LM3554TMXSLASHNOPB datasheet(12/47 Pages) TI1

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LM3554

SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016

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7 Detailed Description

7.1 Overview

The LM3554 is a high-power white-LED flash driver capable of delivering up to 1.2-A of LED current into a single

LED, or up to 600 mA into two parallel LEDs. The device incorporates a 2-MHz constant frequency,

synchronous, current mode PWM boost converter, and two high-side current sources to regulate the LED current

over the 2.5-V to 5.5-V input voltage range.

The LM3554 operates in two modes: LED mode or constant voltage-output mode. In LED mode when the output

voltage is greater than VIN – 150 mV, the PWM converter switches and maintains at least 300 mV (VHR) across

both current sources (LED1 and LED2). This minimum headroom voltage ensures that the current sinks remain

in regulation. When the input voltage is above VLED + VHR, the device operates in pass mode with the device not

switching and the PFET on continuously. In pass mode the difference between (VIN – ILED × RON_P) and VLED is

dropped across the current sources. If the device is operating in pass mode, and VIN drops to a point that forces

the device into switching, the device goes into switching mode one time. The LM3554 remains in switching mode

until the device is shut down and re-enabled. This is true even if VIN rises back above VLED + 300 mV during the

current flash or torch cycle. This prevents the LED current from oscillating when VIN is operating close to VOUT.

In voltage-output mode the LM3554 operates as a voltage output boost converter with selectable output voltages

of 4.5 V and 5 V. In this mode the LM3554 is able to deliver up to typically 5 W 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 four 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 (LEDI), or as a threshold

detector for thermal sensing (NTC). In NTC mode when the threshold (VTRIP) at the LEDI/NTC pin is crossed

(VLEDI/NTC falling), the flash pulse is forced to the torch current setting, or into shutdown depending on the NTC

shutdown bit setting.

The device is controlled via an I2C-compatible interface. This includes switchover from LED to voltage-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, overtemperature

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

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