Although special-purpose frequency-to-voltage chips help you to create an F-to-V circuit block easily, they suffer from a major drawback. For given external components, their gainconstant, and hence voltage output, can vary more than ±5% from piece to piece (e.g, the LM331 and the LM2917). Their currentsource value also varies piece to piece by a similar amount, thus causing piece-to-piece variations in the maximum voltage-output beyond which F-to-V goes into saturation.
This creates problems for mass-produced solutions requiring F-to-V conversion if the application can’t tolerate these variations. The result is messy trimming procedures for external passive timing components for each mass-produced piece.
This is where old workhorse LM555 outshines these special-purpose ICs. The 555 generates a mono-shot time-period equation:
TD (mono-shot delay) = 1.1 × RC
Timing accuracy is typically 1%. (For National Semiconductor’s LM555 and LM55C, it’s 0.5% and 1%, respectively.) Thus, designers can use timing components with precision tolerance to meet an application’s requirements, without unduly worrying about pieceto- piece chip variations.
A mono-shot-based F-to-V works by converting incoming pulses to a fixed-pulse-width, variable-frequency pulse-width-modulation (PWM) signal. The duration of the pulse is fixed at the period of the mono-shot (1.1RC in this case). This duration also forms the on period or duty-cycle of the PWM output.
Lower-frequency signals yield lower-percentage duty cycles, since the period forms a lesser percentage of the total period of the incoming pulse train. As the frequency rises, the output’s duty cycle increases, since the “on-period” is now a greater percentage of the incoming pulse-train period (Fig. 1).
This dc value of the output PWM signal (given by percentage duty cycle) is directly proportional to the incoming pulse-train frequency. However, the situation gets a bit tricky when the incoming pulse train has a period shorter than the on-period. The F-to-V is now supposed to saturate to its maximum value for all of the frequencies with time periods less than the mono-shot on-period.
However, if the mono-shot is not re-triggerable, it will expire “asynchronously” with the incoming pulse triggers, causing random output variations. As a result, the example circuit uses a transistorized discharge-switch to allow the 555 to re-trigger, which it otherwise could not do (Fig. 2).
Transistor Q1 forms a discharge path around capacitor C2. Resistor R4 and capacitor C2 are the timing components of the mono-shot with a time period of about 0.39 ms. Hence for a frequency of about 1 kHz, the output duty cycle is 39%, giving a voltage of about 4.68 V for a VBATT of 12 V. C1, R1, D1, R2, and R3 form the base trigger circuit for Q1. The 555’s internal transistor at pin-7 is not unused in this circuit. Rather than leaving the pin un-terminated, R5 and C3 terminate it as recommended by some CMOS 555 chip vendors. It can also be used as an auxiliary source of a sawtooth wave if required by the application. R6 and C5 form an output low-pass filter to extract dc voltage from the PWM pulse-train appearing at the IC’s pin 3.
The circuit’s output voltage increases linearly at the rate 4.68 V/kHz up to 2.5 kHz (Fig. 3). The output then saturates to its maximum value as desired (as a consequence of the re-triggerable feature of this mono-shot circuit).