UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Low Current Shutdown Circuit:
VIN: 5V-25V
Since the shunt regulator circuitry needs to remain ac-
tive, even when the MODE pin is less than 1V and the
output is not switching, a low current shutdown is not
provided in the UCC3972/3. The following is a simple
on/off control requiring only two transistors with internal
bias resistors to disconnect VIN providing a low current
shutdown. VBAT and BUCK pins will consume a small
current in this mode because they have 430kW of internal
resistance.
SOT23
or
SOT323
47K
PNP
22K
OFF / ON
CONTROL
0V-5V
22K
47K
470
NPN
UCC3972
VDD
8
Cold Cathode Lamp Characteristics
1µF
Before beginning a CCFL converter design, it is impor-
tant to become familiar with the characteristics of the
lamp. The lamp presents a non-linear load to the con-
verter resulting in unique voltage vs. current (VI) charac-
teristics. The length, diameter, and physical construction
of the lamp determine its performance, and thus impact
the design of the converter. Fig. 11 shows the VI charac-
teristics collected from various lengths of 6mm diameter
lamps, where Fig. 12 shows the characteristics of several
3mm-diameter lamps.
6
5
GND
MODE
10uF
Part Numbers (Motorola)
NPN sot-23:
MMUN2234LT1
PNP sot-23:
MMUN2134LT1
NPN sot-323: MUN5234T1
It is interesting to note how the operating and striking
voltages (VSTRIKE) of the lamps are related to length as
well as lamp diameter. Since equal length CCFLs of dif-
ferent diameters have about the same lumens per watt
efficiency, the smaller diameter lamps actually produce
more light when driven at a given current since they op-
erate at a higher voltage. The lamps have regions of pos-
itive and negative resistance with the voltage peaking at
4mA for the 6mm diameter lamps and at 1mA for the
3mm diameter lamps.
PNP sot-323: MUN5134T1
Figure 10. Optional low current shutdown circuit.
enough voltage to keep the lamp operating over the
whole range of operating current, this requirement be-
comes more difficult with longer length and smaller diam-
eter lamps. Since the lamp characteristics will vary with
the manufacturing technique, it is a good idea to collect
data from several lamp manufacturers and to include de-
sign margin for process variations.
In order to successfully dim the lamp, the converter’s
resonant tank and step up transformer must provide
LAMP LENGTH=
LAMP LENGTH=
VSTRIKE=
800
100mm
800V
150mm
750V
250mm
1000V
100mm
800V
150mm
1000V
250mm
1200V
VSTRIKE=
400
350
300
250
200
150
100
50
600
400
200
0
0
0
1
2
3
4
5
LAMP CURRENT (mA)
6
7
8
0
1
2
3
4
5
LAMP CURRENT (mA)
6
7
8
Figure 11. 6mm lamp characteristics (20°C).
Figure 12. 3mm lamp characteristics (20°C).
15
TI [ TEXAS INSTRUMENTS ]
