Table 3. spreadsheet design equations – Rainbow Electronics MAX754 User Manual
Page 11
MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
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11
Table 3. Spreadsheet Design Equations
T1 Center-Tap Peak Voltage
PARAMETER
SYMBOL
MIN
TYP
MAX
V
TAP,PK
Strike Voltage (V
S
)
V
S,RMS
1100
1500
= -SQRT(2) * I
LAMP,RMS(max)
/
(C
BAL
* w * SIN(phi)) / n
Discharging Tube Voltage (V
L
)
V
LAMP,RMS
435
Bias Voltage
V
LCD
= V
LCD(max)
/ 2
32.6
Output Current
I
LCD
0.0245
T1 Turns Ratio (Sec/Pri)
n
171
T1 Resonating Inductance
L
MAG
0.000045
C9 Value
C
RES
2.2E-07
C10 Value
C
BAL
1.5E-11
Royer Frequency
w
= SQRT [1 / (L
MAG
* C
RES
)]
Reference Voltage
V
REF
1.25
Second Volts Constant
sV
0.000008
2.4E-05
R8 Current-Sensing Resistor
R8
= PI() * V
REF
* SQRT(2) /
(2 * I
LAMP,RMS(max)
)
Secondary Voltage Phase vs. Tube
Voltage
phi
= ATAN (-I
LAMP,RMS(max)
/
(C
BAL
* w * + V
LAMP,RMS
)
Secondary Limit Voltage
V
LIM
= V
S,RMS(max)
* 0.9
T1 Center-Tap Limit Peak Voltage
= SQRT(2) * V
LIM
/ n
R5/R6
R
OTP,RATIO
= V
REF
/ (D25 - 0.6 - V
REF
)
V
IN(min)
Full-Load Switching Period
T
FL
= sV(min) / V
IN(min)
+ sV(min) /
(V
LCD(max)
- V
IN(min)
)
L2 Inductance
L2
= L2(max) * 0.8
= sV(min) ^ 2 / (2 * T
FL
*
V
LCD(max)
* I
LCD(min)
)
L2 Peak Currrent
= sV(max) / L2(min)
R4/R3
R
LCD,RATIO
= V
REF
/ (V
LCD(max)
- V
REF
)
Input Voltage
V
IN
= (2 / PI()) * V
TAP,PK
18
Discharging Tube Current (I
L
)
I
LAMP,RMS
= 0.28 *
I
LAMP,RMS(max)
0.005
CCFL Specifications
LCD Contrast Voltage Specifications
Royer Specifications
MAX754 Specifications
CCFL Circuit Calculations
LCD Circuit Calculations
Application Circuit Operating Range