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is exceeded and the lamp ignites. Figure 9 shows the ramping of voltage appearing across the lamp

and also the voltage on capacitor CRAMP. Note that the sudden drop in lamp voltage indicates that

the lamp has ignited. Also note that the voltage on capacitor C12 is still increasing at the point when

the lamp has already ignited meaning the frequency is still ramping down to the final minimum

ignition frequency. This minimum frequency corresponds to the absolute maximum ignition voltage

required by the lamp under all conditions.

During the Ignition Ramp mode the voltage on the CPH pin of IC2 continues to ramp up until the

voltage at the CPH pin of IC2 exceeds the Run mode threshold (5.1V). Over-current sensing is also

enabled at the beginning of the Ignition Ramp mode. A full explanation of the functionality of the

over-current sensing is in the section on Fault Mode.

Run Mode

At the end of the Ignition Ramp mode (CPH pin

> 5.1V) the ballast switches to the Run mode at

which point the frequency is shifted to the run

frequency. The run frequency is determined by

the parallel combination of resistors RT and

RRUN and capacitor CT. Resistor RRUN is

connected in parallel by turning on the internal

open drain MOSFET connected to the RUN pin

of IC2 (see Figure 1, IR21571 block diagram).

The sensing of under-current conditions is also

enabled at the beginning of the Run mode. The

full explanation of the functionality of the under-

current sensing is in the section on Fault Mode.

Figure 10 shows the functionality of the CPH,

RPH and RUN pins of IC2 during Startup, Pre-

heat, Ignition Ramp and Run modes.

The Run mode frequency is that at which the lamp is driven to the lamp manufacturer’s recom-

mended lamp power rating. The running frequency of the lamp resonant output stage for selected

component values is defined as,

Figure 10: Top trace: CPH pin IC2

Middle trace: RPH pine IC2

Bottom trace: RUN pin IC2

2

2

2

2

2

2

2

2

2

1

4

2

1

2

1

2

1

C

L

V

V

CV

P

LC

CV

P

LC

f

Lamp

DCbus

Lamp

Lamp

Lamp

Lamp

run













−

−





























−

+













−

=

π

π

(3)