IR2161 datasheet(9/20 Pages) IRF

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IR2161(S)&(PbF)

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CSD

Set Oscillator

Range

ISS

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.

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.

Figure 3, Typical Cold Lamp Inrush Current.

Figure 4, Cold Lamp Inrush Current with Soft Start.

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.

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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