MAX8759 Maxim Integrated Products, MAX8759 Datasheet - Page 15

• Manufacturer Part Number: MAX8759
• Description: CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

A simplified CCFL inverter circuit is shown in Figure 5
(a). The full-bridge power stage is simplified and repre-
sented as a square-wave AC source. The resonant tank
circuit can be further simplified to Figure 5(b) by
removing the transformer. C
capacitor, C
secondary, C
the transformer turns ratio, L is the transformer sec-
ondary leakage inductance, and R
resistance that models the CCFL in normal operation.
Figure 3. MAX8759 SMBus and Ambient-Light-Sensor Input Block
PWMI
SDA
ALS
SCL
PWMO
Low-Cost, SMBus, CCFL Backlight Controller
BUFFER
S
BUFFER
DIGITAL
’ is the series capacitance reflected to the
P
POT
is the secondary parallel capacitor, N is
0X02
______________________________________________________________________________________
REGISTER
STATUS
FAULT/
A
D
0X04
REGISTER
OFFSET
STATUS
S
ALS
is the primary series
A
D
L
0X05
is an idealized
LOW-LIMIT
REGISTER
ALS
INTERFACE
CLAMP
ALS
SMBus
0X06
HIGH-LIMIT
REGISTER
ALS
SMBus AND AMBIENT-LIGHT-SENSOR INPUT BLOCK
Figure 6 shows the frequency response of the resonant
tank’s voltage gain under different load conditions. The
primary series capacitor is 1µF, the secondary parallel
capacitor is 15pF, the transformer turns ratio is 1:93,
and the secondary leakage inductance is 260mH.
Notice that there are two peaks, f
quency response. The first peak f
nant peak determined by the secondary leakage
inductance (L) and the series capacitor reflected to the
secondary (C’
0X00
"1"
CONTROL
REGISTER
BRIGHT
PWM_MD
MUX
0X01
CONTROL
REGISTER
S
DEVICE
MULTIPLIER
):
DIGITAL
MUX
f
S
ALS_CTL
=
2
π
INVERTER
ON/OFF
1
LC
′
S
S,
S
MUX
is the series reso-
and f
PWM_SEL
P
, in the fre-
DPWM
SETTING
15