LM3554

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SNVS549A – JUNE 2009 – REVISED APRIL 2011

In Voltage Output mode the LM3554 operates as a voltage output boost converter with selectable output

voltages of 4.5V and 5V. In this mode the LM3554 is able to deliver up to typically 5W of output power. At light

loads and in Voltage Output mode the PWM switching converter changes over to a pulsed frequency regulation

mode and only switches as necessary to ensure proper LED current or output voltage regulation. This allows for

improved light load efficiency compared to converters that operate in fixed-frequency PWM mode at all load

currents.

Additional features of the LM3554 include 4 logic inputs, an internal comparator for LED thermal sensing, and a

low-power indicator LED current source. The STROBE input provides a hardware Flash mode enable. The

ENVM/TX2/GPIO2 input is configurable as a hardware Voltage Output mode enable (ENVM), an active high

Flash interrupt that forces the device from FLASH mode to a low-power TORCH mode (TX2), or as a

programmable logic input/output (GPIO2). The TX1 input is configurable as an active high Flash interrupt that

forces the device from FLASH mode to a low-power TORCH mode (TX1), as a hardware Torch mode enable

(TORCH), or as a programmable logic input/output (GPIO1) . The HWEN input provides for an active low

hardware shutdown of the device. Finally, the LEDI/NTC pin is configurable as a low-power indicator LED driver

shutdown depending on the NTC Shutdown bit setting .

Output mode, adjustment of the LED current in TORCH mode, adjustment of the LED current in FLASH mode,

adjustment of the indicator LED currents, changing the flash LED current duration, changing the switch current

limit. Additionally, there are 5 flag bits that can be read back indicating flash current timeout, over-temperature

condition, LED failure (open or short), LED thermal failure, and an input voltage fault.

STARTUP

Turn on of the LM3554 is done through bits [2:0] of the Torch Brightness Register (0xA0), bits [2:0] of the Flash

Brightness Register (0xB0), the ENVM input, or the STROBE input. Bits [1:0] of the Torch Brightness Register or

Flash Brightness Register enables/disables the current sources (LED1, LED2, and LEDI). Bit [2] enables/disables

the voltage output mode. A logic high at STROBE enables Flash mode. A logic high on the ENVM input forces

the LM3554 into Voltage Output mode.

typically 350mA to the output capacitor. During this time all current sources (LED1, LED2, and LEDI) are off.

When the voltage across the output capacitor reaches 2.2V, the current sources can turn on. At turn-on the

current sources step through each FLASH or TORCH level until the target LED current is reached (16 µs/step).

PASS MODE

limit. In this situation the output current must be limited to 2.5A.

LIGHT LOAD DISABLE

Configuration Register 1 bit [0] = 1 disables the light load comparator. With this bit set to 0 (default) the light load

comparator is enabled. Light Load mode only applies when the LM3554 is active in Voltage Output mode. In LED

mode the Light Load Comparator is always disabled. When the light load comparator is disabled the LM3554 will

switching frequency across the entire load current range is desired.

Copyright © 2009–2011, Texas Instruments Incorporated

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Product Folder Links: LM3554