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LM3555

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SNVS594G – DECEMBER 2008 – REVISED APRIL 2016

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Feature Description (continued)

7.3.9.6 Broken Inductor Fault (IP)

An IP fault is triggered when the LM3555 detects that the inductance of the inductor has dropped below an

acceptable value. This fault indicates that the inductor has been damaged. An inductor that has had its ferrite

material damaged could cause this fault to occur. This fault is reported to the IP fault bit in the Fault Register

(bit1 in address 0x05).

7.3.10 Undervoltage Lockout (UVLO)

The LM3555 has a UVLO feature that disables the operation of the device in the event that the input voltage falls

below 2.4 V (typical). In simple control mode, the input voltage must increase to at least 2.47 V (typical), and the

EN1 and EN2 pins must be toggled low (0) before normal operation can resume.

In I2C control mode, the output enable bit in the Control Register (Address 0x04) is set to a 0 in the event of a

UVLO occurrence. The input voltage must rise to at least 2.47 V before the LM3555 becomes fully functional

again.

A UVLO event does not disturb the state of the other registers of the LM3555.

7.3.11 Power-On Reset (POR)

A POR circuit is present on the LM3555 for use in I2C control mode. The POR circuit ensures that the device

starts in a known OFF state and that the registers used in the I2C control interface are initialized to the proper

start-up values once the input voltage reaches a voltage greater than 1.8 V (typical). An input voltage lower than

1.8 V not only places the device into UVLO, but also clears all of the LM3555 registers.

7.4 Device Functional Modes

7.4.1 Single LED Operation

In single LED operation, the LED flash current is allowed to reach the maximum level of 500 mA. By default, the

assist/torch current is set to 80 mA, and the flash current is set to 500 mA.

For input voltages that are higher than the LED forward voltage, the LM3555 operates in a pass mode. As VIN

drops, the LM3555 first transitions from pass mode to the minimum duty-cycle boost mode. In this mode, the

output voltage (VOUT) increases to a level higher than needed to maintain current regulation through the current

source. If VIN continues to decrease, the LM3555 transitions again, this time from minimum duty-cycle boost

mode to standard boost mode. Standard boost mode adjusts the converters duty cycle to maintain 300 mV

across the current source of the device.

Once the LM3555 transitions from pass mode to either boost mode, the device stays in one of those boost

modes until the device is disabled or timed-out and then restarted.

7.4.2 Dual LED Operation

In dual LED operation, the LED flash current is allowed to reach a maximum level of 400 mA. By default, the

assist/torch current is set to 60 mA, and the flash current is set to 320 mA.

During dual LED operation, the output voltage is always greater than the input voltage (assuming standard white

flash LEDs are used), forcing the LM3555 to be in boost mode over the entire input voltage range.

7.4.3 Torch or Assist (Continuous Current) Operation

There are two different continuous current modes on the LM3555: torch and assist.

Torch mode is enabled through the use of the dedicated TORCH pin using both simple and I2C modes (1 =

Torch, 0 = Standby (I2C mode) or shutdown (simple mode). In I2C control mode, the TORCH pin functionality can

be enabled and disabled through by setting the value of the TEN bit in the Control Register (Address 0x04). TEN

= 1 allows an external torch while TEN = 0 does not.

Assist mode is enabled in simple control mode by driving EN1 low (0) and by driving EN2 high (1). In I2C control

mode, assist mode is enabled by setting the output mode bits (OM1 and OM0) to 10 and setting the output

enable bit (OEN) to a 1 in the Control Register (0x04). Assist mode remains active in I2C mode until the OEM bit

is set to 0 or until a flash event occurs.