LM3554 datasheet(21/43 Pages) TI1

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LM3554

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SNVS549A – JUNE 2009 – REVISED APRIL 2011

FLASH TERMINATION (STROBE-INITIATED FLASH)

Bit [7] of the Flash Brightness Register (STR bit) determines how the Flash pulse terminates with STROBE-

initated flash pulses. With the STR bit = 1 the Flash current pulse will only terminate by reaching the end of the

Flash Timeout period. With STR = 0, Flash mode can be terminated by pulling STROBE low, or by allowing the

Flash Timeout period to elapse. If STR = 0 and STROBE is toggled before the end of the Flash Timeout period,

the Timeout period resets on the rising edge of STROBE. See LM3554 TIMING DIAGRAMS regarding the Flash

pulse termination for the different STR bit settings.

After the Flash pulse terminates, either by a flash timeout, or pulling STROBE low, LED1 and LED2 turn

completely off. This happens even when Torch is enabled via the I2C-compatible interface, and the Flash pulse is

turned on by toggling STROBE. After a Flash event ends the EN1, EN0 bits (bits [1:0] of the Torch Brightness

FLASH TERMINATION (I2C-INITIATED FLASH)

For I2C initated flash pulses, the flash LED current can be terminated by either waiting for the timeout duration to

expire or by writing a (0, 0) to bits [1:0] of the Torch Brightness Register, or Flash Brightness Register. If the

timeout duration is allowed to elapse, the flash enable bits of the Torch Brightness and Flash Brightness

Registers are automatically reset to 0.

FLASH TIMEOUT

The Flash Timeout period sets the duration of the flash current pulse. Bits [4:0] of the Flash Duration Register

programs the 32 different Flash Timeout levels in steps of 32ms giving a Flash Timeout range of 32ms to

1024ms (see Table 7).

TORCH MODE

In Torch mode the current sources LED1 and LED2 each provide 8 different current levels (see Table 2). The

Torch currents are adjusted by writing to bits [5:3] of the Torch Brightness Register. Torch mode is activated by

setting Torch Brightness Register bits [1:0] to (1, 0) or Flash Brightness bits [1:0] to (1, 0). Once the Torch mode

is enabled the current sources will ramp up to the programmed Torch current level by stepping through all of the

Torch currents at 16µs/step until the programmed Torch current level is reached.

TX1/TORCH

The TX1/TORCH/GPIO1 input has a triple function. With Configuration Register 1 Bit [7] = 0 (default),

TX1/TORCH/GPIO1 is a Power Amplifier Synchronization input (TX1 mode). This is designed to reduce the

current pulled from the battery during an RF power amplifier transmit event. When the LM3554 is engaged in a

Flash event, and the TX1 pin is pulled high, both LED1 and LED2 are forced into Torch mode at the programmed

Torch current setting. If the TX1 pin is then pulled low before the Flash pulse terminates the LED current will

ramp back to the previous Flash current level. At the end of the Flash timeout whether the TX1 pin is high or low,

the LED current will turn off.

With the Configuration Register Bit [7] = 1, TX1/TORCH/GPIO1 is configured as a hardware Torch mode enable

(TORCH). In this mode a high at TORCH turns on the LED current sources in Torch mode. STROBE (or I2-

initiated flash) will take precedence over the TORCH mode input. Figure 15 details the functionality of the

hardware TORCH mode. Additionally, when a flash pulse is initiated during hardware TORCH mode, the

hardware torch mode bit is reset at the end of the flash pulse. In order to re-enter hardware Torch mode, bit [7] of

Configuration Register 1 would have to be re-written with a 1.

The TX1/TORCH/GPIO1 input can also be configured as a GPIO input/output. for details on this, refer to the

GPIO REGISTER ection of the datasheet.

ENVM/TX2/GPIO2

The ENVM/TX2/GPIO2/INT pin has four functions. In ENVM mode (Configuration Register 1 bit [5] = 0), the

ENVM/TX2/GPIO2/INT pin is an active high logic input that forces the LM3554 into Voltage Output Mode. In TX2

mode (Configuration Register 1 bit [5] = 1), the ENVM/TX2/GPIO2/INT pin is a Power Amplifier Synchronization

input that forces the LM3554 from Flash mode into Torch mode. In GPIO2 mode (GPIO Register Bit [3] = 1) the

ENVM/TX2/GPIO2/INT pin is configured as a general purpose logic input/output and controlled via bits[3:5] of the

GPIO Register. In INT mode the ENVM/TX2/GPIO2/INT pin is a hardware interrupt output which pulls low when

the LM3554 is in NTC mode, and the voltage at LEDI/NTC falls below VTRIP.

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