IR2157 データシートの表示（PDF） - International Rectifier
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IR2157 Datasheet PDF : 20 Pages
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IR2157
ADVANCED INFORMATION
the hot filament-to-cold filament resistance is desired
for maximum lamp life, as shown in Figure 7
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
50
Figure 7: Lamp filament voltage during the
preheat, ignition ramp and run modes.
The Preheat Time is programmed by means of the
preheat capacitor, CPH, an internal 1mA current
source, and an internal threshold on the CPH pin of
4.0V, according to the following formula:
0
0
5
10
15
20
25
30
35
40
RT (K ohms)
Figure 8: fosc versus effective RT (tDEAD = 2.0 usec)
tPH = 4E6⋅ CPH, or
250
CPH = 250E- 9⋅ tPH
At the end of the Preheat Time, the internal, open-
drain transistor holding the RPH pin to ground turns
off, and the voltage on this pin charges exponentially
up to the RT pin 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 pin reaches 5.15V, the ignition ramp is
always 1/4th as long as the preheat time.
When the CPH pin reaches 5.15V, an open-drain
transistor on the RUN pin 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 .
12
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)
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