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MC3423P1G Datasheet(PDF) 4 Page - ON Semiconductor |
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MC3423P1G Datasheet(HTML) 4 Page - ON Semiconductor |
4 / 10 page MC3423 http://onsemi.com 4 APPLICATION INFORMATION Basic Circuit Configuration The basic circuit configuration of the MC3423 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 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 RG, the SCR, Q1, will receive the greatest gate current possible without damaging the MC3423. If lower output currents are required, RG can be increased in value. The switch, S1, 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 propagating delay of 1.0 ms. If faster operation is desired, Pin 3 may be connected to Pin 2 with Pin 4 left floating. This will result in decreasing the propagating delay to approximately 0.5 ms at the expense of a slightly increased TC for the trip voltage value. Configuration for Programmable Minimum Duration of Overvoltage Condition Before Tripping In many instances, the MC3423 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 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 value 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 VZ1 + 1.4 V. Power Supply R1 R2 MC3423 RG (+ Sense Lead) (− Sense Lead) − + 1k Z1 7 3 4 5 2 1 8 C Figure 7. Configuration for Programmable Duration of Overvoltage Condition Before Trip/With Immediate Trip at High Overvoltages Additional Features 1. Activation Indication Output An additional output for use as an indicator of OVP activation is provided by the MC3423. 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 is its remote activation input, Pin 5. If the voltage on this CMOS/TTL compatible input is held below 0.8 V, the MC3423 operates normally. However, if it is 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 pullup 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 can be used to activate another MC3423 if a single transistor inverter is used to interface the former’s indication output to the latter ’s 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. |
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