Rainbow Electronics MAX1839 - Manual (Page 14)

MAX1739/MAX1839
†
Wide Brightness Range
CCFL Backlight Controllers
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brightness code of ob00000 corresponds to a 9.375%
DPWM duty cycle, and a brightness code of ob11111
corresponds to a 100% DPWM duty cycle. The duty
cycle changes by 3.125% per step, except codes
ob00000 to ob00011 all produce 9.375% (Figure 3).
To disable DPWM and always use 100% duty cycle, set
V
MINDAC to VL. Note that with DPWM disabled, the
equations above should assume V MINDAC = 0 instead
of V MINDAC = VL. Table 2 lists MINDAC’s functionality,
and Table 3 shows some typical settings for the bright-
ness adjustment.
In normal operation, V MINDAC is set between 0 and
V REF , and the MAX1739/MAX1839 use both lamp cur-
rent control and DPWM control to vary the lamp bright-
ness (Figure 4). In this mode, lamp current control
regulates the average lamp current during a DPWM on-
cycle and not the overall average lamp current.
Analog Interface and Brightness Code
The MAX1739/MAX1839 analog interface uses an inter-
nal ADC with 1-bit hysteresis to generate the brightness
code used to dim the lamp (see Dimming Range).
CTL/SDA is the ADC’s input, and CRF/SCL is its refer-
ence voltage. The ADC can operate in either positive-
scale ADC mode or negative-scale ADC mode. In
positive-scale ADC mode, the brightness code increas-
es from 0 to 31 as V CTL increases from 0 to V CRF . In
negative-scale mode, the brightness scale decreases
from 31 to 0 as V
CTL increases from 0 to V CRF (Figure 5).
The analog interface’s internal ADC uses 1-bit hystere-
sis to keep the lamp from flickering between two codes.
V
CTL ’s positive threshold (V CTL(TH) ) is the voltage
required to transition the brightness code as V CTL
increases and can be calculated as follows:
V CTL(TH) = (n + 2) / 33 V CRF
(positive-scale ADC mode, MODE = GND)
V CTL(TH) = (33 - n) / 33 V CRF
(negative-scale ADC mode, MODE = REF)
where n is the current selected brightness code. V CTL ’s
negative threshold is the voltage required to transition
the brightness code as V
CTL decreases and can be
calculated as follows:
V CTL(TH) = n / 33 V CRF
(positive-scale ADC mode, MODE = GND)
V CTL(TH) = (31 - n) / 33 V CRF
(negative-scale ADC mode, MODE = REF)
Figure 5 shows a graphic representation of the thresh-
olds. CRF/SDA’s and CTL/SCL’s input voltage range is
2.7V to 5.5V.
MINDAC = VL
DPWM disabled (always on 100% duty cycle). Operates in lamp current control only.
(Use V
MINDAC = 0 in the equations.)
MINDAC = REF
DPWM control enabled, duty cycle ranges from 9% to 100%. Lamp current control is disabled
(always maximum current).
0 ≤ V
MINDAC < VREF The device uses both lamp current control and DPWM.
Table 2. MINDAC Functionality
0
10
20
30
60
70
40
50
80
90
100
0 12 4 8 20 16 24 28 32
BRIGHTNESS CODE
DPWM DUTY CYCLE (%)
DPWM SETTINGS
Figure 3. DPWM Settings
0
10
20
30
60
70
40
50
80
90
100
0 12 4 8 20 16 24 28 32
BRIGHTNESS CODE
COMBINED POWER LEVEL (%)
COMBINED POWER LEVEL
(BOTH DPWM AND
LAMP CONTROL CURRENT)
Figure 4. Combined Power Level