IR2161_15 Datasheet, PDF (9/20 Pages) International Rectifier – HALOGEN CONVERTOR CONTROL IC

IR2161(S) & (PbF)
minimum frequency over a period of around 1s (assuming
CSD=100nF). During this time the external capacitor at the
CSD pin charges from 0V to 5V, controlling the oscillator
frequency through the internal voltage controlled oscillator
(VCO). The value of CSD will determine the duration of the
soft start sweep. However, since it also governs the shut
down circuit delays, the value should be kept at 100nF to
achieve the datasheet operation.
ISS
CSD
Set Oscillator
Range
Figure 4, Cold Lamp Inrush Current with Soft Start.
5V
Figure 2, Halogen Convertor.
It can be seen from Figure 2, that at switch on, the CSD
capacitor is internally switched to the soft start circuit input.
A current source charges CSD linearly to 5V over a period
of 0.5s at which time the comparator output goes high. The
PMOS switch opens and the ISS current source is
disconnected from CSD. The comparator latches high at this
point and this causes the oscillator range to change and the
CSD capacitor to be disconnected from the soft start circuit
and connected to the voltage compensation circuit. The
latching comparator has a built in delay of at least 20uS in
order to prevent false triggering caused by transients.
Run Mode (Voltage Compensation)
When soft start is completed the system switches over to
compensation mode. This function provides some regula-
tion of the output voltage of the convertor from minimum to
maximum load. In this type of system it is desirable that the
voltage supplied to the lamp does not exceed a particular
limit. If the lamp voltage becomes too high the temperature
of the filament runs too high and the life of the lamp is
significantly reduced. The problem is that the output trans-
former is never perfectly coupled so there will always be a
degree of load regulation.
The transformer has to be designed such that the lamp
voltage at maximum load is sufficiently high to ensure
adequate light output.
At minimum load the voltage will consequently be higher
and is likely to exceed the maximum desired lamp voltage.
In the widely used self-oscillating system based around
bipolar power transistors, there is some frequency change
(increasing the frequency reduces the output voltage)
depending on the load that helps to compensate for this,
although this is non-linear and depends on many parameters
in the circuit and so is not easy to predict.
Figure 3, Typical Cold Lamp Inrush Current.
The IR2161 based system includes a function that monitors
the load current through the current sense resistor (RCS).
The peak current is detected and amplified within the IC
then appears at the CSD pin during run mode. The voltage
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