There are many different types of voltage controlled oscillator (VCO) circuits that can use an IC timer. VCO circuits are valuable for instrumentation, electronic music applications, and function generators. The output is a rectangular pulse stream whose frequency is related to the external control voltage. A VCO circuit using the TLC555 is shown in Figure 7.30.
At startup, the voltage at the trigger input (pin 2) is less than the trigger level voltage (1/3 VDD), causing the timer to be triggered via pin 2. The output of the timer (pin 3) goes high, allowing capacitor Ct, to charge very rapidly through diode D1 and resistor R1. The charge time of C, is extremely short and may essentially be neglected. When capacitor C, charges to the upper threshold voltage (2/3 VDD), the flip-flop is reset, the output (pin 3)
goes low, and capacitor Ct discharges through the current mirror, TL0ll. When the voltage at pin 2 reaches 1/3 VDD, the lower threshold or trigger level, the timer triggers again and the cycle is repeated.
The input voltage (VI) determines the constant current output of the current mirror, which is used as a voltage to current converter and sets the discharge rate of capacitor Ct. The discharge time of Ct determines the frequency of the oscillator. As the input voltage is varied from 10 V to 50 V. the output frequency varies at a linear rate.
As an example, assume an application calls for an output midrange frequency of 500 Hz. Since T = l/f, the time between output pulses will be 2 ms. The charge time, which will be less than 1µs, may be neglected. The discharge current of Ct for a specific input control voltage is:
With an input voltage of 10 V to 50 V, the TL011 current will vary linearly from 20µA to 100µA. Figure 7-31 shows the voltage to frequency conversion obtained with two different values for capacitor Ct. With Ct = 0.001 µF, a frequency range of 3.3 kHz to 10 kHz is obtained. When a value of 0.02 µF is used, a frequency range of 187 Hz to 1 kHz is obtained.
Since the capacitor, Ct, discharges from 10 V to 5 V (2/3 VDD to 113 VDD), the capacitor value may be calculated for 500 Hz as follows:
Note that the current mirror is sinking current during both the charge and discharge of Ct. However, the small discharge current is easily overcome during the charge cycle by the lower impedance, high-current charge path from the output pin. For linear ramp applications, the output is obtained across Ct.