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MC34844A
FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
DISABLING LED CHANNELS
The 34844A allows the user to enable and disable each of
the 10 channels separately by writing the corresponding
CHENx bit on Registers 08 and 09 thru I2C.
Since the enable and disable functions reconnects the
feedback circuit of the LED drivers, this shouldn’t be used on
any channel that shuts down either because an open LED
functionality, the 80 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 I 2 C setting bits (PINEN & NINEN), or by tying PIN and NIN
pins high (>2.2 V). The LED Driver maximum current is set to
80 mA by using a 3.48 kohm resistor at the ISET pin.
IDIM [ A ] = ISINK [ A ] × ------------------------
×
( VSET – VNIN ) [ V ]
channel condition or because is was previously POWER
OFF. This could cause instability issues since the voltage on
this open LED driver is not substantially above the DHC
regulation voltage (0.75 V typ) and may interfere with the
operation of the dynamic headroom control (DHC) which can
lead to erratic output voltage regulation
FAIL PIN
Current on LED Channel (PIN mode)
VPIN [ V ]
2
Current on LED Channel (NIN mode)
IDIM [ A ] = ISINK [ A ] ----------------------------------------------------
2
Current on LED Channel (PIN+NIN mode)
Eqn. 8
Eqn. 9
Eqn. 10
( VSET + VPIN – VNIN ) [ V ]
If a LED fails open in any of the LED strings, the voltage in
that particular LED channel will be close to ground and the
IDIM [ A ] = ISINK [ A ]
×
--------------------------------------------------------------------------
2
LED open failure is detected. When this happens, a failure is
registered, the FAIL pin is set to its high-impedance stage,
and the channel is shut down.
The FAIL pin cannot be cleared for manual mode unless a
complete power on reset is applied.
However for I 2 C/SMBUS mode, the FAIL pin can be
cleared by cycling the clear fail bit (CLRFAIL bit = 0 - 1 - 0).
This allows the user to waive any known failure and set the
device for being able to detect any other failure during
operation.
If the fail pin cannot be cleared by software then it indicates
that the failure is because of 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
If I 2 C 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 34844A 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 34844A 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 V to V SET
(2.048 V typ.) input range which affects the current through
the LEDs. The PIN pin increases current as the voltage rises
from 0 to V SET . The NIN pin reduces current as the voltage
rises from 0 - V SET .
A 6.98 kohm resistor or higher value must be used at the
ISET pin if the part is configured to use PIN+NIN control loop
VPIN and VNIN is the voltage applied on PIN and NIN pins
correspondingly.
For ISINK formula please refer to Equation 1 .
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
will go to the OVP level and then come down to the regulation
level to continue powering the rest of the LED strings.
Short LED Protection
If an LED shorted in any of the LED strings, the device will
continue to operate without interruption. However, if the
shorted LED happens to be in the LED string with the highest
forward voltage, the DHC circuit will automatically regulate
the output voltage with respect to the new highest LED
voltage. If more LEDs are shorted in the same LED string, it
may cause excessive power dissipation in the channel which
may cause the OTT circuit to trip which will completely
shutdown the device.
OVER-TEMPERATURE PROTECTION
The 34844A has an on-chip temperature sensor that
measures die temperature. If the IC temperature exceeds the
OTT threshold, the IC will turn off all power sources inside the
IC (LED drivers, boost and internal regulators) until the
temperature falls below the falling OTT threshold. Once it
comes back on, it will operate with the default configuration
(refer to Table 14 ).
SERIAL INTERFACE CONTROL
The 34844A uses an I 2 C interface capable of operating in
standard (100 kHz) or fast (400 kHz) modes.
The A0/SEN pin can be used an address select pin to
allow more than 2 devices in the system. The A0/SEN pin
should be held low on all chips expect the one to be
addressed, where it is taken HIGH.
34844A
Analog Integrated Circuit Device Data
Freescale Semiconductor
47
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