UBA2021 datasheet(4/16 Pages) PHILIPS

4 / 16 page

UBA2021_4

Product data sheet

Rev. 04 — 25 July 2008

4 of 16

NXP Semiconductors

UBA2021

630 V driver IC for CFL and TL lamps

7.2 Initial start-up

Initial start-up is achieved by charging capacitor CS9 with the current applied to the

RHV-pin. At start-up, MOSFET T2 conducts and T1 does not conduct. This ensures Cboot

becomes charged. This start-up state is reached for a supply voltage of VVS(reset). This is

the voltage level on the VS-pin at which the circuit will be reset to its initial state and

maintained until the low voltage supply (VVS) reaches a value of VVS(start). The circuit is

reset to the start-up state.

7.3 Oscillation

When the low voltage supply (VVS) has reached the value of VVS(start) the circuit starts

oscillating in the preheat state. The internal oscillator is a current-controlled circuit which

generates a sawtooth waveform. The frequency of the sawtooth is determined by the

capacitor CCF and the current out of the CF-pin, mainly set by RRREF. The sawtooth

frequency is twice the frequency of the signal across the load. The IC brings MOSFETs

T1 and T2 alternately into conduction with a duty factor of approximately 50 %. Figure 4

represents the timing of the IC. The circuit block 'non-overlap' generates a non-overlap

time tno that ensures conduction periods of exclusively T1 or T2. Time tno is dependent on

the reference current IRREF.

7.4 Operation in the preheat mode

The circuit starts oscillating at approximately 2.5

× f

B (108 kHz). The frequency gradually

decreases until a deﬁned value of current Ishunt is reached (see Figure 5). The slope of the

decrease in frequency is determined by capacitor CCI. The frequency during preheating is

approximately 90 kHz. This frequency is well above the resonant frequency of the load,

which means that the lamp is off, the load only consists of L2, C5 and the electrode

resistance. The preheat time is determined by capacitor CCP. The circuit can be locked in

the preheat state by connecting the CP-pin to ground. During preheating, the circuit

monitors the load current by measuring the voltage drop over external resistor Rshunt at

the end of

conduction of T2 with decision level VRS(ctrl). The frequency is decreased as long as

VRS >VRS(ctrl). The frequency is increased for VRS < VRS(ctrl).

Fig 4.

Oscillator timing

mgs991

time

start-up

V(G1-S1)

V(G2)

tno

internal

clock

VCF

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