LM3554_16 Datasheet, PDF (14/47 Pages) Texas Instruments – Synchronous Boost Converter

LM3554
SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016
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Feature Description (continued)
Because the current limit is sensed in the NMOS switch there is no mechanism to limit the current when the
device operates in pass mode. In situations where there could potentially be large load currents at OUT, and the
LM3554 is operating in Pass mode, the load current must be limited to 2.5 A. In boost mode or pass mode if
V_OUT falls below approximately 2.3 V, the device stops switching, and the PFET operates as a current source
limiting the current to typically 350 mA. This prevents damage to the LM3554 and excessive current draw from
the battery during output short circuit conditions.
7.3.4 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 pulseonly terminates 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
Register, or Flash Brightness Register) are automatically re-written with (0, 0).
7.3.5 Flash Termination (I²C-Initiated Flash)
For I²C-initiated 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.
7.3.6 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 32 ms giving a flash timeout range of 32 ms to 1024 ms
(see Table 4).
7.3.7 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 ramp up to the programmed torch current level by stepping through all of the torch
currents at 16 µs/step until the programmed torch current level is reached.
7.3.8 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 ramps
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 turns 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) takes precedence over the TORCH mode input. Figure 37 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 section of the datasheet.
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