www.irf.com

- 23 -

voltage range can be found to comply within the 3 to 5.5 times cold resistance limits. The cold resistance

can be easily measured with a digital multi-meter (DMM).

When the ballast is running, a true RMS digital voltmeter or differential oscilloscope probe can be

connected across the lower cathode and the voltage can be observed at maximum and minimum brightness.

The cathode voltage normally increases as the ballast is dimmed. The values of COUT1 and COUT2 plus

the resonant output inductor heater windings will control how much. Reducing the capacitance will reduce

the amount by which the voltage rises. The COUT1 and COUT2 values should be chosen to prevent the

voltage exceeding the upper limit at minimum output to prevent premature end blackening of the lamps and

reduced life.

Note that utilizing additional windings on the inductor to provide cathode heating means that

power is transferred through the core and consequently the core losses will increase and hence the core

operating temperature. The core will reach its highest operating temperature when the ballast is running at

minimum brightness.

The component values in this design have been selected for a 28W T5 linear lamp. The circuit will

need to be optimized for the particular lamp used to obtain best performance.

Ignition

The IRS2158D incorporates regulated ignition control.

Fig 16. Lamp Voltage and CPH pin voltage - No Ignition

At the end of the preheat period the IRS2158D VCO pin voltage increases and so the frequency

decreases until the voltage at the CS pin reaches the internal threshold of 1.25 V. At this point the

IRS2158D internally pulls down the VCO pin voltage and therefore regulates the circulating current in the

resonant output circuit of the ballast and maintains a constant voltage across the lamp (green trace). This

voltage will remain while the CPH pin voltage (red trace) charges from 1/3VCC to 1/2VCC. If the lamp has

not ignited at this point the IRS2158D will shut down. If preheat is insufficient the ballast will fail to ignite

and shut down at the end of the ignition phase as shown in Fig 16. In this design the ignition voltage has

been set at 1.25 kV peak and 2 kV peak to peak. This ignition voltage is correct for worst case conditions in

a 28W TL5 lamp and can be set by using an RCS value of 1.5R.

A good lamp normally ignites as the output voltage ramps up to the ignition regulation level as

shown in Fig 17.