MC3423

4

MOTOROLA ANALOG IC DEVICE DATA

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 propogation delay of 1.0

µs. If faster operation

is desired, Pin 3 may be connected to Pin 2 with Pin 4 left

floating. This will result in decreasing the propogation delay to

approximately 0.5

µs 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

3

C

Figure 6. 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 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 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.