Rainbow Electronics MAX1839 - Manual (Page 10)

MAX1739/MAX1839
†
Wide Brightness Range
CCFL Backlight Controllers
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Detailed Description
The MAX1739/MAX1839 regulate the brightness of a
CCFL in three ways:
1) Linearly controlling the lamp current.
2) Digitally pulse-width modulating (or chopping) the
lamp current (DPWM).
3) Using both methods simultaneously for widest dim-
ming range.
DPWM is implemented by pulse-width modulating the
lamp current at a rate faster than the human eye can
detect. Figure 1 shows the current and voltage wave-
forms for the three operating modes with the brightness
control set to 50% of full scale.
The MAX1739/MAX1839 include a 5.3V linear regulator
to power most of the internal circuitry, drivers for the
buck and Royer switches, and the synchronizable
DPWM oscillator. The MAX1739/MAX1839 are very flex-
ible and include a variety of operating modes, an ana-
log interface, an SMBus interface (MAX1739 only), a
shutdown mode, lamp-out detection, and buck-switch
short detection.
Figure 1. Brightness Control Methods
V MINDAC = 0
V CTL = V CRF /2 or BRIGHT[4:0] = 10,000
(DAC SET TO MIDSCALE)
EFFECTIVE BRIGHTNESS IS:
50% IN CONTINUOUS AND DPWM CONTROL
25% IN COMBINED CONTROL
V CTAP
V
CSAV
CURRENT + DPWM CONTROL CONTINUOUS CURRENT CONTROL
DPWM CONTROL
V CTAP
TRANSFORMER
VOLTAGE
V CSAV
LAMP CURRENT