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1.

The signal ground (pin 12) should only be connected to the power ground at one single point to

prevent ground loops from forming.

2.

The point described in (1) should be where the grounds of the current sense resistors for both the

half bridge MOSFETs and the lamp current feedback both meet.

3.

The VCC decoupling capacitor should be placed as close to the IRS2158D VCC (pin 13) and

COM (pin 12) as possible with the shortest possible traces.

4.

The devices; CPH, RMIN, CVCO, CT and CCS should all be located as close to the IRS2158D as

possible with traces to the relevant pins being as short as possible.

5.

The ground connections from the devices listed in (4) should be connected back to the COM pin

of the IRS2158D through the shortest possible traces. These should be connected back to the

COM pin of the IC without joining the power ground trace at any point. In the example shown

above the power ground trace runs along the lower side of the board on the bottom Copper layer

6.

The charge pump diode connection to ground should be made to the power ground not the signal

ground.

7.

The power factor correction section (if used) should be kept apart from the ballast control as

shown in the example above. the power factor correction section is at the left side of the PCB.

Output Inductor Design

The output inductor LRES should be designed to allow a sufficient peak ignition current without

saturating. This is important as the ballast will be unable to ignite if unable to deliver sufficient voltage at

the lamp. The ignition current depends on the type of lamp being used and must be kept to a minimum by

ensuring that the cathodes are sufficiently preheated. To minimize eddy current losses in the inductor

windings multi-stranded wire should be used. Ferrite cores of sufficiently good quality should also be used

to allow a high peak flux density at increased temperature and low core losses. It is important to have a

large enough air gap to produce the highest available peak current before allowing the inductor to saturate.

If the air gap is too large however, losses can occur because the magnetic field emanating from the gap

extends far enough to induce eddy currents in the windings.

When the cores are hot the saturation point and hence the peak current for the inductor will be

lower therefore a poorly designed inductor may result in the ballast failing to ignite the lamp during an

attempted hot re-strike.

The inductor design process can be greatly simplified by using the Ballast Designer software. For

this application it is recommended to fix the core size to EF25 and gap size at 1mm.

Lamp Preheating

It is essential that the lamp be sufficiently preheated before ignition. The correct preheat current

can be determined from published data or from International Rectifier’s Ballast Designer software.

It is necessary in order to achieve the maximum possible lamp life to heat the cathodes to the

correct temperature before ignition. Accelerated deterioration occurs if lamps are ignited when the cathodes

are either not sufficiently heated or over heated. The cathode temperature at ignition is determined by the

preheat current, which in turn is determined by the preheat frequency through RPH and the preheat time set

by CCPH.

Cathode heating after ignition (Run mode)

The lamp filament (Cathode) resistance over the range of dimming levels should be between 3 and

5.5 times the resistance when cold. A simple method for determining the hot resistance is to first connect

one cathode to a DC power supply via an ammeter and slowly increase the voltage from zero, noting the

current at 1 V intervals. This should be done until the cathode can be seen to be glowing red. When this

occurs the voltage should not be increased further or the cathode is likely to overheat and become open

circuit. With these results, the resistance can be calculated for each voltage and hence the acceptable