TDA4700 Datasheet, PDF (6/21 Pages) Siemens Semiconductor Group – Control IC for Single-Ended and Push-Pull Switched-Mode Power Supplies SMPS

TDA 4700
TDA 4718
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_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.
Phase Comparator
If the component is operated without external synchronization, the sync input must be
connected to the sync output for the phase comparator to set the rated voltage at C_filter.
The VCO then oscillates with rated frequency. In the case of external synchronization,
other components can be synchronized with the sync output. The component can be
frequency-synchronized, but not phase-synchronized, with the sync input. The duty
cycle of the squarewave voltage at the sync input is arbitrary. The best stability as to
small phase and frequency interference deviation is achieved with a duty cycle as
offered by the sync output.
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 outputs is low
can be infinitely varied. As the frequency remains constant, this process corresponds to
a change in duty cycle.
Operational Amplifier K1 (TDA 4700; A)
The K1 op amp is a high-quality amplifier. Fluctuations in the output voltage of the power
supply are amplified by K1 and applied to the free + 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.
Semiconductor Group
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