17

LM3555

www.ti.com

SNVS594G – DECEMBER 2008 – REVISED APRIL 2016

Product Folder Links: LM3555

Submit Documentation Feedback

Copyright © 2008–2016, Texas Instruments Incorporated

Feature Description (continued)

7.3.6 TORCH Pin

The TORCH pin of the LM3555, depending on the state and configuration, allows the user to enable torch/assist

mode without having to write the command through the I2C bus or through toggling the EN1 and EN2 pins. In

simple mode, the LM3555 drives 60 mA of LED current if two series LEDs are present and 80 mA is one LED is

present. In I2C mode, the external torch mode bit (TEN in register 0x04) must be set to a 1 to allow an external

torch (default value = 1). In I2C mode, the torch mode current is equal to the Assist mode current level stored in

register 0x03. The TORCH pin has an internal pulldown resistor enabled in both simple mode and I2C mode.

7.3.7 Indicator LED Pin (IND)

The indicator LED current source pin (IND) is able to drive a single red indicator LED when the anode is

connected to the LM3555 and the cathode is connected to ground. In simple logic mode, the default indicator

current is 2.5 mA, and in I2C mode, the indicator LED current can be adjusted to 2.5 mA, 5 mA, 7.5 mA, or 10

mA.

7.3.8 Internal Diode Detection

During the start-up sequence of the LM3555 an internal voltage comparator on the VLED pin monitors the

forward voltage of the LED or LEDs. This measurement occurs when the ramp-up current reaches 80 mA. If, at

this time, the diode voltage exceeds the user-selectable diode detect threshold (Register 0x02 bits VO1 and

VO0), the LM3555 assumes two series LEDs are present and limits the maximum flash current to 400 mA. The

four adjustable levels are; 00 = 4.35 V, 01 = 4.65 V, 10 = 4.05 V and 11 = 4.95 V. This detection feature can be

disabled by setting the diode detect enable bit (DEN) in the Current Set Register (address 0x03) to a 0. The DEN

bit is set to a 1 (enabled) by default.

In all cases during start-up, the diode current first ramps to 80 mA and then proceeds to the target current. If the

torch/assist current is set to 60 mA, the LM3555 first reaches 80 mA and then drop to 60 mA.

The number of LEDs present in the system is recorded in a read-only diode number (DN) bit of the fault register

(address 0x05). In simple mode, the number of LEDs present are output on the STROBE pin (0 = 1 LED, 1 = 2

LEDs).