UBA2021_4 © NXP B.V. 2008. All rights reserved.
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 C
boot
becomes charged. This start-up state is reached for a supply voltage of V
VS(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 (V
VS
) reaches a value of V
VS(start)
. The circuit is
reset to the start-up state.
7.3 Oscillation
When the low voltage supply (V
VS
) has reached the value of V
VS(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 C
CF
and the current out of the CF-pin, mainly set by R
RREF
. 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 t
no
that ensures conduction periods of exclusively T1 or T2. Time t
no
is dependent on
the reference current I
RREF
.
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 I
shunt
is reached (see Figure 5). The slope of the
decrease in frequency is determined by capacitor C
CI
. 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 C
CP
. 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 R
shunt
at
the end of
conduction of T2 with decision level V
RS(ctrl)
. The frequency is decreased as long as
V
RS
>V
RS(ctrl)
. The frequency is increased for V
RS
< V
RS(ctrl)
.
Fig 4. Oscillator timing
mgs991
0
0
0
0
time
start-up
V
(G1-S1)
V
(G2)
t
no
t
no
internal
clock
V
CF
