IR21571 Ver la hoja de datos (PDF) - International Rectifier
Número de pieza
componentes Descripción
Fabricante
IR21571 Datasheet PDF : 17 Pages
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IR21571(S)
The Control Sequence & Timing
Component Selection
The IR21571 uses the following control sequence
(Figure 3) to drive rapid start fluorescent lamps.
fStart
fPH
fRun
fmin
t
5V
VCPH
The heart of this controller is an oscillator which
resembles those found in many popular PWM voltage
regulator ICs. In its simplest form, this oscillator
consists of a timing resistor and capacitor connected
to ground. The voltage across the timing capacitor
CT is a sawtooth, where the rising portion of the ramp
is determined by the current in the RT lead, and the
falling portion of the ramp is determined by an external
deadtime resistor RDT. The oscillograph in Figure 4
illustrates the relationship between the oscillator
capacitor waveform and the gate driver outputs.
2V
VRPH
2V
VRUN
Preheat mode
Ignition Run mode
Ramp
mode
Figure 3: IR21571 control sequence
The control sequence used in the IR21571 allows
the Run Mode operating frequency of the ballast to
be higher than the ignition frequency (i.e., fstart >
fph > frun > fign). This control sequence is
recommended for lamp types where the ignition
frequency is too close to the run frequency to ensure
proper lamp striking for all production resonant LC
component tolerances (please note that it is possible
to use the IR21571 in systems where fstart > fph >
fign > frun, simply by leaving the RUN lead open).
Six leads in the IC are used to control the Startup,
Preheat, Ignition Ramp, and Run modes of
operation, and to allow ballast and lamp engineers
the flexibility to optimize their designs for virtually
any lamp type.
Figure 4
The deadtime can be programmed by means of the
external RDT resistor, given a certain range of CT
capacitor values, using the graph shown in Figure 5.
The RT input is a voltage-controlled current source,
where the voltage is regulated to be approximately
2.0V. In order to maintain proper linearity between
the RT lead current and the CT capacitor charging
current, the value of the RT lead current should be
kept between 50µA and 500µA. The RT lead can
also be used as a feedback point for closed loop
control.
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