MOTOROLA SEMICONDUCTOR aaa TECHNICAL DATA Overvoltage Crowbar Sensing Circuit These overvoltage protection circuits (OVP) protect sensitive electronic circuitry from overvoltage transients or regulator failures when used in conjunction with an external crowbar SCR. They sense the overvoltage condition and quickly crowbar or short circuit the supply, forcing the supply into current limiting or opening the fuse or circuit breaker. The protection voitage threshold is adjustable and the MC3423/3523 can be programmed for minimum duration of overvoltage condition before tripping, thus supplying noise immunity. The MC3423/3523 is essentially a two terminal system, therefore it can be used with either positive or negative supplies. MAXIMUM RATINGS MC3423 MC3523 OVERVOLTAGE SENSING CIRCUIT SILICON MONOLITHIC INTEGRATED CIRCUIT P1 SUFFIX PLASTIC PACKAGE CASE 626 (MC3423 only) U SUFFIX CERAMIC PACKAGE CASE 693 D SUFFIX PLASTIC PACKAGE CASE 751 (SOP-8) Rating Symbol Value Unit Differential Power Supply Voltage Voc-VEE 40 Vde Sense Voltage (1) VSense1 6.5 Vde Sense Voltage (2) VSense2 6.5 Vde Remote Activation Input Voltage Vact 7.0 Vde Output Current lo 300 mA Operating Ambient Temperature Range TA C MC3423 0 to +70 MC3523 55 to +125 Operating Junction Temperature Ty C Plastic Package 125 Ceramic Package 150 Storage Temperature Range Tstg -65 to +150 C Typical Application Vin Vout oF + o Current I et | ca [ove LF Power MC3523/3423 Supply I o4 O PIN CONNECTIONS ft 5) ou [2] Vee Indicator | 3| Output Remote Activation (Top View) ORDERING INFORMATION Device Temperature Range Package MC3423D $0-8 MC3423P1 0 to +70C Plastic DIP MC3423U Ceramic DIP MC3523U -55 to +125C Ceramic DIP MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA 3-111MC3423, MC3523 ELECTRICAL CHARACTERICISTICS (5 V < Vcc VEE $36 V, Tlow < TA, Thigh, unless otherwise noted.) Characteristics Symbol Min Typ Max Unit Supply Voltage Range Voc-VEE 4.5 _ 40 Vde Output Voltage Vo Voc-2.2 | Vcoc-1.8 _ Vde (Io = 100 mA) Indicator Output Voltage Vo_(ind) _ 0.1 0.4 Vde (lO(Ind) = 1-6 mA) Sense Trip Voltage VSense1, 2.45 2.6 2.75 Vdc (Ta = 25C) VSense2 Temperature Coefficient of VSense1 TCVs1 _ 0.06 _ BIC (Figure 2) Remote Activation Input Current pA (VIH = 2.0 V, Voc - Veg = 5.0 V) I _ 5.0 40 (VIL = 0.8 V, Voc - VEE = 5.0 V) ML -120 -180 Source Current ISource 0.1 0.2 0.3 mA Output Current Risetime tr _ 400 - mAs (Ta = 25C) Propagation Delay Time tod _ 0.5 _ ps (Ta = 25C) Supply Current Ip mA MC3423 _ 6.0 10 MC3523 - 5.0 7.0 NOTES: Tiow 10 Thigh = 55 to +125C for MC3523 = 0 to +70C for MC3423 Figure 1. Block Diagram Voc 91 lSource Current 4 Source Sense 1 Output 8 7 VEE 30 Sense2 5 6 Indicator Remote Output Activation Figure 2. Sense Voltage Test Circuit I Vcc Switch 9 1 () e+] 3 8 Switch 1 Switch 2 (B) VSense 1 Position A Closed Switch 2 / 4 VSense 2 Position B Open vy 7.5 , | Ramp V| until output goes high; this is the VSense threshold. MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA 3-112MC3423, MC3523 Figure 3. Basic Circuit Configuration e + Fi (+ Sense | R1 Ri Lead) ; | Virip= Viet (1+ 25 ) = 26V(1+ wp? y | R2-< 10 kQ2 for minimum drift Power 2 8 Supply 3 MC3523 A | To & | Load For minimum value of Rg, see Figure 9 R2 4 \ 7 d > sie *See text for explanation (- Sense Lead) | Figure 4. Circuit Configuration for Supply Voltage Above 36 V + (+ Sense Rg Lead) Vg - 10 rT Rt Rg=( 55 ke 1 Fa Ri RI 8 ] Virip = Veet (1+ 2 }=26V(14 oo ) to *R2<10kQ Power 1N4740 MC3523 2 Load Suppy | inv & M3423 3 Vs an: Vs s = 2Nes04 or exutalet + Ss ; 2N65O5 or eq el 4] *ne Vg < 200 V; 2N6506 or equivalent 7 | 5 | (- Sense Vg < 400 V; 2N6507 or equivalent Lead) Vg < 600 V; 2N6508 or equivalent Vg < 800 V; 2N6509 or equivalent Figure 5. Basic Configuration for Programmable Duration of Overvoltage Condition Before Trip Vec Virip + -= ed +Vcc R3 V10 V Vc Indication tef -- TTT TS a Supply Re 0 7 Vo Vo 0 T i ty! Vref Isource xC=[12 x 105] C (See Figure 10) MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA 3-113MC3423, MC3523 APPLICATION INFORMATION Basic Circuit Configuration The basic circuit configuration of the MC3423/3523 OVP is shown in Figure 3 for supply voltages from 4.5 V to 36 V, and in Figure 4 for trip voltages above 36 V. The threshold or trip voltage at which the MC3423/3523 will trigger and supply gate drive to the crowbar SCR, Q1, is determined by the selection of R1 and R2. Their values can be determined by the equation given in Figures 3 and 4, or by the graph shown in Figure 8. The minimum value of the gate current limiting resistor, Rg, is given in Figure 9. Using this value of Ra, the SCR, Q1, will receive the greatest gate current possible without damaging the MC3423/3523. If lower output currents are required, RG can be increased in value. The switch, $1, shown in Figure 3 may be used to reset the crowbar. Otherwise, the power supply, across which the SCR is connected, must be shut down to reset the crowbar. If a non current-limited supply is used, a fuse or circuit breaker, F1, should be used to protect the SCR and/or the load. The circuit configurations shown in Figures 3 and 4 will have a typical propogation delay of 1.0 us. If faster operation is desired, Pin 3 may be connected to Pin 2 with Pin 4 leftfloating. This will result in decreasing the propogation delay to approximately 0.5 tts at the expense ofa slightly increased TC for the trip voltage value. Configuration for Programmable Minimmum Duration of Overvoltage Condition Before Tripping tn many instances, the MC3423/3523 OVP will be used in a noise environment. To prevent false tripping of the OVP circuit by noise which would not normally harm the load, MC3423/3523 has a programmable delay feature. To implement this feature, the circuit configuration of Figure 5 is used. In this configuration, a capacitor is connected from Pin 3 to VEE. The vaiue of this capacitor determines the minimum duration of the overvoltage condition which is necessary to trip the OVP. The value of C can be found from Figure 10. The circuit operates in the following manner: When Vcc rises above the trip point set by R1 and R2, an internal current source (Pin 4) begins charging the capacitor, C, connected to Pin 3. If the overvoltage condition disappears before this occurs, the capacitor is discharged at a rate = 10 times faster than the charging rate, resetting the timing feature until the next overvoltage condition occurs. Occasionally, it is desired that immediate crowbarring of the supply occur when a high overvoltage condition occurs, while retaining the false tripping immunity of Figure 5. In this case, the circuit of Figure 6 can be used. The circuit will operate as previously described for small overvoltages, but will immediately trip if the power supply voltage exceeds Vz714+1.4V. Figure 6. Configuration for Programmable Duration of Overvoltage Condition Before Trip/With Immediate Trip at High Overvoltages Oo (+ Sense + Lead) 1 Z1 4 Rg y MC3523 Power Supply t 1k T Cc - | (Sense Lead) ; Oo Additional Features 1. Activation Indication Output An additional output for use as an indicator of OVP activation is provided by the MC3423/3523. This output is an open collector transistor which saturates when the OVP is activated. In addition, it can be used to clock an edge triggered flip-flop whose output inhibits or shuts down the power supply when the OVP trips. This reduces or eliminates the heatsinking requirements for the crowbar SCR. 2. Remote Activation Input Another feature of the MC3423/3523 is its remote activation input, Pin 5. !f the voltage on this CMOS/TTL compatible input is held below 0.8 V, the MC3423/3523 operates normally. However, ifitis raised to a voltage above 2.0 V, the OVP output is activated independent of whether or not an overvoltage condition is present. It should be noted that Pin 5 has an internal pull-up current source. This feature can be used to accomplish an orderly and sequenced shutdown of system power supplies during a system fault condition. In addition, the activation indication output of one MC3423/3523 can be used to activate another MC3423/3523 if a single transistor inverter is used to interface the formers indication output to the latters remote activation input, as shown in Figure 7. In this circuit, the indication output (pin 6) of the MC3423 on power supply 1 is used to activate the MC3423 associated with power supply 2. Q1 is any small PNP with adequate voltage rating. MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA 3-114MC3423, MC3523 Figure 7. Circuit Configuration for Activating One MC3523 from Another O + | 1 Power 6 Supply #1 LL 7 Ri 3 10k | 7 O+ 1 Power 5 a Supply fe 1.0k [ 7 e O - Note that both supplies have their negative output leads tied together (i.e., both are positive supplies). If their positive leads are common (two negative supplies) the emitter of Q1 would be moved to the positive lead of supply 1 and R1 would therefore have to be resized to deliver the appropriate drive to Ql. Crowbar SCR Considerations Referring to Figure 11, it can be seen that the crowbar SCR, when activated, is subject to a large current surge from the output capacitance, Coyt. This capacitance consists of the power supply output caps, the loads decoupling caps, and in the case of Figure 11A, the supplys input filter caps. This Surge current is illustrated in Figure 12, and can cause SCR failure or degradation by any one of three mechanisms: di/dt, absolute peak surge, or l@t. The interrelationship of these failure methods and the breadth of the applications make specification of the SCR by the semiconductor manufacturer difficult and expensive. Therefore, the designer must empiri- cally determine the SCR and circuit elements which result in reliable and effective OVP operation. However, an under- standing of the factors which influence the SCRs di/dt and surge capabilities simplifies this task. di/dt As the gate region of the SCR is driven on, its area of conduction takes a finite amount of time to grow, Starting as a very small region and gradually spreading. Since the anode current flows through this turned-on gate region, very high Current densities can occur in the gate region if high anode currents appear quickly (di/dt). This can result in immediate destruction of the SCR or gradual degradation of its forward blocking voltage capabilities depending on the severity of the occasion. Ri, RESISTANCE (k 2) Figure 8. R1 versus Trip Voltage 8 = o 0 5.0 10 15 20 25 30 Vy, TRIP VOLTAGE (V) Figure 9. Minimum Rg versus Supply Voitage 35 Reimin) = 0 = 40 tee tt 8 5 25 S x a 20 a 8 15 10 0 10 20 30 40 50 60 70 80 Rg, GATE CURRENT LIMITING RESISTOR (Q) Figure 10. Capacitance versus Minimum Overvoltage Duration 1 23 571 1.0 c of =a lu 2 = 001 Oo Oo <> 0.001 1 5 2 0.0001 1 0.001 0.01 04 1.0 10 tg, DELAY TIME (ms) MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA 3-115MC3423, MC3523 Figure 11. Typical Crowbar OVP Circuit Configurations i 141A Vin Vout OwU -O DC + OV J | Power |= cou Supply T o DC Power Supply *Needed if supply not current limited Figure 12. Crowbar SCR Surge Current Waveform Surge Due to Output Capacitor Current Limited Supply Output v Figure 13. Circuit Elements Affecting SCR Surge & di/dt To MC3423 Pan _rrm__] Ruead Lead ESR Output Est | Cap + R & LEMPIRICALLY DETERMINED! The usual design compromise then is to use a garden variety fuse (3AG or 3AB style) which cannot be relied on to blow before the thyristor does, and trust that if the SCR does fail, it will fail short circuit. In the majority of the designs, this willbe the case, though this is difficult to guarantee. Of course, a sufficiently high surge will cause an open. These comments also apply to the fuse in Figure 11B. For a complete and detailed treatment of SCR and fuse selection, refer to Motorola Application Note AN-789. The Value of di/dt that an SCR can safely handle is influenced by its construction and the characteristics of the gate drive signal. A center-gate-fire SCR has more di/dt capability than a corner-gate-fire type, and heavily overdriving (3 to 5 times I@T) the SCR gate with a fast < 1.0 us rise time signal will maximize its di/dt capability. A typical maximum number in phase control SCRs of less than 50 A(RMS) rating might be 200 A/us, assuming a gate current of five times IGT and < 1.0 us rise time. If having done this, a di/dt problem is seen to still exist, the designer can also decrease the di/dt of the current waveform by adding inductance in series with the SCR, as shown in Figure 13. Of course, this reduces the circuit's ability to rapidly reduce the DC bus voltage and a tradeoff must be made between speedy voltage reduction and di/dt. Surge Current If the peak current and/or the duration of the surge is excessive, immediate destruction due to device overheating will result. The surge capability of the SCR is directly proportional to its die area. If the surge current cannot be reduced (by adding series resistance see Figure 13) toa safe level which is consistent with the systems requirements for speedy bus voltage reduction, the designer must use a higher current SCR. This may result in the average current capability of the SCR exceeding the steady state current requirements imposed by the DC power supply. A WORD ABOUT FUSING Before leaving the subject of the crowbar SCR, a few words about fuse protection are in order. Referring back to Figure 11A, it will be seen that a fuse is necessary if the power supply to be protected is not output current limited. This fuse is not meant to prevent SCR failure but rather to prevent a fire! In order to protect the SCR, the fuse would have to possess an |2t rating less than that of the SCR and yet have a high enough continuous current rating to survive normal supply output currents. In addition, it must be capable of successfully clearing the high short circuit currents from the supply. Such a fuse as this is quite expensive, and may not even be available. CROWBAR SCR SELECTION GUIDE As an aid in selecting an SCR for crowbar use, the following selection guide is presented. Device Irnms | 'Fsm Package 2N6400 Series 16A 160A T0220 Plastic 2N6504 Series 25A 160A TO220 Plastic 2N1842 Series 16A 125A Metal Stud 2N2573 Series 25A 260A Metal TO-3 Type 2N681 Series 25A 200A Metal Stud MCR3935-1 Series 35A 350A Metal Stud MCR81-5 Series 80A | 1000A Metal Stud MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA 3-116