Datasheet

15
UCC1972/3
UCC2972/3
UCC3972/3
Low Current Shutdown Circuit:
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 V
IN
providing a low current
shutdown. VBAT and BUCK pins will consume a small
current in this mode because they have 430kW of internal
resistance.
Cold Cathode Lamp Characteristics
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.
It is interesting to note how the operating and striking
voltages (V
STRIKE
) 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.
In order to successfully dim the lamp, the converter’s
resonant tank and step up transformer must provide
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.
APPLICATION INFORMATION (cont.)
UCC3972
6
8
5
VDD
GND
MODE
470
1µF
VIN: 5V-25V
10uF
PNP
NPN
22K
22K
47K
47K
SOT23
or
SOT323
OFF / ON
CONTROL
0V-5V
Figure 10. Optional low current shutdown circuit.
NPN sot-23:
MMUN2234LT1
NPN sot-323: MUN5234T1
PNP sot-23:
MMUN2134LT1
PNP sot-323: MUN5134T1
Part Numbers (Motorola)
0
50
100
150
200
250
300
350
400
012345678
LAMP CURRENT (mA)
LAMP VOLTAGE
100mm 150mm 250mm
V
STRIKE= 800V 750V 1000V
LAMP LENGTH=
Figure 11. 6mm lamp characteristics (20°C).
0
200
400
600
800
012345678
LAMP CURRENT (mA)
LAMP VOLTAGE
100mm 150mm 250mm
V
STRIKE= 800V 1000V 1200V
LAMP LENGTH=
Figure 12. 3mm lamp characteristics (20°C).