Mike Mitchell
EDN
The power-up cycle of the supply voltage in embedded-system applications is sometimes not a clean event. This fact holds especially true in battery-operated systems, because the insertion of a battery often causes significant ringing or glitching on the supply line (Figure 1). In products with on-off switches, the contact bounce of the switch can cause an unclean power-up. A power-up cycle such as the one in Figure 1 can often cause a processor to enter a brownout condition. This condition constitutes an errant condition of the processor, which requires a reset to take place before the processor behaves as expected. The processor is often "lost" or "in the weeds" during a brownout condition. Usually, a reset supervisor controls the reset line to the processor and thus avoids the brownout condition. Traditional supply-voltage-supervisor circuits hold the processor in reset until the supply voltage reaches a predetermined value. They also reset the processor if the voltage dips below the predetermined value. However, the level at which the SVS operates often does not suit the system. For example, the level may be lower than the minimum operating voltage of the processor, or it may be higher than the desired operating voltage of the system. The reset circuit in Figure 2 provides a reset to the processor based on stabilization of the supply voltage and not on a predetermined value.
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| Figure 1. | Insertion of a battery supply often results in glitching and ringing. | |
The circuit uses a TLV3491 from Texas Instruments. The comparator draws approximately 1 µA and operates from 1.8 to 5.5 V, making it well-adapted to battery-operated applications. The input to the minus terminal is a simple resistor divider. The resistor values should be relatively high to reduce the power consumption of the circuit. The input to the plus terminal is basically an RC circuit. The RC time constant provides a tunable power-up delay. When you apply power or insert a battery, the output of the comparator is low, holding the processor in the reset condition.
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| Figure 2. | This circuit resets a processor based on the stabilization time of the supply voltage. | |
The plus input of the comparator becomes higher than the minus input only after the supply voltage stabilizes, resulting in a high output state and thus releasing the processor for operation. The stabilization time for the supply voltage depends on the RC-network component values. Here, the use of low-value resistors carries no penalty, because no current flows through the RC network after supply stabilization. By selecting R1, C1, R2, and R3, you can guarantee a reliable reset signal to the processor for a given dV/dt for VCC. The equations for the voltages at the comparator's inputs are:
To hold the processor in reset, you need the condition V–>V+. That condition yields:
Solving for t, you obtain
From the last equation, you can calculate the amount of time the processor stays in reset. Therefore, as long as the supply ramps to a steady state in a shorter time, you're guaranteed a reliable reset. The reverse-biased diode and resistor R4 provide a faster discharge path for the capacitor. This fast discharge allows the circuit to quickly react to negative glitches in the supply voltage during normal operation, in which it may be desirable to reset the processor. R4 allows you to tune the response time of the circuit for any expected supply-voltage glitches. Removal of the resistor yields the fastest response time to supply-voltage glitches but may result in undesired resets for the processor. The pullup resistor at the output of the comparator is necessary because of the comparator's open-drain output. The capacitor at the comparator's output smoothes any fast switching the comparator may encounter.
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| Figure 3. | The circuit in Figure 2 enables the processor well after the stabilization of the power-supply voltage. |
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The current consumption of the circuit in Figure 2 is approximately 1 µA (the current consumption of the comparator) plus the current through R2 and R3. The circuit costs less than many dedicated supply-voltage supervisors. Figure 3 illustrates the performance of the circuit. Figure 3 is a scope capture of the same battery insertion of Figure 1. The top trace is the supply voltage; the next trace is the positive input to the comparator. The negative input to the comparator is the next trace, and the bottom trace is the comparator's output (connected to the microcontroller's reset pin). You can clearly see that the circuit holds the processor in reset until the supply stabilizes. Thus, the perfomance depends not on any predefined supply-voltage level, but rather on stabilization time.
