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MC34844
FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
Table 5. Operation Current Consumption Modes
Mode
Manual
SM Bus
EN Pin
Low
High
Low
Low
Low
Low
SCK/SDA Pins
Low
Low
Low (> 27 ms)
Active
Active
X
I 2 C Bit Command
N/A
N/A
EN bit = X
EN bit = 0
EN bit = 1
SETI2C bit = 1
CLRI2C bit = 0
Current Consumption
Mode
Shutdown
Operational
Shutdown
Sleep
Operational
I 2 C Low Power
(Shutdown)
Comments
Part Doesn’t
Wake-up
EN bit = X
SETI2C bit = 1
I 2 C
High
X
CLRI2C bit = 0
Sleep
EN bit = 0
SETI2C bit = 1
High
X
CLRI2C bit = 0
Operational
EN bit = 1
BOOST
The integrated boost converter operates in non-
synchronous mode and integrates a 2.5 A FET. An integrated
sense circuit is used to sense the voltage at the LED current
mirror inputs and automatically sets the boost output voltage
(DHC) to the minimum voltage needed to keep all LEDs
biased with the required current. The DHC is designed to
operate under specific pulse width conditions in the LED
drivers. It operates for pulse widths higher than 4.0 μ s
If the pulse widths are shorter than specified, the DHC
circuit will not operate and the voltage across the LED drivers
will increase to a value given by the OVP minus the total LED
voltage in the LED string. Therefore it is imperative to select
the proper OVP level to minimize power dissipation.
The OVP can be set from 11 to 62 V, ~4.0 V spaced, using
the I 2 C interface (OVP Register). If I 2 C capability is not
present, the OVP can be controlled by a resistor divider
connected from VOUT to GND with its mid point tied to A0/
SEN pin (threshold = 6.5 V). During an OVP condition, the
output voltage will go to the OVP level which is programmed
via the I 2 C interface or settled by a resistor divider on A0/SEN
pin, or by a zener diode. The formulas to calculate the
hardware OVP using any of the two methods are as follows:
HARDWARE OVP :
The OVP value should be set to greater than the maximum
LED voltage over the whole temperature range. A good
practice is to set it 5.0 V or so above the max LED voltage.
The boost converter also features internal Over-current
Protection (OCP) and has a user programmable Over-
voltage Protection (OVP).
The OCP operates on a cycle by cycle basis. However, if
the OCP condition remains for more than 10 ms then the
device turns off the LED Drivers, the Boost goes to Sleep
mode and the output FAULT pin goes into high-impedance.
The device can only be restarted by recycling the enable or
creating a Power On Reset (POR).
The user can program the boost frequency by I 2 C
(BST[1:0]) only after the IC is powered up and before the
boost circuit is turned ON for the first time (PWM pin low to
high). This sequence avoids boost frequency to be changed
inadvertently during operation. The first I 2 C command has to
wait for 5.0 ms after the part is turned ON, in order to allow
sufficient time for the device power up sequence to be
completed.
The boost controller has an integral track and hold
amplifier with indefinite hold time capability, to enable
Method 1
V OUT
Method 2
immediate LED on cycles after extended off times. During
extended off times, the external LEDs cool down from their
normal quiescent operating temperature and thereby
RUPPER
A0/SEN
VZENER2
experience a forward voltage change, typically an increase in
the forward voltage. This change can be significant for
A0/SEN
RLOWER
OVP = V ZENER2 + 6.5 V
applications with a large number of series LEDs in a string
operating at high current. If the boost controller did not track
OVP = 6.5 V [(R UPPER / R LOWER ) + 1] + (100E-6 x R UPPER )
this increased change, the potential on the LED drivers would
saturate for a few cycles once the LED channels are re-
enabled.
34844
Analog Integrated Circuit Device Data
17
Freescale Semiconductor
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