Analog Integrated Circuit Device Data

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16

Freescale Semiconductor

34844

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

•I2C mode, has to be configured by I2C communication (SETI2C bit = 1) right after the IC is turned ON, it prevents the part from being

turned ON/OFF by the bus. Sleep mode is also present and it is intended to save power, but still keep the IC prepared to communicate

by I2C. By taking the EN bit low and then the EN pin low, the part enters into a shutdown mode.

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 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 I2C (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 I2C

command has to wait for 5.0 ms after the part is turned ON, to allow sufficient time for the device power up sequence to be completed.

Please follow this sequence to change the Boost frequency thru I2C:

1. Take PWM pin low

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)

5. Reconfigure all registers

6. Take PWM pin High

The boost controller has an integral track and hold amplifier with indefinite hold time capability, to enable 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 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 I2C interface (OVP Register). If the I2C 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 goes to the OVP level, which is

programmed via the I2C 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:

Table 5. Operation Current Consumption Modes

MODE

EN Pin

SCK/SDA Pins

I2C Bit Command

Current Consumption Mode

Comments

Manual

Low

Low

N/A

Shutdown

PWM pin = Low

High

Low

N/A

Operational

SM Bus

Low

Low (> 27 ms)

EN bit = 0

Shutdown

Low

Active

EN bit = 0

Sleep

Low

Active

EN bit = 1

Operational

I2C

Low

X

SETI2C bit = 1

I2C Low Power (Shutdown)

Part Doesn’t

Wake-up

CLRI2C bit = 0

EN bit = 0

High

X

SETI2C bit = 1

Sleep

CLRI2C bit = 0

EN bit = 0

High

X

SETI2C bit = 1

Operational

CLRI2C bit = 0

EN bit = 1