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LM3555

SNVS594G – DECEMBER 2008 – REVISED APRIL 2016

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Copyright © 2008–2016, Texas Instruments Incorporated

7.3 Feature Description

7.3.1 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 plus current source headroom voltage the device turns the PFET on continuously (pass mode). In

pass mode the difference between (VIN – ILED × RON_P) and the voltage across the LEDs is dropped across the

current source. When the output voltage (VOUT) is greater than the input voltage (VIN) minus approximately

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 device does not return to pass mode until the

device is disabled and re-enabled. At this point, the converter re-evaluates the conditions and enter the

appropriate mode.

7.3.2 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 automatically sets 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.

7.3.3 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.

7.3.4 SDA/EN2 and SCL/EN1 Pins

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

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

as an I2C SCL (clock) pin. If the I2C/EN pin is equal to a 0, the SDA/EN2 pin functions as the simple control pin

EN2, and the SCL/EN1 pin functions 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.

7.3.5 STROBE Pin

The STROBE pin of the LM3555 provides an external method for initiating a flash event. In most cases, the

STROBE pin is connected to an imaging module so that the image capture and flash event are synchronized.

The STROBE pin is only functional when the LM3555 is placed into I2C control mode (I2C/EN = 1) and the output

on (OEN in 0x04) and strobe signal Mode (SEN in 0x04) bits are set (1). The STROBE pin can be configured to

be an edge sensitive or level sensitive input by setting the strobe signal usage bit (SSU in 0x04. 1 = Level, 0 =

Edge). In edge sensitive mode, a rising edge transition (0 to 1) starts the flash event, and the internal flash timer

terminates the event. In level sensitive mode, a rising edge transition (0 to 1) starts the flash event and a falling

edge transition (1 to 0) or the internal flash timer, whichever occurs first, terminates the event. In I2C mode, there

is an internal pulldown resistor that becomes enabled on the STROBE pin.

In simple control mode, the STROBE pin functions as a output when a pullup resistor is connected, alerting the

user to the number of flash LEDs present in the system. If the STROBE pin is outputting a 1, two LEDs are

present, whereas a 0 indicates a single LED is present.