34844 datasheet(16/37 Pages) FREESCALE

16 / 37 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

34844

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

CURRENT MIRROR

The programmable current mirror matches the current in

10 LED strings to within 2%. The maximum current is set

using a resistor to GND from the ISET pin. This can be scaled

down using the I2C interface to 255 levels.

Zero current is achieved by turning off the LED Driver by

I2C (registers CHENx = 0h) for a Duty Cycle from 0% to 99%

or by pulling PWM pin low regardless of the Duty Cycle.

I2C capability allow the channels to be controlled

individually or in parallel.

Current on LED Channel (PIN and NIN mode disabled)

Eqn. 1

In the off state, the LEDs current is set to 0 and the boost

converter stops switching.

This feature allows to drive more than 50mA of current by

connecting the LED string to 2 or more LED channels in

parallel. For example; if the application requires to drive 5

channels at 100mA, then the bottom of each LED string

should be connected to two channels in order to duplicate the

current capability (Example: CH0+CH1 = 100mA).

PWM GENERATOR

The PWM generator can operate in either master or slave

modes, as set by the M/~S pin.

In master mode, the internal PWM generator frequency is

programmed through the I2C interface (registers FPWM).

The default programmed value set the number of 25kHz

clocks (40

μs) in one PWM cycle. The 18-bit resolution allows

minimum PWM frequencies of 100Hz to be programmed. The

resulting frequency is output on the CK pin.

PWM Frequency

Eqn. 2

In slave mode, the CK pin acts as an input. The internal

digital PLL uses this frequency as the PWM frequency. By

setting one device as master, and connecting the CK output

to the input on a number of slave configured devices, all

PWM frequencies are synchronized together.

The duty cycle of the PWM waveform in both master and

slave modes is set using a second register on the I2C

interface (register DPWM), and can be controlled from 100%

duty cycle to 1/256 Tpwm = 0.39%. Zero percent of duty cycle

is achieved by turning LED Drivers off (register CHENx = 0h)

or pulling PWM pin low.

An external PWM can also be used. The PWM input is

'AND'ed with the internal signal. By setting the serial interface

to 100% duty cycle (default), the external pin has full control

of the PWM duty cycle. This pin can also be used to modulate

the LED at a lower frequency than the PWM dimming

frequency (Minimum pulse width = 150ns).

A pulsed mode can also be programmed using the I2C

interface (STROBE bit = 1). In this mode, each rising edge of

the PWM signal turns on the next channel, while turning off

all other channels. The duration that the channel is

illuminated is set by the duty cycle of the PWM input pin. This

can be used to scan the output channels.

FAIL PIN

If an LED fails open, the voltage at the LED channel will be

pulled to GND and the LED string open is detected. An error

is registered for that channel, the fail output is set high, and

that channel is turned off. The malfunction channel can be re-

enabled by I2C commands, first clearing the fail (CLRFAIL bit

=1), removing the failure and then re-enabling the channel

driver (Register CHEN). All fails are cleared when the device

is powered up.

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

shutdown the part (refer to Table 5).

If I2C communication is not present, FAIL condition should

be reset by removing the failure and re-enabling the device

through the EN pin.

OPTICAL AND TEMPERATURE CONTROL LOOP

The 34844 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 34844 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 0-2.048V input

range which affects the current through the LEDs. The PIN

pin increases current as the voltage rises from 0-2.048V. The

NIN pin reduces current as the voltage rises from 0-2.048V.

A 10.2k resistor or higher value must be used at the ISET

pin if the part is configured to use PIN+NIN control loop

functionality, the 50mA maximum current is achieved at the

higher allowed level of PIN/NIN pins, ensuring the maximum

current of the LED Drivers are not exceeded.

Current A

[]

ICH RegisterValue

[]

RSET ohms

[]

-----------------------------------------------------------

PWMFrequency Hz

[]

19.2Mhz

FPWM RegisterValue

[]

--------------------------------------------------------------------

34844 Datasheet(PDF) 16 Page - Freescale Semiconductor, Inc

34844 Datasheet(HTML) 16 Page - Freescale Semiconductor, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Datasheet Download

Link URL