TDA 4714 C

TDA 4716 C

Semiconductor Group

4

Circuit Description

The following is a description of the individual functional units and their interaction.

Voltage Controlled Oscillator (VCO)

The VCO generates a sawtooth voltage. The duration of the falling edge is determined

by the value of

C

approximately the frequency, is determined by the value of

R

VCO provides a trigger signal for the ramp generator, as well as an L signal for a number

of IC parts to be controlled.

Ramp Generator

The ramp generator is triggered by the VCO and oscillates at the same frequency. The

duration of the falling edge of the ramp generator waveform is to be shorter than the fall

time of the VCO. To control the pulse width at the output, the voltage of the rising edge

of the ramp generator signal is compared with a DC voltage at comparator K2. The slope

of the rising edge of the ramp generator signal is controlled by the current through

R

This offers the possibility of an additional, superimposed control of the output duty cycle.

This additional control capability, called “feed-forward control”, is utilized to compensate

for known interference such as ripple on the input voltage.

Push-Pull Flipflop

The push-pull flipflop is switched by the falling edge of the VCO. This ensures that only

one output of the two push-pull outputs is enabled at a time.

Comparator K2

The two plus inputs of the comparator are switched such that the lower plus level is

always compared with the level of the minus input. As soon as the voltage of the rising

sawtooth edge exceeds the lower of the two plus levels, both outputs are disabled via

the pulse turn-OFF flipflop. The period during which the respective, active output is low

can be infinitely varied. As the frequency remains constant, this process corresponds to

a change in duty cycle.

Operational Amplifier K1 (TDA 4716 C)

The op amp K1 is a high-quality amplifier. Fluctuations in the output voltage of the power

supply are amplified by K1 and applied to the free positive input of comparator K2.

Variations in output voltage are, in this way, converted to a corresponding change in

output duty cycle. K1 has a common-mode input voltage range between 0 V and + 5 V.