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ML4876 Datasheet(PDF) 8 Page - Micro Linear Corporation |
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ML4876 Datasheet(HTML) 8 Page - Micro Linear Corporation |
8 / 9 page ML4876 8 CONTRAST CONTROL GENERATOR The contrast voltage generator is a separate regulator in a flyback configuration. In conjunction with the external transformer (T3), MOSFET (Q1), diode (D1), and assorted capacitors and resistors, it provides an adjustable DC output contrast voltage necessary to drive LCD screens. The voltage is adjusted by controlling the voltage applied to R5 at the contrast adjustment point. The contrast voltage can be made either positive or negative simply by changing the connection of the external components. The schematic shown in Figure 1 is connected for a negative voltage. Please refer to Application Note 32 for the circuit connection for a positive output voltage. OSCILLATOR The frequency of the oscillator in the ML4876 is set by selecting the value of CT. Figure 4 shows the oscillator frequency versus the value of CT. This nomograph may be used to select the appropriate value of CT to achieve the desired oscillator frequency. LINEAR REGULATOR A linear voltage regulator is provided to power the low voltage and low current control circuitry on the ML4876. This is typically used when there is no separate 5V supply available at the inverter board. For operation up to 18V the linear regulator is used by connecting HVDD to the input battery voltage. For operation over 18V, a MOSFET and a resistor (Q2 and R1, Figure 1) are connected as shown. The MOSFET is required to stand off the high voltage. LAMP OUT DETECT In those cases when there is no lamp connected, or the connection is faulty, the output voltage of the lamp driver circuit will tend to rise to a high level in an attempt to start the nonexistent lamp. The lamp out detect circuit on the ML4876 will detect this condition by sensing the center tap voltage on the primary of the output transformer (T1) on the L RTD pin. When this voltage exceeds 16V, an internal latch is set and the lamp driver goes into a shutdown mode. The logic control pin L ON must be cycled low, then high to reset the latch and return the lamp driver to the normal state. LOGIC CONTROL The ML4876 is controlled by a two logic inputs, L ON and ON/OFF. A logic level high on the L ON pin enables just the lamp driver. A logic zero on the L ON pin disable the lamp driver only. A logic level high on the ON/OFF pin enable the complete circuit. A logic level low on the ON/OFF pin puts the circuit into a very low power state. POWER SEQUENCING It is important to observe correct power and logic input sequencing when powering up the ML4876. The following procedure must be observed to avoid damaging the device. 1. Apply the battery power to HVDD 2. Apply the VDD voltage (if HVDD is not used). With HVDD connected this voltage is supplied by the internal regulator on the ML4876. 3. Apply a logic high to the ON/OFF input. This will enable the internal linear regulator to ensure the VDD supply is on (when HVDD is used). 4. Apply a logic high to the L ON input. FUNCTIONAL DESCRIPTION (Continued) 200KHz 100KHz 70KHz 50KHz 40KHz 30KHz CT 20pF 30pF 40pF 50pF 70pF 100pF 200pF Figure 4. Frequency vs. CT |
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Similar Description - ML4876 |
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