34844 datasheet(14/36 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34844

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

Table 4. Operation Current Consumption Modes

BOOST

The integrated boost converter operates in non-

synchronous mode and integrates a 3A 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

μs

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 62V, ~4V spaced, using

the I2C interface (OVP Register). If I2C 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 = 6.5V). During an OVP condition, the

output voltage will go 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:

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 5V 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 10ms 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 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 5.0ms 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

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.

MODE

EN Pin

SCK/SDA Pins

I2C Bit Command

Current Consumption

Mode

Comments

Manual

Low

N/A

Shutdown

High

Low

N/A

Operational

SM-Bus

Low

Low (> 27ms)

EN bit = X

Shutdown

Low

Active

EN bit = 0

Sleep

Low

Active

EN bit = 1

Operational

I2C

Low

SETI2C bit = 1

I2C Low Power

(Shutdown)

Part Doesn’t

Wake-up

CLRI2C bit = 0

EN bit = X

High

SETI2C bit = 1

Sleep

CLRI2C bit = 0

EN bit = 0

High

SETI2C bit = 1

Operational

CLRI2C bit = 0

EN bit = 1

OUT

Method 1

Method 2

A0/SEN

OVP = VZENER2 + 6.5V

OVP = 6.5V [(RUPPER / RLOWER) + 1] + (100E-6 x RUPPER)

RUPPER

RLOWER

VZENER2

34844 Datasheet(PDF) 14 Page - Freescale Semiconductor, Inc

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