LM3554 datasheet(22/43 Pages) TI1

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LM3554

SNVS549A – JUNE 2009 – REVISED APRIL 2011

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In TX2 mode, when Configuration Register 1 bit [6] = 0 the ENVM/TX2/GPIO2 pin is an active low transmit

interrupt input. Under this condition, when the LM3554 is engaged in a Flash event, and ENVM/TX2/GPIO2 is

pulled low, both LED1 and LED2 are forced into either Torch mode or LED shutdown depending on the logic

state of Configuration Register 2 bit [0]. In TX2 mode with Configuration Register 1 bit [6] = 1, the

ENVM/TX2/GPIO2 pin is an active high transmit interrupt. Under this condition when the LM3554 is engaged in a

Flash event, and the TX2 pin is driven high, both LED1 and LED2 are forced into Torch mode or LED shutdown,

depending on the logic state of Configuration Register 2 bit [0]. After a TX2 event, if the ENVM/TX2/GPIO2 pin is

disengaged, and the TX2 Shutdown bit is set to force Torch mode, the LED current will ramp back to the

previous Flash current level. If the TX2 shutdown bit is programmed to force LED shutdown upon a TX2 event

the Flags Register must be read to resume normal LED operation. Table 2, Figure 11 and Figure 12 detail the

functionality of the ENVM/TX2 input.

ENVM/TX2/GPIO2/INT as an Interrupt Output

In GPIO2 mode the ENVM/TX2/GPIO2 pin can be made to reflect the inverse of the LED Thermal Fault flag

(bit[5] in the Flags Register). Configure the LM3554 for this feature by:

set GPIO Register Bit [6] = 1 (NTC External Flag)

set GPIO Register Bit [3] = 1 (GPIO2 mode)

set GPIO Register Bit [4] = 1 (GPIO2 is an output)

set Configuration Register 1 Bit [3] = 1 (NTC mode)

When the voltage at the LEDI/NTC pin falls below VTRIP (1.05V typical), the LED Thermal Fault Flag (bit [5] in the

Flags Register) is set, and the ENVM/TX2/GPIO2/INT pin is forced low. In this mode the interrupt can only be

reset to the open-drain state by reading back the Flags register.

INDICATOR LED/THERMISTOR (LEDI/NTC)

The LEDI/NTC pin serves a dual function, either as an LED indicator driver or as a threshold detector for a

negative temperature coefficient (NTC) thermistor.

Led Indicator Mode (LEDI)

LEDI/NTC is configured as an LED indicator driver by setting Configuration Register 1 bit [3] = (0) and Torch

Brightness Register bits [1:0] = (0, 1), or Flash Brightness Register bits [1:0] = (0, 1). In Indicator mode there are

4 different current levels available (2.3mA, 4.6mA, 6.9mA, 8.2mA). Bits [7:6] of the Torch Brightness Register set

the 4 different indicator current levels. The LEDI current source has a 1V typical headroom voltage.

Thermal Comparator Mode (NTC)

Writing a (1) to Configuration Register 1 bit [3] disables the indicator current source and configures the LEDI/NTC

pin as a detector for an NTC thermistor. In this mode LEDI/NTC becomes the negative input of an internal

comparator with the positive input internally connected to a reference (VTRIP = 1.05V typical). Additionally,

Configuration Register 2 bit [1] determines the action the device takes if the voltage at LEDI/NTC falls below

VTRIP (while the device is in NTC mode). With Configuration register 2 bit [1] = 0, the LM3554 will be forced into

Torch mode when the voltage at LEDI/NTC falls below VTRIP. With Configuration Register 2 bit [1] = 1 the device

will shut down the current sources when VLEDI/NTC falls below VTRIP. When the LM3554 is forced from Flash into

Torch (by VLEDI/NTC falling below VTRIP), normal LED operation (during the same Flash pulse) can only be re-

started by reading from the Flags Register (0xD0) and ensuring the voltage at VLEDI/NTC is above VTRIP. When

VLEDI/NTC falls below VTRIP, and the Flags register is cleared, the LM3554 will go through a 250µs deglitch

time before the flash current falls to either torch mode or goes into shutdown.

ALTERNATIVE EXTERNAL TORCH (AET MODE)

Configuration Register 2 bit [2] programs the LM3554 for Alternative External Torch mode. With this bit set to (0)

(default) TX1/TORCH is a transmit interrupt that forces Torch mode only during a Flash event. For example, if

TX1/TORCH goes high during a Flash event then the LEDs will be forced into Torch mode only for the duration

of the timeout counter. At the end of the timeout counter the LEDs will turn off.

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