LM3555

SNVS594E – JANUARY 2009 – REVISED NOVEMBER 2011

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Circuit Description

OVERVIEW

The LM3555 is a high power white LED flash driver capable of delivering up to 500 mA of LED current into a

single LED, or up to 400 mA into two series LEDs. The device incorporates a 2MHz constant frequency,

synchronous, current mode PWM boost converter, and a single high side current source to regulate the LED

diode detection (single LED or two LEDs in series) make the LM3555 highly adaptable to a large variety of

designs

CIRCUIT COMPONENTS

Synchronous Boost Converter

The LM3555 operates in two modes: LED boost mode or LED pass mode. When the input voltage is above the

LED voltage + current source headroom voltage the device turns the PFET on continuously (Pass mode). In

PWM converter switches and maintains at least 300 mV across the current source (LED Boost mode). This

minimum headroom voltage ensures that the current sinks remain in regulation.

Once the LM3555 transitions from Pass Mode to Boost Mode, the part will not return to Pass Mode until the part

is disabled and re-enabled. At this point, the converter will re-evaluate the conditions and enter the appropriate

mode

High-Side Current Source

The High-Side Current Source of the LM3555 is capable of driving one or two LEDs in series. Depending on the

configuration, the LM3555 will automatically set default diode current levels and diode current limits. For a single

LED, the flash current range is 200 mA to 500 mA in 20 mA steps with a default current equal to 500 mA. For

two LEDs in series, the flash current range is 200 mA to 400 mA in 20 mA steps with a default current equal to

320 mA.

Additionally, the high-side current source is capable of supporting Assist/Torch current levels (continuous current)

between 60 mA and 160 mA in 20 mA levels.

The I2C/EN pin on the LM3555 changes the control interface depending on its state. To use the LM3555 in the

recommended.

SDA/EN2 and SCL/EN1 Pins

Depending on the state of the I2C/EN pin, the SDA/EN2 and SCL/EN1 pins will function in different ways. If the

I2C/EN pin is equal to a '1', the SDA/EN2 pin will function as an I2C SDA (data) pin and the SCL/EN1 pin will

function as an I2C SCL (Clock) pin. If the I2C/EN pin is equal to a '0', the SDA/EN2 pin will function as the simple

control pin EN2 and the SCL/EN1 pin will function as the simple control pin EN1.

When using the Simple Control Mode, the Flash, Torch and Indicator Modes can be enabled. In Simple Control

pulldowns become disabled.

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