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UBA20260 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 2 — 10 October 2011 9 of 30
NXP Semiconductors
UBA20260
600 V driver IC for step dimmable CFLs
In the boost state, a lamp current feedback control is implemented to improve lamp
stability (see Section 7.2.6
). The lamp current has a fixed ratio of 1.5 compared to the
burn state to boost the slow luminescence increase of a cold CFL lamp. In the boost to
burn transition time, there is a slow 15-step ratio decrease from 1.5 down to 1. The
preheat timer is reused for the transition to burn time and the boost ratio is gradually
decreased in 15 steps from 1.5 to 1, within 32 saw-tooth pulses on the CP pin. Using the
application values for C
CB
and R
ext(RREF),
a boost time of more than 300 s is possible. In
addition to boost bypass at a temperature of T
j(bp)bst
( 80 C), a temperature protection
function is implemented during boost state of T
j(end)bst
( 120 C). If the temperature
passes this level during boost, the transition timer is immediately started to enter the burn
state faster. Effectively this reduces the boost time (see Figure 4
[B]).
The boost state current boost does not start in dim modes DIM_2, DIM_3 or MDL;
see Figure 4
[A].
Remark: If the CB pin is short circuited to ground, the boost function is disabled. In such a
situation, the bottom frequency f
bridge(min)
is 1.8 times higher than the boost bottom
frequency f
bridge(bst)min
.
7.2.6 Burn state
After the boost state or when it is bypassed, the burn state starts. The lamp current sensor
circuit remains enabled (see Figure 4
[A]). The CSI (Current Sense Input) pin measures
the voltage across sense resistor R
CSI
. It is then passed through a Double-Sided Rectifier
(DSR) circuit and fed towards an Operational Transconductor Amplifier (OTA).
When the RMS voltage on the CSI pin reaches the actual internal reference level, the
lamp current sensor circuit takes over control of the lamp current. The internal current
output of the OTA is transferred using an integrator on the CI pin to the input for Voltage
Controlled Oscillator (VCO). The VCO regulates the frequency and as a result, the lamp
current.
Fig 6. Boost timing