Nuts and Volts - October 2015

piezoelectric flapper manufactured by Measurement Specialties, Inc. The LDT0-028K is labeled as a vibration sensor or as a piezoelectric switch. It consists of Polyvinylidene Fluoride (PVDF) — a piezoelectric plastic with metallic electrodes. If its plastic tab is bent over 90 degrees, the LTD0-028K will generate up to 7 nC of charge, or 90 volts. In Figure 3, we use a plastic piezoelectric capacitor to switch a ceramic ferroelectric capacitor in an event detector circuit.

There are issues using the LDT-028K bender as shown in Figure 3. It is a capacitor, so when sharing charge with the Type AD ferroelectric capacitor, it will generate the voltage set by the ferroelectric capacitor for the amount of charge being deposited. If the ferroelectric capacitor starts DOWN, a bend of the plastic piezoelectric switch generates 7 nC to force the capacitor UP. According to

Figure 2, the voltage will only reach about six volts.

However, if the ferroelectric capacitor is already UP and does not switch, the switch could generate a far higher voltage across the ferroelectric capacitor and damage it. Even more important, if the plastic piezoelectric switch is bent the wrong way, it will generate negative voltages!

Another issue with the simple circuit in Figure 3 is the status of P0 and P1 when the microprocessor is powered down. All pins on a microprocessor package will have clamping diodes to the power rails (Vcc and ground) to dissipate static discharge inadvertently applied to a pin.

When the LDT0-028K attempts to generate a voltage on P0 to switch the ferroelectric capacitor UP while the microprocessor is powered off, the clamping diodes on P0 will limit the voltage range to ±0.7 volts.

A blocking transistor must be placed on P0 to isolate the external components from the clamping diodes of P0 inside the package when the microprocessor is turned off. On the other hand, we can use this property to our advantage on P1. When the piezoelectric switch attempts to change the state of the ferroelectric capacitor and the microprocessor is off, the ferroelectric charge will flow into CSense until the voltage on CSense exceeds the clamping voltage on P1; whereupon, the excess current will flow to ground. We will not have to worry about back voltage from CSense fighting the voltage across the ferroelectric capacitor applied by the switch. To solve these issues, we add three components to the event detector circuit in Figure 3 to create a realistic real world circuit in Figure 4.

T0 isolates the common node A from P0 and is only enabled by the microprocessor when it wants to write or read CFE. When power is off to the microprocessor, T0 will be off to allow the piezoelectric switch (LDT0-028K) to generate a voltage across the ferroelectric capacitor and not be clamped to ground by P0. When power is on, event detection will still function.

T0 is turned off during detection. P0 and P1 are left at zero volts. Sensing pulses from the LDT0-028K are blocked from P0 by T0, but current from CFE can flow into P1. D1 — at all times — blocks negative voltages from the sensor, and the TVS limits the voltage across the ferroelectric capacitor to no more than + 6 volts.