LM3555TLE Datasheet, PDF (16/33 Pages) Texas Instruments – LM3555 Synchronous Boost Converter with 500 mA High Side LED Driver and Dual-Mode Control Interface

LM3555
SNVS594E – JANUARY 2009 – REVISED NOVEMBER 2011
Circuit Description
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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
current over the 2.5V to 5.5V input voltage range. Dual control interfaces (Simple ENABLE Control or I²C) and
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
Pass mode the difference between (V_IN - I_LED×R_{ON_P}) and the voltage across the LEDs is dropped across the
current source. When the output voltage (V_OUT) is greater than the input voltage (V_IN) minus approx. 200 mV, the
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.
I²C/EN Pin
The I2C/EN pin on the LM3555 changes the control interface depending on its state. To use the LM3555 in the
"Simple Control" mode, the I2C/EN pin must be tied low. To use the LM3555 in "I²C Control" mode, the I2C/EN
pin must be tied high. Toggling this pin between Simple Control Mode and I²C Control Mode is not
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
Mode, internal pulldown resistors on the SDA/EN2 and SCL/EN1 pins become active. In I²C Control Mode, these
pulldowns become disabled.
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