FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
reach instability problems. This will allow detecting another
failure from the connected channels. (See Figure 5)
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.
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)
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
by I2C setting bits (PINEN & NINEN), or by tying PIN and NIN
pins high (>2.2 V) it is called VSET mode, and the LED Driver
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
(VPIN × ICH[RegisterValue])
Current[A] = ---------------------------------------------------------------------------------------
RSET[ohms]
Current on LED Channel (NIN mode)
Eqn. 4
(2.048 – VNIN) × ICH[RegisterValue]
Current[A] = ------------------------------------------------------------------------------------------------------------
RSET[ohms]
Current on LED Channel (PIN+NIN mode)
Eqn. 5
(2.048 – VNIN + VPIN) × ICH[RegisterValue]
Current[A] = ----------------------------------------------------------------------------------------------------------------------------------
RSET[ohms]
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.
LED FAILURE PROTECTION
Open LED Protection
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 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
34844
Analog Integrated Circuit Device Data
Freescale Semiconductor
16
FREESCALE [ Freescale ]
