Analog Integrated Circuit Device Data



Freescale Semiconductor

19

34844

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

A 6.98 k

resistor or higher value must be used at the ISET pin if the part is configured to use PIN+NIN control loop functionality. The

80 mA maximum current is achieved at the higher allowed level of PIN/NIN pins, ensuring the maximum current of the LED Drivers are

not exceeded.

The optical and temperature control loop can be disabled by the I2C setting bits (PINEN & NINEN), or by tying PIN and NIN pins high

(>2.2 V). The LED Driver maximum current is set to 80 mA by using a 3.48 k

 resistor at the ISET pin.

Current on LED Channel (PIN mode)

Eqn. 3

Current on LED Channel (NIN mode)

Eqn. 4

Current on LED Channel (PIN+NIN mode)

Eqn. 5

VPIN and VNIN is the voltage applied on PIN and NIN pins correspondingly.

For ISINK formula, refer to Equation 1.

LED FAILURE PROTECTION

Open LED Protection

If an LED fails open in any of the LED strings, the voltage on that channel is pulled close to zero, which causes the channel to be

disabled. As a result, the boost output voltage goes to the OVP level and comes down to the regulation level, to continue powering the

rest of the LED strings.

Short LED Protection

If an LED is shorted in any of the LED strings, the device continues to operate without interruption. However, if the shorted LED happens

to be in the LED string with the highest forward voltage, the DHC circuit automatically regulates the output voltage with respect to the new

highest LED voltage. If more LEDs are shorted in the same LED string, it may cause excessive power dissipation in the channel, which

may cause the OTT circuit to trip completely shutting down the device.

OVERTEMPERATURE PROTECTION

The 34844 has an on-chip temperature sensor measuring die temperature. If the IC temperature exceeds the OTT threshold, the IC

turns off all power sources inside the IC (LED drivers, boost and internal regulators) until the temperature falls below the falling OTT

threshold. Once the chip is back on, it operates with the default configuration (refer to Table 7).

SERIAL INTERFACE CONTROL

The 34844 uses an I2C interface capable of operating in standard (100 kHz) or fast (400 kHz) modes. The A0/SEN pin can be used as

an address select pin to allow more than two devices in the system. The A0/SEN pin should be held low on all chips except the one to be

addressed, where it is taken HIGH.

IDIM A



ISINK A

 VPIN V



2

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



=

IDIM A



ISINK A



VSET VNIN

–

 V



2

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



=

IDIM A



ISINK A



VSET VPIN VNIN

–

+

 V



2

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



=