IR21593 datasheet(18/25 Pages) IRF

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IR21592/IR21593(S)

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circuit measures the voltage at the CS pin and

compares it against the voltage at the IPH pin.

During the rising ignition ramp, the voltage at

the IPH pin is increased to 20% above its value

5.1V

CPH

VCO

IGN

DIM

DIM & C TPH

TIME CONSTANT

IGN-TO-DIM

TIME

Figure 10, IR21592/IR21593 ignition detection.

during preheat mode. When the voltage on the

CS pin exceeds this voltage, the voltage on the

IPH pin is decreased to VIPH Pre-Heat +10% and

the ignition detection circuit is then active (See

Figure 10). When the lamp ignites, the voltage on

the CS pin will then fall below the voltage on the

IPH pin and the IC enters DIM Mode and the phase

control loop is closed. In order for the ignition

detection circuit to function properly and for the IC

to enter DIM mode, the voltage on the CS pin must

first rise above VIPH Pre-Heat + 20% during the

ignition ramp to activate the circuit, and then

decrease below VIPH Pre-Heat +10% when the lamp

ignites.

Ignition-to-Dim (IGN-to-DIM)

When the IR21592/IR21593 enters dim mode, the

phase control loop is closed and the phase of the

load current is regulated against the user control

input on pin DIM. To control the rate at which the

dim setting changes from maximum brightness

to the user setting (IGN-TO-DIM time, Figure 11),

pin DIM is connected internally to pin CPH when

the IR21592/IR21593 enters DIM mode. The

resistor on pin DIM (RDIM) discharges the

capacitor on pin CPH down to the user dim

setting. The resistor can be selected for a fast

time constant to minimize the amount of flash

visible over the lamp just after ignition, or, a

long time constant such that the brightness

ramps down smoothly to the user setting. Should

the ignition-to-dim time be too fast, however, the

loop can respond faster than the ionization

constant of the lamp (milliseconds) causing the

VCO to over-shoot.

This can result in a

frequency that is higher than the minimum

brightness frequency and can extinguish the

lamp. The capacitor on pin CPH serves multiple

functions by setting the preheat time, the travel

rate just after ignition (together with resistor

RDIM), and, serving as a filter capacitor on pin

Figure 11, IR21592/IR21593 ignition timing diagram.

IPH

IPH+ 20%

IPH + 10%

IGN

DIM

IR21593 Datasheet(PDF) 18 Page - International Rectifier

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