Reference Design Data Sheet intended for design information only.
Subjected to changes without prior notice.
2
IRPLCFL1
Functional Description
The circuit is centered around the IR51H420 Ballast Driver Hybrid which contains the
IR2151 Ballast Driver IC and two 500 volt size 2 HEXFET’s in a half bridge configuration. With
a 120 volt AC line input (AC1-N), the voltage is rectified and doubled to provide a bus voltage
of approximately 300 volts. With a 220 volt AC line input (AC1-AC2), the voltage is rectified
but 3not doubled and again provides a bus voltage of approximately 300 volts.The start up
resistor R2 is sized such that it can supply enough current to start the oscillator in the
IR51H420 but not enough to cause the shunt clamp to regulate and maintain constant
oscillation. With this constraint the power dissipation in resistor R2 is low enough so that a 1/4
watt unit will suffice. A charge pump circuit, consisting of capacitor C10 and diodes D5 and
D6, is used so that when the IR51H420 begins to oscillate, the charge pump circuit supplies
the current to increase the voltage on Vcc to cause the shunt clamp to regulate. If the lamp is
removed from the circuit there is no longer a path for the charge pump capacitor C10. This
causes the voltage at Vcc of the IR51H420 to begin falling. When the voltage at Vcc of the
IR51H420 falls below the negative undervoltage lockout threshold the oscillator stops
switching. At this point the voltage will begin to rise again and when the voltage reaches the
positive undervoltage lockout threshold the IC again begins to oscillate. If there is no lamp
installed in the circuit there will be no path for the charge pump circuit to supply current and
the voltage at Vcc will again fall below the negative undervoltage lockout threshold. The
circuit will continue this sequence indefinitely until the power is removed or a lamp is
reinserted into the circuit. If a lamp is reinserted into the circuit, the lamp will light.
To provide long life and to insure soft-starting of the lamp, the cathodes must be pre-
heated so that their hot resistance is approximately three to four times that of the cold
resistance value. This is performed by using a three step start-up sequence; the three steps
being three oscillator frequency settings. The oscillator is started at a frequency well above
the resonant point of the LC circuit formed by inductor L1 and capacitor C9. This is done to
insure that the initial voltage applied across the lamp is below the strike potential. The second
frequency step, below step 1, was chosen to provide a current through the cathodes large
enough to heat them in the pre-heat time while also maintaining the voltage across the lamp
below the strike potential. The third step is to move the oscillator to the final running
frequency. At this point the voltage across the lamp becomes large enough to strike the arc
and the resonant point of the circuit shifts lower and the current in the lamp is limited by the
inductor L1.
