UBA2024 データシートの表示（PDF） - NXP Semiconductors.

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UBA2024

Half-bridge power IC for CFL lamps

NXP Semiconductors. 

NXP Semiconductors

UBA2024

Half-bridge power IC for CFL lamps

7.5 Oscillation

The oscillation is based upon the 555-timer function. A self oscillating circuit is made with

the external resistor ROSC and the capacitor COSC (see Figure 4).

To realize an accurate 50 % duty cycle, an internal divider is used. This reduces the bridge

frequency to half the oscillator frequency.

The output voltage of the bridge will change at the falling edge of the signal on pin RC.

The design equation for the half-bridge frequency is:

f osc = -k----x----R----O---S---C-1----x-----C-----O---S---C-

An overview of the oscillator signal, internal LS and HS drive signals and the output is

given in Figure 4.

VRC

0

time

HS

drive

0

time

LS

drive

0

time

VOUT

half

bridge

0

Fig 4. Oscillator, drivers and output signals

time

014aaa658

When entering the sweep mode (VSW = 0 V), the bridge oscillator starts at 2.5 times the

nominal bridge frequency and sweeps down to the nominal frequency (bridge), set by

ROSC and COSC. During the sweep mode the amplitude of the RC oscillator on pin RC, will

swing between Vtrip(osc)low and VSW + 0.4Vtrip(osc)high. The amplitude of the RC oscillator

will continue to increase until VSW + 0.4Vtrip(osc)high = Vtrip(osc)high, this determines the end

of the sweep time. The voltage on pin SW however will continue to rise until it reaches

supply voltage level.

During this continuous decrease in frequency, the circuit approaches the resonance

frequency of the load, and this causes a high voltage across the load, which ignites the

lamp. The sweep to resonance time should be much larger than the settling time of the

supply voltage on pin HV, to guarantee that the full high-voltage is present at the moment

of ignition. See Figure 5.

UBA2024_4

Product data sheet

Rev. 04 — 17 September 2009

© NXP B.V. 2009. All rights reserved.

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