10/94
BLOCK DIAGRAM
1
A ICC when Disabled
PWM Control for LCD Supply
Zero Voltage Switched (ZVS) on
Push-Pull Drivers
Open Lamp Detect Circuitry
4.5V to 20V Operation
Non-saturating Transformer
Topology
Smooth 100% Duty Cycle on
Buck PWM and 0% to 95% on
Flyback PWM
The UC1871 Family of IC's is optimized for highly efficient fluorescent lamp
control. An additional PWM controller is integrated on the IC for applications re-
quiring an additional supply, as in LCD displays. When disabled the IC draws
only 1
A, providing a true disconnect feature, which is optimum for battery
powered systems. The switching frequency of all outputs are synchronized to
the resonant frequency of the external passive network, which provides Zero
Voltage Switching on the Push-Pull drivers.
Soft-Start and open lamp detect circuitry have been incorporated to minimize
component stress. An open lamp is detected on the completion of a soft-start
cycle.
The Buck controller is optimized for smooth duty cycle control to 100%, while
the flyback control ensures a maximum duty cycle of 95%.
Other features include a precision 1% reference, under voltage lockout, flyback
current limit, and accurate minimum and maximum frequency control.
Resonant Fluorescent Lamp Driver
FEATURES
DESCRIPTION
UDG-92061-1
UC1871
UC2871
UC3871
Note: Pin numbers refer to DIL-18 and SOIC-18 packages only.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Reference Section
Output Voltage
T
J
=25
C
2.963
3.000
3.037
V
Overtemp
2.940
3.000
3.060
V
Line Regulation
V
CC
= 4.75V to 18V
10
mV
Load Regulation
Io=0 to -5mA
10
mV
Oscillator Section
Free Running Freq
T
J
=25
C
57
68
78
kHz
Max Sync Frequency
T
J
=25
C
160
200
240
kHz
Charge Current
V
CT
= 1.5V
180
200
220
A
Voltage Stability
2
%
Temperature Stability
4
8
%
Zero Detect Threshold
0.46
0.5
0.56
V
Error Amp 1 Section
Input Voltage
V
O
= 2V
1.445
1.475
1.505
V
Input Bias Current
-0.4
-2
A
Open Loop Gain
V
O
= 0.5 to 3V
65
90
dB
Output High
V
EA
(-) = 1.3V
3.1
3.5
3.9
V
Output Low
V
EA
(-) = 1.7V
0.1
0.2
V
Output Source Current
V
EA
(-) = 1.3V, Vo = 2V
-350
-500
A
Output Sink Current
V
EA
(-) = 1.7V, Vo = 2V
10
20
mA
Common Mode Range
0
V
IN
-1V
V
Unity Gain Bandwidth
T
J
= 25
C (Note 4)
1
MHz
Maximum Source Impedance
Note 5
100k
UC1871
UC2871
UC3871
ELECTRICAL CHARACTERISTICS
Unless otherwise stated, these parameters apply for T
A
= -55
C to +125
C for the
UC1871; -25
C to +85
C for the UC2871; 0
C to +70
C for the UC3871; V
CC
= 5V,
V
C
= 15V, V
ENABLE
= 5V, C
T
= 1nF, Zero Det = 1V.
ABSOLUTE MAXIMUM RATINGS
Analog Inputs . . . . . . . . . . . . . . . . . . . . . . . . .
-
0.3 to +10V
V
CC
, V
C
Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +20V
Zero Detect Input Current
High Impedance Source . . . . . . . . . . . . . . . . . . +10mA
Zero Detect
Low Impedance Source. . . . . . . . . . . . . . . . . . . . . +20V
Power Dissipation at T
A
= 25
C . . . . . . . . . . . . . . . . . . . 1W
Storage Temperature . . . . . . . . . . . . . . . . -65
C to +150
C
Lead Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 300
C
Note 1: Currents are positive into, negative out of the specified ter-
minal.
Note 2: Consult Packaging Section of Databook for thermal
limitations and considerations of package.
DIL-18, SOIC-18 (TOP VIEW)
J or N, DW Package
PACKAGE PIN FUNCTION
FUNCTION
PIN
Gnd
1
B Out
2
A Out
3
V
C
4
E/A 1 Comp
5
SS
6
E/A 1(-)
7
N/C
8
C
T
9
Zero Detect
10
N/C
11
V
REF
12
E/A 2(+)
13
E/A 2(-)
14
E/A 2 Comp
15
V
CC
16
Enable
17
Flyback I
SENSE
18
D Out
19
C Out
20
PLCC-20 (Top View)
Q Package
CONNECTION DIAGRAMS
2
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Open Lamp Detect Section
Soft Start Threshold
V
EA
(-) = 0V
2.9
3.4
3.8
V
Error Amp Threshold
V
SS
= 4.2V
0.7
1.0
1.3
V
Soft Start Current
V
SS
= 2V
10
20
40
A
Error Amp 2 Section
Input Offset Voltage
V
O
= 2V
0
10
mV
Input Bias Current
-0.2
-1
A
Input Offset Current
0.5
A
Open Loop Gain
V
O
= 0.5 to 3V
65
90
dB
Output High
V
ID
= 100mV, V
O
= 2V
3.6
4
4.4
V
Output Low
V
ID
= -100mv, V
O
= 2V
0.1
0.2
V
Output Source Current
V
ID
= 100mV, V
O
= 2V
-350
-500
A
Output Sink Current
V
ID
= -100mV, V
O
= 2V
10
20
mA
Common Mode Range
0
V
IN
-2V
V
Unity Gain Bandwidth
T
J
= 25
C (Note 4)
1
MHz
Isense Section
Threshold
0.475
0.525
0.575
V
Output Section
Output Low Level
I
OUT
= 0, Outputs A and B
0.05
0.2
V
I
OUT
= 10mA
0.1
0.4
V
I
OUT
= 100mA
1.5
2.2
V
Output High Level
I
OUT
= 0, Outputs C and D
14.7
14.9
V
I
OUT
= -10mA
13.5
14.3
V
I
OUT
= -100mA
12.5
13.5
V
Rise Time
T
J
= 25
C, Cl = 1nF(Note 4)
30
80
ns
Fall Time
T
J
= 25
C, Cl = 1nF(Note 4)
30
80
ns
Output Dynamics
Out A and B Duty Cycle
48
49.9
50
%
Out C Max Duty Cycle
V
EA
1(-) = 1V
100
%
Out C Min Duty Cycle
V
EA
1(-) = 2V
0
%
Out D Max Duty Cycle
V
EA
2(+)- V
EA
2(-) = 100mV
92
96
%
Out D Min Duty Cycle
V
EA
2(+)- V
EA
2(-) = -100mV
0
%
Under Voltage Lockout Section
Start-Up Threshold
3.7
4.2
4.5
V
Hysterisis
120
200
280
mV
Enable Section
Input High Threshold
2
V
Input low Threshold
0.8
V
Input Current
V
ENABLE
= 5V
150
400
A
Supply Current Section
VCC Supply Current
V
CC
= 20V
8
14
mA
VC Supply Current
V
C
=20V
7
12
mA
ICC Disabled
V
CC
= 20V, V
ENABLE
= 0V
1
10
A
UC1871
UC2871
UC3871
ELECTRICAL
CHARACTERISTICS (cont.)
Unless otherwise stated, these parameters apply for T
A
= -55
C to +125
C for the UC1871;
-25
C to +85
C for the UC2871; 0
C to +70
C for the UC3871; V
CC
= 5V, V
C
= 15V,
V
ENABLE
= 5V, C
T
= 1nF, Zero Det = 1V.
Note 3: Unless otherwise specified, all voltages are with respect to ground.
Currents are positive into, and negative out of the specified terminal.
Note 4: Guaranteed by design but not 100% tested in production.
Note 5: Impedance below specified maximum guarantees proper operation of the Open Lamp Detect.
3
Figure 1
UC1871
UC2871
UC3871
Figure 1 shows a complete application circuit using the
UC3871 Resonant Fluorescent lamp and LCD driver.
The IC provides all drive, control and housekeeping func-
tions to implement CCFL and LCD converters. The buck
output voltage (transformer center-tap) provides the zero
crossing and synchronization signal. The LCD supply
modulator is also synchronized to the resonant tank.
The buck modulator drives a P-channel MOSFET di-
rectly, and operates over a 0-100% duty-cycle range. The
modulation range includes 100%, allowing operation with
minimal headroom. The LCD supply modulator also di-
rectly drives a P-channel MOSFET, but it's duty-cycle is
limited to 95% to prevent flyback supply foldback.
The oscillator and synchronization circuitry are shown in
Figure 2. The oscillator is designed to synchronize over a
3:1 frequency range. In an actual application however,
the frequency range is only about 1.5:1. A zero detect
comparator senses the primary center-tap voltage, gen-
erating a synchronization pulse when the resonant wave-
form falls to zero. The actual threshold is 0.5 volts,
providing a small amount of anticipation to offset propa-
gation delay.
The synchronization pulse width is the time that the 4mA
current sink takes to discharge the timing capacitor to 0.1
volts. This pulse width sets the LCD supply modulator
minimum off time, and also limits the minimum linear
control range of the buck modulator. The 200
A current
source charges the capacitor to a maximum of 3 volts. A
comparator blanks the zero detect signal until the capaci-
tor voltage exceeds 1 volt, preventing multiple synchroni-
zation pulse generation and setting the maximum
frequency. If the capacitor voltage reaches 3 volts (a zero
detection has not occurred) an internal clock pulse is
generated to limit the minimum frequency.
TYPICAL APPLICATION
APPLICATION INFORMATION
4
A unique protection feature incorporated in the UC3871
is the Open Lamp Detect circuit. An open lamp interrupts
the current feedback loop and causes very high secon-
dary voltage. Operation in this mode will usually break-
down the transformer's insulation, causing permanent
damage to the converter. The open lamp detect circuit,
shown in Figure 3 senses the lamp current feedback sig-
nal at the error amplifiers input, and shuts down the out-
puts if insufficient signal is present. Soft-start circuitry
limits initial turn-on currents and blanks the open lamp
detect signal.
Other features are included to minimize external circuitry
requirements. A logic level enable pin shuts down the IC,
allowing direct connection to the battery. During shut-
down, the IC typically draws less than 1
A. The UC3871,
operating from 4.5V to 20V, is compatible with almost all
battery voltages used in portable computers. Under-volt-
age lockout circuitry disables operation until sufficient
supply voltage is available, and a 1% voltage reference
insures accurate operation. Both inputs to the LCD sup-
ply error amplifier are uncommitted, allowing positive or
negative supply loop closure without additional circuitry.
The LCD supply modulator also incorporates cycle-by-
cycle current limiting for added protection.
Figure 3
UC1871
UC2871
UC3871
UNITRODE INTEGRATED CIRCUITS
7 CONTINENTAL BLVD.
MERRIMACK, NH 03054
TEL. (603) 424-2410
FAX (603) 424-3460
APPLICATION INFORMATION (cont.)
Figure 2
UC1871 Open Lamp Detect Circuitry
UC3871 Oscillator Section
5
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Copyright
1999, Texas Instruments Incorporated
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