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5

IRPLMB1E

6. Functional Description

Figure 6.1 shows the voltage across the lamp and

the current in the resonant inductor LRES during

Startup, Preheat, Ignition and Run Mode.

Fig. 6.1:

Voltage across the lamp (yellow waveform) and current in

the resonant inductor (green waveform) during Startup,

Preheat, Ignition and Run Mode

When power is turned on, the IR2520D goes into

Under Voltage Lockout (UVLO) mode.

The UVLO mode is designed to maintain a very low

(<200uA) supply current and to guarantee that the IC

is fully functional before the high- and low-side out-

put drivers are activated. During UVLO, the high- and

low-side driver outputs (LO and HO) are both low

and pin VCO is pulled down to COM for resetting the

starting frequency to the maximum.

Once VCC reaches the startup threshold (UVLO+),

the IR2520D turns on and the half-bridge FETs start

to oscillate. The IC goes into Frequency Sweep Mode.

At startup, VCO is 0V and the frequency is very high

(about 2.5 times fmin). This minimizes voltage spikes

and lamp flash at startup. The frequency ramps down

towards the resonant frequency of the high-Q ballast

output stage, causing the lamp voltage and lamp

current to increase. During this time, the filaments of

the lamp are pre-heated to the emission tempera-

ture to guarantee a long lamp life. The frequency

keeps decreasing until the lamp ignites. If the lamp

ignites successfully, the IR2520D enters RUN Mode.

If the minimum frequency has been chosen below

or very close to the resonant frequency, the IC will

work near resonance and will adjust the frequency

continuously to maintain ZVS at the half-bridge and

to minimize the losses in the FETs. If the minimum

frequency has been chosen higher than the reso-

nant frequency the IR2520D will work at the mini-

mum frequency.

Figure 6.2 shows the current across the resonant

inductor and the voltage across the lamp filaments

at the startup.

Figure 6.2:

Voltage across the lamp filaments (yellow) and current

in the resonant inductor (green) at the startup.