By Grant Smith, National Semiconductor Corporation

Small microprocessor voltage monitors have been around for a long time, but they haven't always had the same features and parameters that they do now. This tends to make these devices even more versatile when it comes to using them in other ways. With many sources, a typical 1.0 μA quiescent current, on-board reference, built in hysteresis, and the ability to sink milliamps of current, they can often do the job of a comparator with far fewer components and less cost and current draw. In some cases the built-in hysteresis may not be adequate, but an extra component or two will likly solve this and the circuit is still leaner than most alternatives.

The examples below demonstrate two of the most common alternative uses for these parts. Both examples use the device as a comparator. In this case, one is an overvoltage lockout and the other is an undervoltage lockout. The overvoltage lockout prevents the output voltage from running away if the load is somehow removed, such as with an LED driver when an LED goes open circuit. The undervoltage lockout prevents the PWM controller from attempting to operate at too low an input voltage and restarts the controller should the input momentarily dip.

The circuit in Figure 1 demonstrates the use of a 3.0V voltage monitor as an overvoltage lockout capable of restarting U1, the PWM controller. The monitored circuit is an off-line negative buck converter such as might be used to drive high current LEDs. The soft-start capacitor, C3, is a required part for soft-start. The overvoltage monitor components consist of U2 voltage monitor, and three resistors, R1, R2 and R3. Only two actual resistances are required, but the voltage rating of surface mount resistors is not usually above 200VDC, so two in series are required. The overvoltage level is set by configuring a 325VDC undervoltage lockout between Vout- and ground. This amounts to an overvoltage level of 75VDC across the output, Vout+ to Vout-, since Vout+ to ground is a regulated 400VDC. The voltage rating of C2 would normally be 100VDC in this application, but it would need to be at least 400VDC without overvoltage protection. From both, cost and board area perspectives, the overvoltage lockout is worthwhile.

  

Figure 1. 3.0V Voltage monitor as an overvoltage lockout capable of restarting the PWM controller

There is one important precaution to be taken. The current which is level shifted to provide feedback across R4 must be compensated for by some kind of loading of the output voltage. Otherwise it could pull down on Vout- and create an overvoltage on C2 even without U1 working. In most case the feedback level shifter itself will provide this, but it is something to take note of.

The circuit in Figure 2 demonstrates the more common undervoltage lockout, again using the same 3.0V voltage monitor. It also uses the soft-start pin as an enable so that if a fault condition occurs, a complete restart is initiated. In this case, there are only two resistors needed, R1 and R2. Two additional components, D1 and C2, are required because the hysteresis of U2 may not be enough. During startup the input voltage might dip because of the increased load. At low input voltages the circuit would continually restart as this load is removed and re-applied over and over. A more typical hysteresis resistor would be used except that the soft-start pin of U1 may not supply enough current.

Figure 2. Typical undervoltage lockout, using the same 3.0V voltage monitor

Conclusion:
Voltage monitors are probably the most effective way to get a comparator with hysteresis and a voltage reference in one small inexpensive package requiring the fewest possible additional components. They are easy to use and quite versatile.

About the author
Grant Smith is an applications designer for National Semiconductor's Design Center (Phoenix), where he has worked for four years. The center's products include next-generation PWM power controllers, gate drivers, and hot-swap controllers. Before joining National Semiconductor, Grant worked as a power and analog signal processing designer for over 25 years. In his spare time, he enjoys bicycling, swimming, the Web, people, and his dog Duke.