TDA 4700

TDA 4718

Semiconductor Group

7

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

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

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

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

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

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.