Analog Integrated Circuit Device Data

16

Freescale Semiconductor

34844

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

reach instability problems. This will allow detecting another

failure from the connected channels. (See Figure 5)

Figure 5. Single Channel Disconnect Circuit.

For applications where multiple failure detection is

required, then one 100 kohm resistor must be placed from

each channel to a diode (D2) connected to VDC2. The

resistor will provide a pull up voltage to the disconnected

channels so that they do not interfere with the DHC circuit.

The diode (D2) ensures that when the connected channels

are in PWM off state the LED strings do no conduct current

to VDC2. (See Figure 6)

Figure 6. Resistor/Diode placement for multiple open

circuit detection

If the fail pin cannot be cleared by software then it indicates

that the failure is because of t an over-current in the Boost.

Since this is a critical failure the only way to clear it is by

releasing the part from the over-current condition and then

shutdown the part (Refer to Table 4)

If I2C communication is not present, FAIL condition should

be reset by removing the failure and re-enabling the device

thru the EN pin.

OPTICAL AND TEMPERATURE CONTROL LOOP

The 34844 supports both optical and temperature loop

control.

For temperature loop control, the LED brightness can be

adjusted depending on the temperature of the LEDs.

For optical loop control, the 34844 supports both optical

closed loop backlight control, where the brightness of the

backlight is maintained at a required level by adjusting the

light output, until the desired level is achieved, or with

ambient light control, where the backlight brightness

increases as ambient light increases.

Both temperature and optical loops are supported through

the PIN and NIN pins. Each pin supports a 0-2.048 V input

range which affects the current through the LEDs. The PIN

pin increases current as the voltage rises from 0-2.048 V.

The NIN pin reduces current as the voltage rises from 0-

2.048 V.

A 10.2 k resistor or higher value must be used at the ISET

pin if the part is configured to use PIN+NIN control loop

functionality, the 50 mA maximum current is achieved at the

higher allowed level of PIN/NIN pins, ensuring the maximum

current of the LED Drivers are not exceeded.

The optical and temperature control loop can be disabled

maximum current is set to 50 mA by using a 5.1 k resistor at

the ISET pin.

Current on LED Channel (PIN mode)

Eqn. 3

Current on LED Channel (NIN mode)

Eqn. 4

Current on LED Channel (PIN+NIN mode)

Eqn. 5

LED FAILURE PROTECTION

Open LED Protection

If LED fails open in any of the LED strings, the voltage in

that channel will be pulled close to zero, which will cause the

channel to be disabled. As a result, the boost output voltage

Current A

[]

VPIN

ICH RegisterValue

[]

×

()

RSET ohms

[]

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=

Current A

[]

2.048 VNIN

–

()

ICH RegisterValue

[]

×

RSET ohms

[]

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=

Current A

[]

2.048 VNIN

–

VPIN

+

()

ICH RegisterValue

[]

×

RSET ohms

[]

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=