Rainbow-electronics MAX8759 Bedienungsanleitung PDF herunterladen (Seite 15)

MAX8759

Low-Cost, SMBus, CCFL Backlight Controller

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Figure 3. MAX8759 SMBus and Ambient-Light-Sensor Input Block

ALS

STATUS

ALS

LOW-LIMIT

ALS

HIGH-LIMIT

ALS

CLAMP

BRIGHT

CONTROL

DEVICE

CONTROL

MUX

DIGITAL

MULTIPLIER

BUFFER

DIGITAL

POT

SMBus

INTERFACE

DPWM

SETTING

PWMI

SDA

SCL

0X04 0X00 0X010X060X05

MUX

"1"

PWMO

ALS

SMBus AND AMBIENT-LIGHT-SENSOR INPUT BLOCK

INVERTER

ON/OFF

MUX

PWM_SEL

ALS_CTL

PWM_MD

BUFFER

FAULT/

STATUS

0X02

OFFSET

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

is the primary series

capacitor, C

’ is the series capacitance reflected to the

secondary, C

is the secondary parallel capacitor, N is

the transformer turns ratio, L is the transformer sec-

ondary leakage inductance, and R

is an idealized

resistance that models the CCFL in normal operation.

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

and f

, in the fre-

quency response. The first peak f

is the series reso-

nant peak determined by the secondary leakage

inductance (L) and the series capacitor reflected to the

secondary (C’

2LC

′

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 30 31

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