Analog Integrated Circuit Device Data

Freescale Semiconductor

17

34844

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

The optical and temperature control loop can be disabled

pins high (>2.2V) it is called Vset mode, and the LED Driver

maximum current is set to 50mA by using a 5.1k 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

LED FAILURE PROTECTION

Open LED Protection

If LED fails open in any of the LED strings, the voltage in

that channel will be pulled close to zero, which will cause the

channel to be disabled. As a result, the boost output voltage

will go to the OVP level and then come down to the regulation

level to continue powering the rest of the LED strings.

Short LED Protection

If an LED shorted in any of the LED strings, the device will

continue to operate without interruption. However, if the

shorted LED happens to be in the LED string with the highest

forward voltage, the dynamic headroom control circuit will

automatically regulate 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 which

will completely shutdown the device.

OVER-TEMPERATURE PROTECTION

The 34844 has an on-chip temperature sensor that

measures die temperature. If the IC temperature exceeds the

OTT threshold, the IC will turn off all power sources inside the

IC (LED drivers, boost and internal regulators) until the

temperature falls below the falling OTT threshold. Once it

comes back on, it will operate with the default configuration

(please refer to Table 7).

SERIAL INTERFACE CONTROL

standard (100kHz) or fast (400kHz) modes.

The A0/SEN pin can be used an address select pin to

allow more than 2 devices in the system. The A0/SEN pin

should be held low on all chips expect the one to be

addressed, where it is taken HIGH.

Current A

[]

VPIN Volts

[] ICH

×

RegisterValue

[]

()

RSET ohms

[]

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=

Current A

[]

2.048 VNIN

–

() Volts

[]

()

RSET ohms

[]

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=

Current A

[]

2.048 VNIN

–

VPIN

+

() Volts

[]

()

RSET ohms

[]

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=