L6574

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Oscillator Section:

A voltage controlled oscillator, with the selected frequencies fpre and fing, drives the output half bridge.

Independently selected, fpre is effective during tpre and fing is effective during normal lamp burning. When

working open loop, fpre and fing are the highest and lowest allowed oscillation frequencies.

Closed loop control of the lamp current under normal operation can be achieved with the L6574. This is

accomplished by automatic adjustment of the oscillator frequency. The OP AMP output is fed through a

resistor diode network to the Ring pin. See AN 993.

OP AMP Section:

The integrated OP AMP offers low output impedance, wide bandwidth, high input impedance and wide

common mode range. It can be readily used to implement closed loop control (see Oscillator Section) of

the lamp current.

EN1, EN2 Comparators:

Two CMOS comparators, with thresholds set at 0.6 V (typical) are available to implement protection meth-

ods (such as overvoltage, lamp removal, etc.). Short pulses (>200nsec) at the comparator inputs are rec-

ognized.

The EN1 input (active high) forces the L6574 in the shut down state (e.g. LVG low, HVG low, oscillator

stopped) in the event of an undervoltage condition. Normal operating condition is resumed after a power-

off power-on sequence or when EN2 input is high.

The EN2 input (active high) also restarts a preheat sequence (see timing diagrams).

TIMING DIAGRAMS

LVG

HVG

EN1

D97IN490

EN2

D97IN491B