Abstract: The DS3881, DS3882, DS3984, DS3988, DS3991, DS3992, and DS3994 are CCFL (cold-cathode fluorescent lamp)controllers that backlight LCDs (liquid crystal displays). Each controller channel has two input pins for lamp monitoring and current regulation, LCM and OVD. There are four thresholds associated with these two inputs. This application note describes how the thresholds are specified and how to determine correct threshold values based on the application.
<-- ======================================================================= --><-- CONTENT: DB HTML --><-- ======================================================================= -->
Each channel of the DS3881, DS3882, DS3984, DS3988, DS3991, DS3992, and DS3994 CCFL (cold-cathode fluorescent lamp) controllers has two inputs, LCM for lamp current monitoring and OVD for overvoltage detection. There are four thresholds associated with LCM and OVD. Table 1 summarizes the description and symbol for each threshold.
Table 1. Threshold Description and Symbols
|LCM||VLOT||Lamp off threshold|
|VLOC||Lamp overcurrent threshold|
|VLRT||Lamp regulation threshold|
|OVD||VOVDT||Lamp overvoltage threshold|
Since the threshold values can differ depending on the reference used, the user needs to correctly determine the reference (either VDCB or ground) and thus select the correct external component values. The general rule is to use VDCB as the reference if the LCM and OVD signals are AC coupled. This is typically the case for a single-lamp-per-channel application. However, the thresholds should be referenced to ground if no AC coupling is used. This guideline typically applies to applications using multiple lamps per channel applications. Two examples explain this principle.
In this application the internal DC bias voltage at the LCM and OVD pins allows for an AC-coupled input. This design can make the external circuitry very simple. A typical operating circuit for this application is shown in Figure 1.
Figure 1. Typical operating circuit with a single lamp per channel.
In this application, the thresholds are referenced to VDCB. The values are listed in Table 2 and may be different for other parts.
Table 2. DS3994 LCM and OVD Thresholds (for Figure 1)
|Lamp off threshold||VLOT||0.30||0.40||0.50||V|
|Lamp overcurrent threshold||VLOC||1.80||2.00||2.20||V|
|Lamp regulation threshold||VLRT||0.94||1.00||1.06||V|
Figure 2 shows a typical operating circuit with multiple lamps per channel using the DS3994. In this application, the lamp currents and voltages must be wire-ORed and fed to the LCM and OVD inputs on the DS3994. Some external circuitry, including voltage-dividers and peak detectors, is used. Unlike the application with a single lamp per channel, the application with multiple lamps per channel does not use an AC-coupling capacitor at the LCM input. The DS3994 controls the lamp current based on the peak signal measured at the LCM input. With no AC-coupling capacitor, the peak control level will be the DC-bias voltage (1.35V) plus the lamp-regulation threshold (1.0V) or 2.35V nominal. Hence, the peak voltage level created by the lamp's current-feedback resistor must be attenuated to a target value of 2.35Vpeak at the LCM input so the device can control the lamp current to the proper level. Similarly, the OVD threshold in this application is 2.35Vpeak with respect to ground. More detail can be found in Maxim's application note 3615, "DS3984/DS3988 Multiple Lamp Drive Scheme."
Figure 2. Typical operating circuit with multiple lamps per channel.
The thresholds used in Figure 2 are summarized in Table 3, voltages with respect to ground. These values may differ for other parts.
Table 3. DS3994 LCM and OVD Thresholds (for Figure 2)
|Lamp off threshold||VLOT||1.65||1.75||1.85||V|
|Lamp overcurrent threshold||VLOC||3.15||3.35||3.55||V|
|Lamp regulation threshold||VLRT||2.29||2.35||2.41||V|