ML4876 データシートの表示（PDF） - Micro Linear Corporation

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ML4876

LCD Backlight Lamp Driver with Contrast

Micro Linear Corporation 

ML4876

FUNCTIONAL DESCRIPTION (Continued)

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.

200KHz

100KHz

70KHz

50KHz

40KHz

30KHz

20pF

30pF

40pF 50pF

CT

70pF

100pF

Figure 4. Frequency vs. CT

200pF

8

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