MAX8759 Maxim Integrated Products, MAX8759 Datasheet - Page 18

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

Low-Cost, SMBus, CCFL Backlight Controller
The MAX8759 controls the brightness of the CCFL by
“chopping” the lamp current on and off using a low-fre-
quency (between 100Hz and 350Hz) DPWM signal.
The frequency of the internal DPWM oscillator is
adjustable through a resistor connected between the
FREQ pin and GND. The CCFL brightness is propor-
tional to the DPWM duty cycle, which can be adjusted
from 10.15% to 100%.
In DPWM operation, the COMP voltage controls the
dynamics of the lamp-current envelope. At the begin-
ning of the DPWM ON cycle, the average value of the
lamp-current feedback signal is below the regulation
point, so the transconductance error amplifier sources
current into the COMP capacitor. The switch on-time
(t
vides soft-start. At the end of the DPWM ON cycle, the
MAX8759 turns on a 110µA internal current source. The
current source linearly discharges the COMP capacitor,
gradually decreasing t
The DPWM frequency can be set with an external resis-
tor. Connect a resistor between FREQ and GND. The
DPWM frequency is given by the following equation:
The adjustable range of the DPWM frequency is
between 100Hz and 350Hz (R
and 350kΩ).
The MAX8759 has three ways for brightness control.
The brightness can be controlled by a 2-wire serial
interface (SMBus), by an external PWM signal, or by an
external ambient-light sensor signal. There are five
operating modes, which can be selected by setting bits
1 to 3 in device control register 0x01 (see the SMBus
Register Definitions section for details).
The MAX8759 can work with several types of ambient-
light sensors. The ideal ambient-light sensors should
have a linear response to ambient light and should
have a spectral response equivalent to that of the
human eye. Ambient-light sensors must provide filtering
of low-frequency harmonics found in the electrical
spectrum of the many light sources. The ALS’s output
should be a DC analog voltage that is linearly propor-
tional to the ambient luminance.
In ALS mode, the MAX8759 sets the brightness based
on the analog voltage on the ALS pin. The ALS pin is
connected to the output of an external ambient-light
sensor. The usable input-voltage range of the ALS pin
18
ON
) gradually increases as V
______________________________________________________________________________________
f
DPWM
=
210
ON
Hz
, and providing soft-stop.
×
169 Ω /
FREQ
COMP
k
Dimming Control
R
is between 100kΩ
FREQ
rises, which pro-
ALS Mode
is 0 to 1.8V. The MAX8759 compares the ALS input
voltage against user-programmable low and high limits.
When the ALS input voltage is below the low limit, the
brightness is clamped to the ALS low limit. When the
ALS input voltage is above the high limit, the brightness is
clamped to the ALS high limit. If the minimum ALS setting
is below 10%, the brightness is clamped to 10%. Figure 7
shows the brightness change as a function of the ALS
voltage.
The ALS input voltage is sampled every DPWM period
and is loaded in ALS status register 0x04. The analog
voltage on the ALS pin is converted into an 8-bit digital
code. The total number of brightness levels is 256. One
step change results in a 0.391% change in the DPWM
duty cycle.
In PWM mode, the MAX8759 sets the brightness based
on the duty cycle of the PWMI signal. The absolute min-
imum brightness is 10%. If the PWMI duty cycle is less
than 10%, the brightness stays at 10%. The frequency
range of the PWMI signal is between 5kHz and 50kHz
when the PWMO capacitor is 1µF.
In SMBus mode, the MAX8759 sets the brightness
based on the brightness control register (0x00). The
brightness control register contains 8 bits and supports
256 brightness levels. A setting of 0xFF for register
0x00 sets the inverter to the maximum brightness. A
setting of 0x00 for register 0x00 sets the inverter to the
minimum brightness (10%).
In ALS with DPST mode, the MAX8759 sets the bright-
ness based on the analog voltage on the ALS pin and
duty cycle at the PWMI pin. The MAX8759 lowers the
ALS brightness setting by an additional amount that is
proportional to the duty cycle of the PWMI signal. For
example, if the ALS brightness setting is 80% and the
duty cycle of PWMI signal is 60%, the resulting bright-
ness setting is 80% x 60% = 48%.
In SMBus with DPST mode, the MAX8759 sets the
brightness based on the brightness control register
(0x00). The MAX8759 lowers the SMBus brightness set-
ting by an additional amount that is proportional to the
duty cycle of the PWMI signal. For example, if the
brightness control register is set to 0x80 (correspond-
ing to 50% brightness setting) and the duty cycle of the
PWMI signal is 60%, the resulting brightness setting is
50% x 60% = 30%.
SMBus with DPST Mode
ALS with DPST Mode
SMBus Mode
PWM Mode