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MC34844A
FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
Table 12. 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 = 0
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
PWM pin = Low
Part Doesn’t
Wake-up
EN bit = 0
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 for pulse widths > 400 ns in the LED drivers.
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 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.
Please follow this sequence in order to change the Boost
frequency thru I2C:
1. Take PWM pin low
normal quiescent operating temperature and thereby
experience a forward voltage change, typically an increase in
the forward voltage. This change can be significant for
applications with a large number of series LEDs in a string
operating at high current. If the boost controller did not track
this increased change, the potential on the LED drivers would
saturate for a few cycles once the LED channels are re-
enabled.
HARDWARE AND SOFTWARE OVP :
The OVP value should be set to a higher value 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 OVP can be set from 11 to 62 V, ~4.0 V spaced, using
the I 2 C interface (OVP Register). If the I 2 C capability is not
present, the OVP can be controlled either by a resistor divider
connected from VOUT to GND, with its mid point tied to the
A0/SEN pin, or by a zener diode from VOUT to the 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:
2. Disable the part by software (EN bit = low)
3. Write the new Boost frequency data (BST[1:0])
4. Enable the part by software (EN bit = high)
Method 1
V OUT
Method 2
V OUT
5. Reconfigure all registers
6. Take PWM pin High
The boost controller has an integral track and hold
A0/SEN
RUPPER
RLOWER
A0/SEN
VZENER2
OVP = V ZENER2 + 6.5 V
amplifier with indefinite hold time capability, to enable
immediate LED on cycles after extended off times. During
OVP = 6.5 V [(R UPPER / R LOWER ) + 1] + (100E-6 x R UPPER )
extended off times, the external LEDs cool down from their
34844A
Analog Integrated Circuit Device Data
Freescale Semiconductor
45
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