TDA4700 Datasheet, PDF (7/21 Pages) Siemens Semiconductor Group – Control IC for Single-Ended and Push-Pull Switched-Mode Power Supplies SMPS

TDA 4700
TDA 4718
Pulse-Turn-OFF Flipflop
The pulse turn-OFF flipflop enables the outputs at the start of each half cycle. If an error
signal from comparator K7 or a turn-off signal from K2 is present, the outputs will
immediately be switched off.
Comparator K3
Comparator K3 limits the voltage at capacitance C_{soft start} (and also at K2) to a maximum
of + 5 V. The voltage at the ramp generator output may, however rise to 5.5 V. With a
corresponding slope of the rising ramp generator edge, the duty cycle can be limited to
a desired maximum value.
Comparator K4
The comparator has its switching threshold at 1.5 V and sets the error flipflop with its
output if the voltage at capacitance C_{soft start} is below 1.5 V. However, the error flipflop
accepts the set signal only if no reset pulse (error) is applied. In this way the outputs
cannot be turned on again as long as an error signal is present.
Soft Start
The lower one of the two voltages at the plus inputs of K2 is a measure for the duty cycle
at the output. At the instant of turning on the component, the voltage at capacitor C_{soft start}
equals 0 V. As long as no error is present, this capacitor is charged with a current of 6 µA
to the maximum value of 5 V. In case of an error, C_{soft start} is discharged with a current of
2 µA. A set signal is pending at the error flipflop below a charge of 1.5 V and the outputs
are enabled if no reset signal is pending simultaneously. As the minimum ramp
generator voltage, however, is 1.8 V, the duty cycle at the outputs is actually increased
slowly and continuously not before the voltage at C_{soft start} exceeds 1.8 V.
Error Flipflop
Error signals, which are led to input R of the error flipflop cause an immediate disabling
of the outputs, and after the error has been eliminated, cause the component to switch
on again by the soft start.
Comparator K5, K6, K8, V_REF Overcurrent Load
These are error detectors which cause immediate disabling of the outputs via the error
flipflop when an error occurs. After elimination of the error, the component switches on
again using the soft start. The output of K5 can be fed back to the input. This causes the
IC output stage to remain disabled even after elimination of the overvoltage. However, it
requires high-ohmic overvoltage coupling.
Semiconductor Group
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