IR21571 Ver la hoja de datos (PDF) - International Rectifier
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IR21571 Datasheet PDF : 17 Pages
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IR21571(S)
The following graphs, Figures 8 and 9, illustrate the
relationship between the effective RT resistance (i.e.,
the parallel combination of resistors which programs
the CT capacitor charging current) and the operating
frequency.
150
FREQ
(KHz)
100
CT=220pF,RDT=11K
CT=470pF,RDT=6.2K
CT=1nF,RDT=3K
Figure 7: Lamp filament voltage during the
50
preheat, ignition ramp and run modes.
The Preheat Time is programmed by means of the
preheat capacitor, CPH, an internal 1µA current
source, and an internal threshold on the CPH lead of
4.0V, according to the following formula:
tPH = 4E6⋅ CPH, or
CPH = 250E - 9⋅ tPH
At the end of the Preheat Time, the internal, open-
drain transistor holding the RPH lead to ground turns
off, and the voltage on this lead charges exponentially
up to the RT lead potential. During this Ignition Ramp
Mode, the output frequency exponentially decays to
a minimum value. The rate of decay of this frequency
is a function of the RPH * CPH time constant. Because
the Ignition Ramp Mode ends when the voltage on
the CPH lead reaches 5.15V, the Ignition Ramp Mode
is always 1/4th as long as the preheat time.
When the CPH lead reaches 5.15V, an open-drain
transistor on the RUN lead turns on, and the external
RRUN resistor is then in parallel with the RT resistor.
The Run Mode operating frequency is therefore a
function of the parallel combination of RRUN and RT,
and this means that the operating power of the lamp
can be programmed by means of RRUN.
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0
0
5
10
15
20
25
30
35
40
RT (K ohms)
Figure 8: fosc versus effective RT (tDEAD = 2.0 usec)
250
200
150
FREQ
(KHz)
100
CT=220pF, RDT=5.6K
CT=470pF, RDT=2.7K
CT=1nF, RDT=1.2K
50
0
0
5
10
15
20
25
30
35
40
RT (K ohms)
Figure 9: fosc versus effective RT (tDEAD = 1.0 usec)
11
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