ETC UCC3972PW

UCC1972/3
UCC2972/3
UCC3972/3
BiCMOS Cold Cathode Fluorescent Lamp Driver Controller
FEATURES
DESCRIPTION
• 1mA Typical Supply Current
Design goals for a Cold Cathode Fluorescent Lamp (CCFL) converter used
in a notebook computer or portable application include small size, high efficiency, and low cost. The UCC3972/3 CCFL controllers provide the necessary circuit blocks to implement a highly efficient CCFL backlight power
supply in a small footprint 8 pin TSSOP package. The BiCMOS controllers
typically consume less than 1mA of operating current, improving overall
system efficiency when compared to bipolar controllers requiring 5mA to
10mA of operating current.
• Accurate Lamp Current Control
• Analog or Low Frequency Dimming
Capability
• Open Lamp Protection
• Programmable Startup Delay
• 4.5V to 25V Operation
External parts count is minimized and system cost is reduced by integrating such features as a feedback controlled PWM driver stage, open lamp
protection, startup delay and synchronization circuitry between the buck
and push-pull stages. The UCC3972/3 include an internal shunt regulator,
allowing the part to operate with input voltages from 4.5V up to 25V. The
part supports both analog and externally generated low frequency dimming
modes of operation.
• PWM Frequency Synchronized to
External Resonant Tank
• 8 Pin TSSOP and SOIC Packages
Available
• Internal Voltage Clamp Protects
Transformer from Over-voltage
(UCC3973)
The UCC3973 adds a programmable voltage clamp at the BUCK pin. This
feature can be used to protect the transformer from overvoltage during
startup or when an open lamp occurs. Transformer voltage is controlled by
reducing duty cycle when an over-voltage is detected.
TYPICAL APPLICATION CIRCUIT
C6 27pF
UCC3972 NO INTERNAL VOLTAGE CLAMP
UCC3973
T1
INTERNAL VOLTAGE CLAMP LIMITS TRANSFORMER
VOLTAGE AT START-UP OR DURING FAULT
SYSTEM VOLTAGE
(4.5V TO 25V)
R2 1k
R1
1kΩ
8
C1
6.8µF
D1
VBAT
Q2
2
C7
0.1µF
6
GND
5
MODE
3
R10
VDD
C2
1µF
C3
1µF
C5 0.1µF
UCC3972/3
COMP
BUCK
LAMP
HV
Q3
R11
LAMP
LV
L1
68µH
1
ANALOG
DIMMING
R6 75Ω
OUT
7
R5 10k
R3 68k
FB
4
LOW FREQUENCY DIMMING
C4 33nF
DLFD
RLFD 68k
0V-5V LOW FREQUENCY CONTROL SIGNAL
SLUS252B - OCTOBER 1998 - REVISED NOVEMBER 2000
D2
R4 750
UDG-99154
UCC1972/3
UCC2972/3
UCC3972/3
CONNECTION DIAGRAMS
ABSOLUTE MAXIMUM RATINGS
VBAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +27V
VDD Maximum Forced Current . . . . . . . . . . . . . . . . . . . . 30mA
Maximum Forced Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . 17V
BUCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –5V to VBAT
MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to 4.0V
MODE Maximum Forced Current . . . . . . . . . . . . . . . . . . 300µA
Operating Junction Temperature . . . . . . . . . . –55°C to +150°C
Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
TSSOP-8 (TOP VIEW)
PW Package
Unless otherwise indicated, currents are positive into, negative
out of the specified terminal. Pulse is defined as less than 10%
duty cycle with a maximum duration of 500 s. Consult Packaging Section of Databook for thermal limitations and considerations of packages. All voltages are referenced to GND.
DIL-8 (TOP VIEW)
J, N Packages
OUT
1
8
GND
VDD
2
7
MODE
BUCK
3
6
FB
VBAT
4
5
COMP
ELECTRICAL CHARACTERISTICS: Unless otherwise specified these specifications hold for TA=0°C to +70°C for the
UC3972/3, –40°C to +85°C for the UC2972/3, and –55°C to +125°C for the UC1972/3; TA=TJ; VDD=VBAT=VBUCK=12V;
MODE=OPEN. For any tests with VBAT>17V, place a 1k resistor from VBAT to VDD.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX UNITS
Input supply
VDD Supply Current
VBAT Supply Current
VDD = 12V
1
1.5
mA
VBAT = 25V
7
10.5
mA
VBAT = 12V
30
60
A
VBAT = 25V
70
140
A
VDD Regulator Turn-on Voltage
ISOURCE = 2mA to 10mA
17
18
19
V
VDD UVLO Threshold
Low to high
3.6
4
4.4
V
100
200
300
mV
UVLO Threshold Hysteresis
Output Section
Pull Down Resistance
ISINK = 10mA to 100mA
25
50
Pull Up Resistance
ISOURCE = 10mA to 100mA
25
50
Output Clamp Voltage
VBAT = 25V, Shunt Regulator on
16
18
0.2
V
Output Low
MODE = 0.5V, ISINK = 1mA
0.05
Rise Time
CL = 1nF, Note 1
200
ns
Fall Time
CL = 1nF, Note 1
200
ns
2
V
UCC1972/3
UCC2972/3
UCC3972/3
ELECTRICAL CHARACTERISTICS: Unless otherwise specified these specifications hold for TA=0°C to +70°C for the
UC3972/3, –40°C to +85°C for the UC2972/3, and –55°C to +125°C for the UC1972/3; TA=TJ; VDD=VBAT=VBUCK=12V;
MODE=OPEN. For any tests with VBAT>17V, place a 1k resistor from VBAT to VDD.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX UNITS
BUCK = VBAT– 2, VBAT = 12V to 25V,
TA = –40°C to +85°C
52
66
80
kHz
BUCK = VBAT–2, VBAT = 12V to 25V
TA = –55°C to +125°C
44
66
80
kHz
BUCK = VBAT, VBAT = 12V to 25V
TA = –40°C to +85°C
160
220
280
kHz
BUCK = VBAT, VBAT = 12V to 25V
TA = –55°C to +125°C
145
220
280
kHz
FB = 1V, TA < 0°C
84
FB = 1V, TA = 0°C to 70°C
90
Oscillator Section
Minimum Frequency
Maximum Synchronizable Frequency
Maximum Duty Cycle
Minimum Duty Cycle
FB = 2V
BUCK Input Bias Current
BUCK = VBAT = 12V
40
%
0
%
90
A
80
110
µA
Measured at BUCK w/respect to VBAT,
VBAT=12V to 25V, TA < 0°C
–2.4
–1
–0.3
V
Measured at BUCK w/respect to VBAT,
VBAT=12V to 25V, TA = 0°C to 70°C
–2.0
–1
–0.3
V
COMP = 2V, TA = 0° C to +70° C
1.465
1.5
1.535
V
COMP = 2V
1.455
1.545
V
10
mV
BUCK = VBAT = 25V
Zero Detect Threshold
%
95
Error Amplifier
Input Voltage
Line Regulation
Input Bias Current
–2
2
–500
–100
nA
Open Loop Gain
COMP = 0.5V to 3.0V
60
80
Output High Voltage
FB = 1V
3.3
3.7
4.1
dB
Output Low Voltage
FB = 2V
0.15
0.35
V
Output Source Current
FB = 1V, COMP = 2V
–1.2
–0.4
mA
Output Sink Current
FB = 2V, COMP = 2V
2
Output Source Current
FB = 1V, COMP = 2V, MODE = 0.5V
–1
Output Sink Current
FB = 2V, COMP = 2V, MODE = 0.5V
–1
Unity Gain Bandwidth
TJ = 25C, Note 1
4
V
mA
1
1
2
A
A
MHz
Mode Select
Output Enable Threshold
0.85
Open Lamp Detect Enable Threshold
1
1.15
V
2.75
3
3.25
V
Mode Output Current
MODE = 0.5V
15
20
25
A
MODE Clamp Voltage
MODE = OPEN
3.3
3.7
4
V
Measured at BUCK with respect to VBAT,
VBAT=12V to 25V
–8
–7
–6
V
–10.3
–9
–7.7
V
Open Lamp
Open Lamp Detect Threshold
Over-voltage Clamp Threshold (UCC3973) Measured at BUCK with respect to VBAT,
VBAT=12V to 25V, IFB = 100µA
Note 1. Guaranteed by design. Not 100% tested in production.
3
UCC1972/3
UCC2972/3
UCC3972/3
PIN DESCRIPTIONS
BUCK: Senses the voltage on the top side of the inductor feeding the resonant tank. The voltage at this point
is used to synchronize the internally generated ramp
and to detect whether an open lamp condition exists.
An open lamp condition exists when this voltage is below the specified threshold for seven clock cycles. If the
MODE pin is held below the open lamp detect enable
threshold, this protective feature is disabled.
GND: Ground reference for the IC.
On the UCC3973, this pin is also used to sense an
over-voltage across the transformer primary. If the voltage at this pin exceeds the clamp threshold, current will
be sourced fron the FB pin.
When the voltage is between 1V and 3V, OUT is enabled
and the error amplifier output is connected to COMP. Open
lamp detection is still disabled and a constant 20 A current
is sourced from this pin. Placing an appropriate value external capacitor between this pin and ground allows the
user to disable open lamp detection for a set period of time
at start-up to allow the lamp to strike.
COMP: Output of the error amplifier.Compensation
components set the bandwidth of the entire system and
are normally connected between COMP and FB. The
error amplifier averages lamp current against a fixed internal reference. The resulting voltage on the COMP
pin is compared to an internally generated ramp, setting
the PWM duty cycle. During UVLO, this pin is actively
pulled low.
CURRENT OUT OF FB (uA)
FB: This pin is the inverting input to the error amplifier.
On the UCC3973, current is sourced form this pin if the
clamp threshold is exceeded at the BUCK pin (see below). The sourced current will reduce OUT duty cycle to
control transformer primary voltage. The source current
is disabled on the UCC3972.
MODE: The voltage on this pin is used to control start-up
and various modes of operation for the part (refer to the table in the block diagram).
When the voltage is below 1V, OUT is forced low, open
lamp detection is disabled and the error amplifier is
tri-stated.
When MODE reaches 3V, open lamp detection is enabled
and normal operation is activated.
OUT: Drives the buck regulator N-channel MOSFET. OUT
turn-on is synchronized to twice the tank resonant frequency. OUT is actively pulled low when in UVLO, an open
lamp condition has been detected or MODE is less than
1V.
VBAT: Positive input supply to power stage. This voltage is
required by internal control circuitry to provide open-lamp
detection and synchronization. Operating range is from
4.5V to 25V.
VDD: This pin connects to the battery voltage from which
the CCFL inverter will operate. If the potential on VBAT can
exceed 18V in the application, a series resistor must be
placed between VBAT and this pin (see applications section). The voltage at the VDD pin will then be regulated to
18V. This pin should be bypassed with a minimum capacitance of 0.1 F.
800
600
400
200
0
8.7
9.2
9.7
VBAT - VBUCK
Clamp current vs. tank voltage for UCC3973.
4
UCC1972/3
UCC2972/3
UCC3972/3
BLOCK DIAGRAM
VBAT
VDD
UVLO
4.0V/3.8V
8
18V
+
TO S3
–
UVLO=1
OPEN LAMP DETECT
COMPARATOR
7.0V
S2
+
VREF
3V REF
+
3 BIT
UP-DOWN
COUNTER
66kHz-200kHz
OSCILLATOR
–
+
1.0V
–
+
SYNC
UVLO
0.2V
Q
7
OUT
6
GND
4
FB
VDD
UVLO
+
R
+
PWM
–
S Q
OUTPUT OFF
(FROM MODE SELECT)
FROM
CLAMP
COMP
VDD
20µA
S1
5
BUCK
R
S
MODE
1
ZERO DETECT
COMPARATOR
FROM MODE SELECT
RAMP OUT
2
+
OVER-VOLTAGE
CLAMP COMPARATOR 9.0V
*MODE
SELECT
ICLAMP
ERROR
AMPLIFIER
+
1.5V
S3
–
(ALWAYS OPEN ON UCC3972)
3
UDG-98154
COMP
*MODE
Output
<1V
1V< MODE< 3V
>3V
OFF
ON
ON
Open Lamp
Detection
DISABLED
DISABLED
ENABLED
S2
Error Amplifier Output
S1
OPEN
OPEN
CLOSED
DISCONNECTED FROM COMP
CONNECTED TO COMP
CONNECTED TO COMP
OPEN
CLOSED
CLOSED
APPLICATION INFORMATION
backlight converter must produce the high voltage
needed to strike and operate the lamp. Although CCFLs
can be operated with a DC voltage, a symmetrical AC
operating voltage is recommended to maintain the rated
life of the lamp. Sinusiodal voltage and current lamp
waveforms are also recommended to achieve optimal
electrical to light conversion and to reduce high voltage
electromagnetic interference (EMI). A topology that provides these requirements while maintaining efficient operation is presented below.
Introduction
Cold Cathode Fluorescent Lamps (CCFL) are frequently
used as the backlight source for Liquid Crystal Displays
(LCDs). These displays are found in numerous applications such as notebook computers, portable instrumentation, automotive displays, and retail terminals.
Fluorescent lamps provide superior light output efficiency, making their use ideal for power sensitive portable applications where the backlight circuit can consume
a significant portion of the battery’s capacity. The
5
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
The resonant tank consisting of CRES and T1 produces
sinusoidal currents (IRES) and voltages and is fed by a
controlled DC current (IBUCK) from the buck stage. Note
that the BUCK node voltage is ½ the primary tank voltage, as VBAT is located at the center tap of the transformer. The high turns ratio transformer (T1) amplifies the
sinusoidal tank voltage to produce a sinusoidal secondary voltage that is divided between the lamp and ballast
capacitor.
Circuit Operation
A current fed push-pull topology is used to power the
CCFL backlight shown in Fig. 1. This topology accommodates a wide input voltage and dimming range while retaining sinusoidal operation of the lamp. The converter
consists of a resonant push-pull stage, a high voltage
output stage, and a buck pre-stage used to regulate current in the converter.
Referring to Fig. 1, the push-pull stage consists of CRES,
Q1, Q2, RB, and T1’s primary and auxiliary windings.
The output stage consists of CBALLAST, the lamp, the current sense resistor RS, and T1’s secondary. The resonant
frequency of the tank is set by the primary inductance of
T1, along with the resonant capacitor (CRES), and the reflected secondary impedance. The secondary impedance
includes the lamp, the ballast capacitor (CBALLAST), the
distributed winding capacitance of T1, and the stray capacitance which forms between the lamp, lamp wires,
and the backlight reflector. Since the lamp impedance is
nonlinear with operating current, the tank resonant frequency will vary slightly with load (typically 1.5:1).
Transistors Q1 and Q2 are driven out of phase at 50 percent duty cycle with an auxiliary winding on T1. The
winding provides a floating AC voltage source at the resonant frequency that is used to drive the transistor bases
alternately on and off. One leg of the auxiliary winding is
tied to the input voltage through base resistor RB, which
is sized to provide sufficient base current to the transistors. The transistors channel the buck inductor current
into opposing ends of the tank at the resonant frequency,
supplying energy for the lamp and system losses.
The buck power stage consists of inductor LBUCK,
MOSFET switch SBUCK, and flyback diode DBUCK. In order to prevent interactions between multiple switching
OUTPUT STAGE
RESONANT PUSH-PULL STAGE
T1 PRIMARY
T1 SECONDARY
CBALLAST
VBAT
CCFL
ILAMP
VBAT
CRES
RB
T1
AUXILIARY
VBAT FB
IRES
RS
Q2
Q1
VBUCK
VBAT
DBUCK
LBUCK
VBAT
IBUCK
SBUCK
Q1
ON
Q2
ON
Q1
ON
OUT
Q2
ON
VBUCK
GND
BUCK STAGE
Figure 1. Push-pull, output, and buck stages.
6
UDG-98157
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
lamp, providing a high impedance sinusoidal current
source with which to drive the CCFL. This approach improves the optical efficiency of the system, as capacitive
leakage effects are minimized due to reduced harmonic
content in the voltage waveforms. Unfortunately, from an
electrical efficiency standpoint, an increased tank voltage
produces increased flux losses in the transformer and increased circulating currents in the tank. In practice, the
voltage drop across the ballast capacitor is selected to
be approximately twice the lamp voltage (750V in our
case) at rated lamp current. Assuming a 50kHz resonant
frequency and 5mA operating current, a ballast capacitance of 22pF is selected. Since the lamp and ballast capacitor impedance are 90 degrees out of phase, the
vector sum of lamp and capacitor voltages determine the
secondary voltage on the transformer.
frequencies, the UCC3972/3 synchronizes the buck frequency to the frequency of the push-pull stage. The traditional buck topology is inverted to take advantage of
the lower RDS(on) characteristics of an N-Channel
MOSFET switch (SBUCK). With a sinusoidal voltage
across the tank, the resulting output of the buck stage
(VBUCK) becomes a full-wave rectified voltage referenced
to VBAT as shown in Fig. 1.
Lamp current is sensed directly with RS and a parallel diode on each half cycle. The resulting voltage across the
sense resistor RS is kept at a 1.5V average by the error
amplifier, which in turn controls the duty cycle of SBUCK.
The buck converter typically operates in continuous cury a 4mode
cell Lith
rent
but can operate with discontinuous current as
the CCFL is dimmed.
Design Procedure
V SEC =
A notebook computer backlight circuit will be presented
here to illustrate a design based on the UCC3972/3 controller. The converter will be designed to drive a single
cold cathode fluorescent lamp (CCFL) with the following
specifications:
250mm (10”)
6mm
1000V (PEAK)
375V (RMS)
5mA
1.9W
2
FRESONANT =
(2)
(3)
1
2π
Resonant Tank and Output Circuit
The selection of components to be used in the resonant
tank of the converter is critical in trading off the electrical
and optical efficiencies of the system. The value of the
output circuit’s ballast capacitor plays a key role in this
trade-off. The voltage across the ballast capacitor is a
function of the resonant frequency and secondary lamp
current:
ILAMP
2 • π • CBALLAST • FRESONANT
+ (VLAMP )
Once the ballast capacitor is selected, the resonant frequency of the push-pull stage can be determined from
the transformer’s inductance (L), turns ratio (N), and the
selection of resonating capacitor (CRES).
Input Voltage Range:
The notebook computer will be powered b
ium-Ion battery pack with an operational voltage range of
10V to 16.8V. When the pack is being charged, the back
light converter is powered from an AC adapter whose DC
output voltage can be as high as 22V.
VCB =
2
The resulting secondary voltage at rated lamp current is
820V. Since the capacitor dominates the secondary impedance, the lamp current maintains a sinusiodal shape
despite the non-linear behavior of the lamp. As the CCFL
is dimmed, lamp voltage begins to dominate the secondary impedance and current becomes less sinusiodal.
Transformer secondary voltage is reduced, however, so
high frequency capacitive losses are less pronounced.
The value of ballast capacitor has no effect on current
regulation since the average lamp current is sensed directly by the controller.
Table 1. Lamp Specifications
Lamp Length
Lamp Diameter
Striking Voltage (20° C)
Operating Voltage (5mA)
Full Rated Current
Full Rated Power
(VCB )
LPRIMARY (CRES + (N 2 • CBALLAST ))
Output distortion is minimized by keeping the independent resonant frequencies of the primary and secondary
circuits equal. This is achieved by making the resonant
capacitor equal to the ballast capacitance times the turns
ratio squared:
CRES = N 2 • CBALLAST = (67) 2 • 22pF = 0 .1 µF
(1)
(4)
The resulting resonant frequency is about 50kHz, this
frequency will vary depending upon the lamp load and
amount of stray capacitance in the system. Since the
UCC3972/3 has an internal oscillator, it is important that
the operating frequencies of a particular design are
within the synchronizable frequencies of the controller.
A voltage drop across CBALLAST many times the lamp
voltage will make the secondary current insensitive to
distortions caused by the non-linear behavior of the
7
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Component Selection for the Resonant Tank and
Output Circuit
Since high efficiency is a primary goal of the backlight
converter design, the selection of each component
must be carefully evaluated. Losses in the ballast capacitor are usually insignificant, however, its value determines the tank voltage which influences the losses in
the resonant capacitor and transformer. Since the resonant capacitor has high circulating currents, a capacitor
with low dissipation factor should be selected. Power
loss in the resonant tank capacitor will be:
CRES _ LOSS (watts) =
(VTANK )
2
(5)
• 2π • FRESONANT • CRES • Dissipation Factor
Polypropylene foil film capacitors give the lowest loss;
metalized polypropylene or even NPO ceramic may
give acceptable performance in a lower cost surface
mount (SMT) package. Table 2 gives possible choices
for the resonant and high voltage ballast capacitors.
The transformer is physically the largest component in
the converter, making the tradeoff of transformer size
and efficiency a critical choice. The transformer’s efficiency will be determined by a combination of wire and
core losses. A Coiltronics transformer (CTX110600)
was chosen for this application because of its small
size, low profile, and overall losses of about 5% at 1W.
Low profile CCFL transformers are also available from
Toko (847)-297-0070 in Mt. Prospect, IL or Sumida
(408)-982-9660 in Santa Clara, CA.
Wire losses are determined by the RMS current and
the ESR of the windings. The primary winding resistance for the Coiltronics transformer is 0.16Ω. The RMS
current of the primary winding includes the sinusoidal
resonant current and the DC buck current on alternate
half cycles (i.e. only ½ of the primary winding sees the
buck current depending upon which transistor is on). Maximum resonant current is equal to:
IRES =
VPRIMARY
LPRIMARY
CRES
=
820
67 •
44
0 .1
(6)
= 600 mA
Buck inductor current is calculated in the next section.
Secondary current is simply the lamp current, the secondary winding has 176Ω of resistance.
Core losses are a function of core material, cross sectional
area of the core, operating frequency, and transformer voltage. For ferrite material, the hysteresis core losses increase with voltage by a cubed factor; for a given core
cross sectional area, doubling the tank voltage will cause
the losses to increase by a factor of 8. This makes the selection of the ballast capacitor a critical decision for efficiency.
Other elements influencing the resonant tank and output
circuit efficiency include the push-pull transistors, the base
drive and sense resistors, as well as the lamp. High gain
low VCESAT bipolar transistor such as Zetek’s FZT849 allow high efficiency operation of the push-pull stage. These
SOT223 package parts have a typical current transfer ratio
(hFE) of 200 and a forward drop (VCESAT) of just 35mV at
500mA. Rohm’s 2SC5001 transistors provide similar performance. For low power, size sensitive applications, a
SOT23 transistor is available from Zetek (FFMT619) with
approximately twice the forward drop at 500mA. The base
drive resistor RB is sized to provide full VCE saturation for
all operating conditions assuming a worst case hFE. For efficiency reasons, the base resistor should be selected to
have the highest possible value. A 1kΩ resistor was selected in this application. Losses scale with buck voltage
as:
RB(LOSS) =
V 2 BUCK
RB
(7)
Table 2. Capacitor selection
Manufacturer
Ballast Capacitor
Cera-Mite (414) 377-3500
NOVA-CAP (805) 295-5920
Murata Electronics
Resonant Capacitor
Wima (914)347-2474
Paccom (800)426-6254
NOVA-CAP
Capacitance Type
High Voltage Disk Capacitor (3kV)
SMT 1808 (3kV)
SMT 1808 (3kV)
Polypropylene foil film FKP02
Metalized Polypropylene
SMT Metalized polyphenylene-sulfide
SMT Metalized polyphenylene-sulfide
SMT Ceramic
8
Series
Dissipation Factor
(1kHz)
564C
COG
GHM
FKP02
MKP2
MKI
CHE
COG
0.0003
0.0005
0.0015
0.0006
0.001
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
The current sense resistor RS provides direct control of
lamp current. Since the current sense resistor voltage is
controlled to a 1.5V reference, its power loss is inversely
proportional to its value at a given lamp current.
VBUCK _ AVE = VBAT −
= VBAT −
V SEC • 2
N•π
(9)
820 • 2
= VBAT − 5 .5 • Volts
67 • π
Synchronizing the Stages
The approximate on time using the maximum 22V input
voltage (VBUCK_AVE = 16.4), a 100kHz switching frequency (two times the resonant frequency), and ignoring
the diode drop can be calculated from the following:
An internal comparator at the BUCK node is used to synchronize the PWM buck frequency to twice the resonant
tank frequency. Synchronization is accomplished with
sync pulse that is generated each time the BUCK node
voltage is within 1.0V of VBAT; the UCC3972/3 uses this
sync pulse to reset the PWM oscillator’s saw-tooth ramp.
The sync circuit will operate with typical PWM frequencies between 66kHz and 200kHz, corresponding to a
33kHz to 100kHz tank frequency.
VBAT − VBUCK _ AVE
t ON
=
T − t ON
VBUCK _ AVE
The resulting on time is 2.5µs. A 150µH inductor will result in a peak to peak ripple current of 280mA. Average
inductor current (with maximum lamp current) can be calculated by taking the lamp power divided by the tank efficiency and the RMS buck voltage.
Buck Stage Design
The PWM output controls current in the buck inductor.
The UCC3972/3’s buck power stage differs from a traditional buck topology in a few respects:
IBUCK =
• The output voltage is a full wave rectified sinewave at
the switching frequency, rather than DC.
The resulting inductor ripple is less than 50%. A list of
possible inductors are given below along with ESR and
current rating (losses in the inductor are calculated with
RMS current).
Referring back to Fig. 1, when OUT turns SBUCK on, the
BUCK node voltage VBUCK is placed across the inductor.
This voltage is typically positive and current ramps up in
the inductor (it is possible for the BUCK node voltage to
go negative if VBAT is low and the lamp current is near
is
turned
off,
maximum).
When
SBUCK
VBAT-VBUCK+VDBUCK is placed across the inductor with
opposite polarity. As with any buck converter, the
volt-seconds across the inductor must be reversed on
each switching cycle to maintain constant current. The
duty cycle (D) relationship is complicated somewhat by
the fact the output voltage is changing within a switching
cycle. The equations below determine the relationship
between on and off times in continuous conduction mode
where T is the switching period, D = tON/T, and tOFF = TtON.
∫ VBUCK
0
T
• dt =
∫ (VBAT − VBUCK
+ VD ) • dt
(11)
VLAMP • ILAMP  •  2 • N  = 375 • 0 .005 • 2 • 67
0 .8 • 820
 Efficiency  V SEC 
= 380 mA
• The topology is inverted using a ground referenced
N-Channel MOSFET rather than a VDD referenced
P-Channel.
t ON
(10)
The choice of a MOSFET for the buck switch should take
into consideration conduction and switching losses. The
RDS(on) and gate charge are typically at odds, however,
where minimizing one will typically result in the other increasing. An International Rectifier IRFL014 was selected (SOT-223 package) in this application with a gate
charge of 11nC and RDS(on) of 0.2Ω. A Schottky diode
should be used for the buck diode in order to minimize
forward drop.
Table 3. Inductor Suppliers
Vendor
(8)
L
Part Number ESR Current
Rating
150 H DO3316-154 0.38
1A
Coilcraft
(847) 639-6400
Coiltronics (407)
150 H CTX150-4 0.175
241-7876
Sumida
150 H CDR125-151 0.4
(847) 956-0666
Toko (847) 297-0070 150 H 646CY-151 0.73
t ON
Selecting the buck inductor:
Maximum ripple current in the inductor occurs when frequency and duty cycle are at a minimum, which corresponds to VBAT and lamp current being a maximum. The
average value of VBUCK at rated lamp current is equal to:
9
0.72A
0.85A
0.4A
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Dimming Techniques
Analog Dimming:
A control circuit that implements analog dimming with a
potentiometer (RADJ) is shown in Fig. 2. When the secondary has a positive polarity current, D1 is reversed biased and lamp current is sensed directly through RL and
RADJ. When the current reverses direction, D1 conducts
and the voltage on the sense node VX is clamped to the
forward drop of the diode. The resulting waveform at VX
is a half wave rectified sinusoid whose voltage is proportional to lamp current.
ILAMP =
VD 
15
π
. +

2 
(12)
Figure 3. Analog dimming control from microprocessor.
2(RL + R ADJ )
VX
CFB
0V
SECONDARY
CBALLAST
Low Frequency Dimming (LFD):
Analog dimming techniques described previously can
provide excellent dimming over a 10:1 range, depending
upon the physical layout and the amount of stray capacitance in the backlight's secondary circuitry. Beyond this
level the lamp may begin to exhibit the "thermometer effect" causing uneven illumination across the tube.
RFB
VX
RADJ
D1
FB
COMP
Low frequency dimming (LFD) is accomplished by operating the lamp at rated current and gating the lamp on
and off at a low frequency. Since the lamp is operated at
full intensity when on, the system layout has little effect
on dimming performance. The average lamp intensity is a
function of the duty cycle and period of the gating signal.
The duty cycle can be controlled to a low minimum value,
allowing a very wide dimming range. Low frequency dimming can be implemented by summing a PWM signal
into the feedback node to turn the lamp off as shown in
Fig. 4. A 68kΩ resistor is used for RFB and RLFD, CFB is
reduced to 6.8nF to speed up the lamp re-strike. The repetition rate of the signal should be greater than 120Hz to
avoid visible flicker.
1.5V
RL
Figure 2. Analog dimmer with potentiometer.
This voltage is averaged by the feedback components
(RFB, CFB) and held to 1.5V by the error amplifier when
the control loop is active. The resulting voltage at the output of the error amplifier (COMP) sets the duty cycle of
PWM stage. Average lamp current is controlled by adjusting RADJ to the appropriate value. Resistor RL sets the
high current level of the lamp.
Analog Dimming by PWM or D/A Control Signal:
Analog dimming control of the lamp can be achieved by
providing a digital pulse stream (or DC control voltage)
from the system microprocessor as shown in Fig. 3. For
this technique, the lamp current sense resistor (R1) is
fixed and the VX node voltage is averaged against the
digital pulse stream of the microprocessor. The averaging
circuit consists of R2, R3, and CFB. A higher average
value from the pulse stream will result in less average
lamp current. The frequency of the digital pulse stream
should be high enough to maintain a constant DC value
across the feedback capacitor. If a D/A converter is available in the system, a DC output can be used in place of
the pulse stream.
Figure 4. Low frequency dimming by forcing lamp
current off through the FB pin.
10
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Referring to Fig. 5, at time t0 the control signal is
brought low and the voltage in the resonant tank begins
to build. At time t1 there is sufficient voltage for the
lamp to strike and the feedback loop controls the lamp
at rated current using a fixed current sense resistor.
When the LFD signal is brought low at time T2, the
COMP output is low and the OUT pin stops switching.
The resonant tank voltage decays until the lamp extinguishes. If the on time were extended to t3 the average
lamp intensity would be increased accordingly, the next
low frequency cycle begins at time t4.
LAMP
VOLTAGE
LAMP
CURRENT
LFD
CONTROL 5V
SIGNAL 0V
ON
t0 t1
OFF
t2
t3
t4
Figure 5. Low frequency dimming timing waveforms.
The time relationship between the resonant and gating
frequency has been exaggerated so that the sinusoidal
waveforms can be depicted. In order to avoid visible
lamp flicker, the low frequency gating rate (t0-t4) should
be greater than 100Hz. To prevent “beat” frequency interference, it may be advantageous to synchronize the
gating frequency to a multiple of the monitor scan rate
of the LCD display. This can be accomplished by con-
trolling the duty cycle with a timer routine within the LCD’s
software program.
LFD waveforms at 200Hz and 50% duty cycle are shown
in Fig. 6a. Fig. 6b show a time expanded photo of the
same waveforms. Channel 1 is lamp voltage at 500V /div,
Channel 2 is lamp current at 20mA / div, and Channel 3 is
the LFD control voltage. Since the photos are from a digital
oscilloscope, alias exists in the waveforms.
Lamp Current Control Loop
The current control loop for the CCFL circuit is discussed
in detail in Unitrode Application Note U-148 and is briefly
repeated here for completeness. A block diagram for the
current control loop is shown in Fig. 7.
The PWM modulator small signal gain is inversely proportional to the internal saw tooth ramp and proportional to
the input voltage (the inductor’s current slope increases as
VBAT increases). The resonant tank and buck inductor
form a RLC filter at the center point of the push pull transformer. The effective L of the filter is dominated by buck inductor and the effective C is approximately 8 times the
resonant capacitor (CRES) value. This occurs because the
reflected ballast capacitance is equal to CRES and the
equivalent capacitance at the push-pull center point is four
times the capacitance across the tank. The equivalent resistance at the push-pull center point is equal to ¼ the
tank voltage squared divided by the lamp power. The corner frequency and Q of the filter are:
FCORNER =
Q=
1
2π LBUCK • 8 • CRES
2πFFILTER LBUCK
RFILTER
(13)
(14)
Figure 6b. Time expanded showing lamp strike and
feedback delay.
Figure 6a. LFD at 50% duty cycle.
11
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
CFB
ERROR
AMPLIFIER
PWM
MODULATOR
VBAT
VSAW
1.5V
2 POLE
RLC
FILTER
TRANSFORMER
2NS
NP
SECONDARY
IMPEDANCE
RLAMP2+XC2
RFB
0.5RS
Figure 7. Current control loop block diagram.
The resulting gain of the filter is unity below the 15kHz
corner frequency, peaking up at the corner frequency
with Q, and rolling off with a 2-pole response above the
corner frequency. As shown in Fig. 7, the transformer
turns ratio provides a voltage gain and the output circuit
(whose impedance includes the lamp and ballast capacitor) converts the voltage into a current. The current
sense resistor produces a voltage on each half cycle,
leaving the error amplifier as the final gain block.
Striking the Lamp
Before the lamp is struck, the lamp presents an impedance much larger than the ballast capacitor and the full
output voltage of the transformer secondary is across the
lamp. Since the buck converter must reverse the volt-seconds on the buck inductor, the average tank voltage at
the primary can be no greater than the DC input voltage.
This constraint along with the turns ratio of the push-pull
transformer sets the peak voltage available to strike the
lamp:
Loop gain is greatest at minimum lamp current and maximum input voltage. With a 22V input, a 2V saw-tooth,
and 1:67 turns transformer, the low frequency voltage
gain of the PWM, RLC filter, and Transformer is 1500.
With a 375V lamp and 1mA of lamp current (using a
22pF ballast capacitor and 50kHz switching frequency)
the secondary impedance is 400kΩ. RSENSE at 1mA is
4kΩ (equation 12), resulting in a low frequency power
loop gain of 7.5. The error amplifier is configured as an
integrator, giving a single pole roll-off and a high gain at
DC. A 68k resistor and 33nF capacitor give a 70Hz
crossover frequency for the feedback network, yielding a
maximum crossover frequency of 500Hz for the total loop
avoiding stability problems with the Q of the resonant
tank. For 5mA of lamp current with a 22V input the total
loop crossover is 200Hz, for low frequency dimming applications CFB can be reduced to 6.8nF with no instability
(1kHz crossover).
V STRIKE = N S:P • π • VINPUT
(15)
The Coiltronics transformer has a 67:1 turns ratio, giving
2100 peak volts available to strike the lamp with the minimum 10V input. In our example this is more than sufficient for the 1000V required to strike the lamp. With the
22V maximum charger input, the available striking voltage could theoretically reach 5000V! The possibility of
breaking down the transformer’s secondary insulation
becomes a real concern at this voltage.
Voltage Clamp Circuit (UCC3972)
An external voltage clamp circuit consisting of D4, Q4,
R7, R8, and R9 can be added to the typical application
circuit as shown in Fig. 8. This circuit limits the maximum
transformer voltage during startup, allowing an extended
time period for striking the lamp while protecting the
transformer from over voltage. For fixed input voltage designs, this circuit is optional since the transformer turns
can be optimized at one voltage.
12
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
VBAT
2
T1
LAMP
C5
R9
DCLAMP
Q2
R7
Q1
R8
VBUCK
D4
R3
Q4
2N3906
D2
R4
UCC3972 EXTERNAL
VOLTAGE CLAMP
L1
4
FB
UDG-99161
Figure 8. Optional voltage clamp circuit. For UCC3972. (Not required for UCC3973)
The clamp circuit works as follows:
If the voltage at the base of Q4 is equal to the zener (D4)
voltage plus the VBE of Q4, the clamp circuit will activate
limiting the voltage in the resonant tank. When the clamp
activates, Q4 is turned on and additional current (set by
R9) is allowed into the feedback capacitor. The peak
clamp voltage is given by:
VCLAMP = VIN – VBUCK =
R7 + R8
• (V ZENER + VBEQ 4 ) PEAK
R7
An optional zener diode DCLAMP can be added to either
UCC3972 or UCC3973 designs as shown in Fig. 8. The
zener provides a high speed clamp when power is initially applied to the circuit and before the voltage clamp
can regulate the feedback loop. DCLAMP can be a small
250mW zener since it will only conduct for a few resonant cycles before the voltage clamp takes effect.
DCLAMP’s value should be a few volts greater than the
voltage clamp.
(16a)
Setting the Time Period for Blanking Open Lamp Detection
Internal Voltage Clamp Circuit for UCC3973
A capacitor on the MODE pin of the UCC3972/3 is used
to blank the open lamp protection circuitry during the initial lamp startup. When the IC is initially powered-up, a
20µA current out of the MODE pin charges the capacitor
CMODE from ground potential. Since the PWM output is
disabled when the MODE pin is between 0V-1V, open
lamp blanking occurs as CMODE is charged from 1V-3V,
giving a soft start period of:
The over-voltage function is provided internally on the
UCC3973. As shown in the block diagram of the
UCC3972/3, an internal comparitor monitors the instantaneous voltage between VBAT and BUCK. If this voltage
exceeds the over-voltage clamp level (9V nominal), a
current will be sourced from the FB pin to reduce duty cycle. The source current level increases with over-voltage,
but is typically 100µA at the threshold voltage. As with
the Open Lamp Trip Level, the Voltage Clamp Threshold
is programmed with external resistors R10 and R11.
VCLAMP = 

R10 + R11
• 9 VPEAK
R10 
TSS =
(16b)
CMODE
• SEC
10µF
(17)
The time required for lamp strike is application dependent, and a 10µF capacitor allows 1 second in which to
strike the lamp. Fig. 9 shows the voltage at the VBUCK
node with a 20V input and a 13.5V peak level for the
internal voltage clamp (UCC3972 requires and external
clamp) under an open lamp fault condition. After the 1
second period, the open lamp detection circuit trips and
the UCC3972/3 shuts down until power is cycled on the
chip.
A 2k resistor for R10 and a 1k resistor for R11 will result
in a peak (VBAT–VBUCK) level of 13.5V. With a 1:67 turns
ration transformer, the secondary voltage will be clamped
to 1280 VRMS.
The FB pin source current is disabled in the UCC3972.
13
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
ing the converter would shut down after the one second
blank time if a true open lamp existed. If the open lamp
voltage is increased, the peak clamp circuit voltage
(equation 16) would need to be increased accordingly. A
peak VBAT-Vbuck voltage of 10.5V has been set for
open lamp detection in this example. (R10 = 2k,
R11 = 1k).
Voltage Regulator
The UCC3972/3 controller contains an internal 18V
shunt regulator that provides a 5% accurate voltage
clamp for the MOSFET gate drive while allowing the controller to operate in applications with input voltages up to
25V. Since only the VBAT and BUCK pins are rated for
25V, the shunt regulator limits the voltage on the VDD
and OUT pins to 18V. The MODE, CS, and COMP pin
voltages are typically less than 5V. If the UCC3972/3 is to
be used in an application with input voltages greater than
18V, a resistor from VBAT to VDD is required to limit the
current into the VDD pin. The resistor should be sized to
allow sufficient current to operate the controller and drive
the external MOSFET gate, while minimizing the voltage
drop across the resistor. A bypass capacitor should be
connected at the VDD pin to provide a constant operating voltage.
Figure 9. VBUCK and MODE pin voltages during an
open lamp fault start-up.
Normal Startup
In practice, the lamp will typically strike in much less than
1 second (usually within the first few cycles) and the voltage at the transformer voltage will collapse to below the
open lamp trip level. Difficulty in striking the lamp usually
results from one or a combination of the following:
Selecting the Shunt Resistor:
The first step in selecting the shunt resistor is to determine the current requirements for the application. With a
100kHz switching frequency and a maximum gate
charge of 11nC for the IRFL014 , the gate drive circuit requires 1.1mA of average current. The UCC3972/3 requires an additional maximum quiescent current of
1.5mA. The shunt resistor must therefore supply 2.6mA
of current over the operating voltage of the part.
• Insufficient transformer turns ratio or input voltage.
• Increase in required striking voltage at cold
temperature.
• The lamp has set for a long period of time.
• Transformer secondary voltage is reduced due to
voltage division between parasitic secondary
capacitance and the ballast capacitor.
The application’s maximum input voltage is 22V. With a
regulator clamp voltage of 18V, the maximum value for
the shunt resistor becomes 1.5kΩ [(22-18)V/2.6mA]. This
resistor will minimize losses at maximum input voltage,
but could produce a 4V drop (from VBAT to VDD) even
when the regulator is not clamped. This drop reduces the
available gate drive voltage, leaving only 6V with the minimum input voltage of 10V. Since the efficiency of the
shunt regulator is not of primary importance when the
charger is running, a smaller value of shunt resistor is selected to improve the available gate drive voltage. A 470Ω
shunt resistor will produce a maximum 1.2V drop from
VBAT to VDD when the shunt regulator is not clamped.
When the regulator is clamped at 18V and the charger
voltage is at its maximum of 22V, the power across the
shunt resistor will be 35mW [(4V x 4V)/470].
Setting the Open Lamp Trip Level
The buck voltage is monitored by an internal 7V comparator to detect an open lamp. The actual trip voltage
across the resonant tank is set with an external resistor
divider R10 and R11.
VOPENLAMP = VIN – VBUCK
(18)
R10 + R11
=
• 7V PEAK
 R10 
R10 and R11 should be in the 1kΩ-5kΩ range, to guarantee sharp zero crossing edges at the buck pin of the IC.
In most applications the peak clamp voltage would be set
to a higher level than the open lamp trip voltage, ensur-
14
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Low Current Shutdown Circuit:
Since the shunt regulator circuitry needs to remain active, 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 430kΩ of internal resistance.
VIN: 5V-25V
SOT23
or
SOT323
22K
47K
PNP
OFF / ON
CONTROL
0V-5V
22K
NPN
470
47K
Cold Cathode Lamp Characteristics
UCC3972
8
VDD
6
GND
5
MODE
1µF
Before beginning a CCFL converter design, it is important to become familiar with the characteristics of the
lamp. The lamp presents a non-linear load to the converter resulting in unique voltage vs. current (VI) characteristics. 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 characteristics collected from various lengths of 6mm diameter
lamps, where Fig. 12 shows the characteristics of several
3mm-diameter lamps.
10uF
Part Numbers (Motorola)
NPN sot-23:
MMUN2234LT1
PNP sot-23:
MMUN2134LT1
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 different 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 operate at a higher voltage. The lamps have regions of positive and negative resistance with the voltage peaking at
4mA for the 6mm diameter lamps and at 1mA for the
3mm diameter lamps.
NPN sot-323: MUN5234T1
PNP sot-323: MUN5134T1
Figure 10. Optional low current shutdown circuit.
voltage to keep the lamp operating over the whole range
of operating current, this requirement becomes more difficult with longer length and smaller diameter 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 design margin for process variations.
In order to successfully dim the lamp, the converter’s resonant tank and step up transformer must provide enough
Figure 11. 6mm lamp characteristics (20 C).
Figure 12. 3mm lamp characteristics (20 C).
15
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Since a fluorescent lamp is a pressurized gas filled tube
(usually Argon and Mercury vapor), it shouldn’t be surprising that temperature plays a major role in the lamp
characteristics. Fig. 13 depicts the variations in striking
and operating voltage for a 150 x 3mm lamp over temperature, illustrating the importance of taking temperature effects into account when designing the converter.
The lumen output of the backlight system is temperature
dependent as well, and may need to be accounted for in
applications requiring tight lumens regulation over a wide
temperature range. Fig. 14 shows the temperature effects
on lumens for the lamp operated at 5mA.
less pronounced as the lamp is over-driven as shown in
Fig. 15. The expected life of the lamp will also degrade,
as illustrated in Fig. 16, when the lamp is operated above
rated current.
Cold Cathode Fluorescent Lamp Efficiency
Trade-Offs
Although CCFLs offer high output light efficiency compared to other lamp types such as incandescent, only a
percentage of the input energy is converted to light. As illustrated in Fig. 17, 35% of the energy is lost in the electrodes, 26% as conducted heat along the tube. A portion
of the Ultra Violet energy gets converted into visible light
by the lamp phosphor, where the remainder is converted
into radiated heat. Finally, Mercury atoms convert 3% of
the initial energy into visible light. The result is typically
15% overall electrical to optical energy conversion in the
lamp.
Since lamp current is roughly proportional to luminosity, it
may be tempting to operate the lamp at a RMS current
higher than specified in the manufacturer’s data sheet.
While the lamp will continue to operate tens of percent
above the rated current, the luminosity gain becomes
5mA
LUMINANCE PERFORMANCE (%)
Striking Voltage
1200
LAMP VOLTAGE (rms)
1000
800
600
400
200
0
0
20
40
AMBIENT TEMPERATURE (°C)
120
100
80
60
40
20
0
60
0
Figure 13. Temperature effects on voltage.
20
40
60
AMBIENT TEMPERATURE (°C)
80
Figure 14. Temperature effects on lumens.
120
100
100
5mA
RATED
LAMP
80
HOURS (1000's)
LUMINANCE PERFORMANCE (%)
140
60
40
10
20
1
0
0
2
4
6
8
LAMP CURRENT (mA)
Figure 15. Lumens output versus current.
50
10
75
100
125
150
175
% RATED LAMP CURRENT
Figure 16. Lamp life versus current.
16
200
UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
In a practical backlight design, the physical spacing between the lamp and high voltage secondary wiring with
respect to the foil reflector and LCD frame can be tight.
With this tight spacing, distributed stray capacitance will
form as shown in Fig. 17. The stray capacitance causes
leakage currents from the high voltage secondary to circuit ground. Although the current through stray capacitance doesn’t directly translate into losses, the extra
current through the transformer, primary resonant tank,
and switching devices does. A poor layout with excessive
stray capacitance can reduce system efficiency by tens
of percent. High frequency harmonics in the secondary
voltage waveform impact efficiency even further, since
capacitive reactance decreases as frequency increases.
This is why a pure sinusoid gives the best electrical to
optical efficiency, minimizing harmonic losses. Sinusoidal
waveforms require more circulating current in the resonant tank, however, lowering the electrical efficiency of
the converter.
The trade-off of electrical and optical efficiencies must be
optimized to achieve the best design. System electrical
efficiencies of 75-85% are easily achievable in a typical
UCC3972/3 based design while still maintaining good
optical conversion. Efficiencies will vary with external
component selection, input voltage, and lamp power. Fig.
18 and 19 show system electrical efficiencies versus input voltage and output power for the 375V lamp design.
CBALLAST
SECONDARY
LAMP POWER 100%
CSTRAY
CSTRAY
L
A
M
P
CSTRAY
POSITIVE COLUMNS 65%
CSTRAY
ELECTRODE
LOSS 35%
HEAT LOSS
26%
HEAT LOSS 85%
CSTRAY
Hg VISIBLE
LIGHT 3%
UV RADIATION
36%
VISIBLE LIGHT
15%
UDG-98165
Figure 17. Lamp and stray capacitor losses.
Figure 18. Design example efficiency vs. input
voltage at 2W.
Figure 19. Design efficiency vs. output power.
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 • FAX (603) 424-3460
17
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