Unit %/°C
ISource
-120
-180 mA/µs
Figure Representative Block Diagram
ISource Sense
Current Source Output
Vref 2.6V
Sense
Remote Activation
Indicator Output
Figure Sense Voltage Test Circuit
Switch Switch Switch VSense VSense Ramp until output goes high; this VSense threshold. Position Position Switch Closed Open
MC3423
MOTOROLA ANALOG DEVICE DATA
MC3423
Figure Basic Circuit Configuration
Power Supply Sense Lead) MC3423 Sense Lead) *See text explanation. Load Vtrip
Vref
minimum drift
minimum value Figure
Figure Circuit Configuration Supply Voltage Above
Power Supply 1N4740 10µF MC3423 Sense Lead) Sense Lead) Load 10µF R1R2 Vtrip Vref
2N6504 equivalent 2N6505 equivalent 2N6506 equivalent 2N6507 equivalent 2N6508 equivalent 2N6509 equivalent
Figure Basic Configuration Programmable Duration Overvoltage Condition Before Trip
Vtrip +VCC Power Supply MC3423 Indication Vref
Vtrip
Vref 103] Isource
(See Figure
MOTOROLA ANALOG DEVICE DATA
MC3423
APPLICATION INFORMATION
Basic Circuit Configuration basic circuit configuration MC3423 shown Figure supply voltages from Figure trip voltages above threshold trip voltage which MC3423 will trigger supply gate drive crowbar SCR, determined selection Their values determined equation given Figures graph shown Figure minimum value gate current limiting resistor, given Figure Using this value SCR, will receive greatest gate current possible without damaging MC3423. lower output currents required, increased value. switch, shown Figure used reset crowbar. Otherwise, power supply, across which connected, must shut down reset crowbar. current-limited supply used, fuse circuit breaker, should used protect and/or load. circuit configurations shown Figures will have typical propogation delay faster operation desired, connected with left floating. This will result decreasing propogation delay approximately expense slightly increased trip voltage value. Configuration Programmable Minimum Duration Overvoltage Condition Before Tripping many instances, MC3423 will used noise environment. prevent false tripping circuit noise which would normally harm load, MC3423 programmable delay feature. implement this feature, circuit configuration Figure used. this configuration, capacitor connected from VEE. value this capacitor determines minimum duration overvoltage condition which necessary trip OVP. value found from Figure circuit operates following manner: When rises above trip point internal current source (Pin begins charging capacitor, connected overvoltage condition disappears before this occurs, capacitor discharged rate times faster than charging rate, resetting timing feature until next overvoltage condition occurs. Occasionally, desired that immediate crowbarring supply occur when high overvoltage condition occurs, while retaining false tripping immunity Figure this case, circuit Figure used. circuit will operate previously described small overvoltages, will immediately trip power supply voltage exceeds Figure Configuration Programmable Duration Overvoltage Condition Before Trip/With Immediate Trip High Overvoltages
Sense Lead) Power Supply MC3423 Sense Lead)
Additional Features Activation Indication Output additional output indicator activation provided MC3423. This output open collector transistor which saturates when activated. addition, used clock edge triggered flip-flop whose output inhibits shuts down power supply when trips. This reduces eliminates heatsinking requirements crowbar SCR. Remote Activation Input Another feature MC3423 remote activation input, voltage this CMOS/TTL compatible input held below MC3423 operates normally. However, raised voltage above output activated independent whether overvoltage condition present. should noted that internal pull-up current source. This feature used accomplish orderly sequenced shutdown system power supplies during system fault condition. addition, activation indication output MC3423 used activate another MC3423 single transistor inverter used interface former's indication output latter's remote activation input, shown Figure this circuit, indication output (Pin MC3423 power supply used activate MC3423 associated with power supply small with adequate voltage rating.
MOTOROLA ANALOG DEVICE DATA
MC3423
Figure Circuit Configuration Activating MC3423 from Another
Power Supply RESISTANCE
Figure versus Trip Voltage
1.0k
Power Supply
TRIP VOLTAGE
Figure Minimum versus Supply Voltage Note that both supplies have their negative output leads tied together (i.e., both positive supplies). their positive leads common (two negative supplies) emitter would moved positive lead supply would therefore have resized deliver appropriate drive Crowbar Considerations Referring Figure seen that crowbar SCR, when activated, subject large current surge from output capacitance, Cout. This capacitance consists power supply output caps, load's decoupling caps, case Figure 11A, supply's input filter caps. This surge current illustrated Figure cause failure degradation three mechanisms: di/dt, absolute peak surge, I2t. interrelationship these failure methods breadth applications make specification semiconductor manufacturer difficult expensive. Therefore, designer must empirically determine circuit elements which result reliable effective operation. However, understanding factors which influence SCR's di/dt surge capabilities simplifies this task. di/dt gate region driven area conduction takes finite amount time grow, starting very small region gradually spreading. Since anode current flows through this turned-on gate region, very high current densities occur gate region high anode currents appear quickly (di/dt). This result immediate destruction gradual degradation forward blocking voltage capabilities depending severity occasion.
SUPPLY VOLTAGE RG(min)
GATE CURRENT LIMITING RESISTOR
Figure Capacitance versus Minimum Overvoltage Duration
CAPACITANCE
0.01
0.001
0.0001 0.001
0.01 DELAY TIME (ms)
MOTOROLA ANALOG DEVICE DATA
MC3423
Figure Typical Crowbar Circuit Configurations
(11A) Power Supply Cout Sense Vout
Power Supply
(11B) Cout Sense
Vout
*Needed supply current limited
will case, though this difficult guarantee. course, sufficiently high surge will cause open. These comments also apply fuse Figure 11B. value di/dt that safely handle influenced construction characteristics gate drive signal. center-gate-fire more di/dt capability than corner-gate-fire type, heavily overdriving times IGT) gate with fast rise time signal will maximize di/dt capability. typical maximum number phase control SCRs less than A(RMS) rating might A/µs, assuming gate current five times rise time. having done this, di/dt problem seen still exist, designer also decrease di/dt current waveform adding inductance series with SCR, shown Figure course, this reduces circuit's ability rapidly reduce voltage tradeoff must made between speedy voltage reduction di/dt. Surge Current peak current and/or duration surge excessive, immediate destruction device overheating will result. surge capability directly proportional area. surge current cannot reduced adding series resistance Figure safe level which consistent with systems requirements speedy voltage reduction, designer must higher current SCR. This result average current capability exceeding steady state current requirements imposed power supply. WORD ABOUT FUSING Before leaving subject crowbar SCR, words about fuse protection order. Referring back Figure 11A, will seen that fuse necessary power supply protected output current limited. This fuse meant prevent failure rather prevent fire! order protect SCR, fuse would have possess rating less than that have high enough continuous current rating survive normal supply output currents. addition, must capable successfully clearing high short circuit currents from supply. Such fuse this quite expensive, even available. CROWBAR SELECTION GUIDE selecting crowbar use, following selection guide presented.
Device 2N6400 Series 2N6504 Series 2N1842 Series 2N2573 Series 2N681 Series MCR3935-1 Series MCR81-5 Series IRMS IFSM 1000 Package TO-220 Plastic TO-220 Plastic Metal Stud Metal TO-3 Type Metal Stud Metal Stud Metal Stud
Figure Crowbar Surge Current Waveform
Surge Output Capacitor Current Limited Supply Output
Figure Circuit Elements Affecting Surge di/dt
RLead Output MC3423 LLead
EMPIRICALLY DETERMINED!
usual design compromise then garden variety fuse (3AG style) which cannot relied blow before thyristor does, trust that does fail, will fail short circuit. majority designs, this
MOTOROLA ANALOG DEVICE DATA
MC3423
OUTLINE DIMENSIONS
SUFFIX PLASTIC PACKAGE CASE 626-05 ISSUE
NOTE
NOTES: DIMENSION CENTER LEAD WHEN FORMED PARALLEL. PACKAGE CONTOUR OPTIONAL (ROUND SQUARE CORNERS). DIMENSIONING TOLERANCING ANSI Y14.5M, 1982. MILLIMETERS 9.40 10.16 6.10 6.60 3.94 4.45 0.38 0.51 1.02 1.78 2.54 0.76 1.27 0.20 0.30 2.92 3.43 7.62 0.76 1.01 INCHES 0.370 0.400 0.240 0.260 0.155 0.175 0.015 0.020 0.040 0.070 0.100 0.030 0.050 0.008 0.012 0.115 0.135 0.300 0.030 0.040
SEATING PLANE
0.13 (0.005)
SUFFIX PLASTIC PACKAGE CASE 751-05 (SOP-8) ISSUE
NOTES: DIMENSIONING TOLERANCING ASME Y14.5M, 1994. DIMENSIONS MILLIMETERS. DIMENSION INCLUDE MOLD PROTRUSION. MAXIMUM MOLD PROTRUSION 0.15 SIDE. DIMENSION DOES INCLUDE MOLD PROTRUSION. ALLOWABLE DAMBAR PROTRUSION SHALL 0.127 TOTAL EXCESS DIMENSION MAXIMUM MATERIAL CONDITION. MILLIMETERS 1.35 1.75 0.10 0.25 0.35 0.49 0.18 0.25 4.80 5.00 3.80 4.00 1.27 5.80 6.20 0.25 0.50 0.40 1.25
0.25
SEATING PLANE
0.10 0.25
MOTOROLA ANALOG DEVICE DATA
MC3423
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MOTOROLA ANALOG DEVICE DATA MC3423/D
*MC3423/D*
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