Semiconductor Group

4

Circuit Description

This device has a conditioning circuit for the internal power supply. It allows standby operation with

very low current consumption (less than 0.5 mA), a hysteresis between enable and switch-off levels

and an internal voltage stabilization. An integrated Z-diode limits the voltage on V

current is fed.

The output driver (Q SIP) is controlled by detector input and current comparator.

The detector input (I DET) which is highly resistive in the operating state reacts on hysteresis-

determined voltage levels. To keep down the amount of circuitry required, clamping diodes are

provided which allow control by a current source.

The operating state of the boost converter choke is sensed via the detector input. H-level means

that the choke discharges and the output driver is inhibited. H-level sets a flip-flop, which stores the

switch-off instruction of the current comparator to reduce susceptibility to interference. As soon as

demagnetization is finished the choke voltage reverses and the detector input is set to L-level, thus

enabling the output driver. This ensures that the choke is always currentless when the SIPMOS

transistor switches on and that no current gaps appear.

The nominal voltage of the multiplier output is compared to the voltage derived from the actual line

current (– I COMP), thus setting the switch-off threshold of the comparator. The current comparator

blocks the output driver when the nominal peak value of the choke current given by the multiplier

output is reached.

This state is maintained in the flip-flop until H-level appears at detector input which takes over the

hold function and resets the flip-flop.

Operating states might occur without any useful detector signal. This is the case with magnetic

saturation of the choke and when the input voltage approaches or exceeds the output voltage as,

for example, during switch-on. The driver remains inhibited for the flip-flop due to the absent set

signal.

The trigger signal can be derived from the subsequent lamp generator or a SMPS control device.

The trigger signal level should be so low that with standard operation the signal from the detector

winding dominates. The multiplier delivers the preset nominal value for the current comparator by

multiplying the input voltage (IM1), which determines the nominal waveform and the output voltage

of the control amplifier.

The control amplifier stabilizes the output dc voltage of the active harmonic filter in the event of load

and input voltage changes. The control amplifier compares the actual output voltage to a

reference voltage which is provided in the IC and stable with temperature.

Output Driver

The output driver is intended to drive a SIPMOS transistor directly.

It is designed as a push-pull stage.

Both the capacitive input impedance and keeping the gate level at zero potential in standby

operation by an internal 10-k

Ω-resistor are taken into account. Possible effects on the output driver

by line inductances or capacitive couplings via SIPMOS transistor Miller capacitance are limited by

diodes connected to ground and supply voltage.

TDA 4817