LM3555TLE Datasheet, PDF (24/33 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
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In all cases during startup, the diode current will first ramp to 80 mA and then proceed to the target current. If the
torch/assist current is set to 60 mA, the LM3555 will first reach 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).
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 will operate in a pass-mode. As V_IN
drops, the LM3555 will first transition from pass-mode to the minimum duty-cycle boost mode. In this mode, the
output voltage (V_OUT) will increase to a level higher than needed to maintain current regulation through the
current source. If V_IN continues to decrease, the LM3555 will transition 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 LM3555's current source.
Once the LM3555 transitions from pass mode to either boost mode, the part will stay in one of the boost modes
until the part is disabled or timed-out and then restarted.
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 will always be 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.
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 will remain active in I²C mode until
the OEM bit is set to '0' or until a flash event occurs.
The LM3555 can drive one or two LEDs at continuous current levels ranging from 60 mA to 160 mA in 20 mA
steps. In Simple Control mode, the torch and assist current levels are equal to 60 mA for two LEDs or 80 mA for
a single LED. In I²C mode, the current is set in the Current Set Register (Address 0x30, AC2-AC0 bits).
FLASH (PULSED CURRENT) OPERATION
A flash event using the LM3555 can be initiated though the dedicated control interface in both Simple and I²C
modes, and through the use of the Strobe pin in I²C mode.
By driving both EN1 and EN2 high ('1') in Simple Mode, the part will enter flash mode and remain there until the
control pins are driven low '0', or a timeout event occurs. In Simple Mode, the flash current is equal to 500 mA
when driving a single LED and 320 mA when two LEDs are present. The default timeout duration is 850 ms.
When placed into I²C Control mode, a flash event is initiated when the Output Mode bits (OM1 and OM0) are set
to '11' and the Output Enable bit (OEN) is set to a '1' in the Control Register (0x04). In I²C mode, the flash event
will remain active as long as the OEN bit is set to a '1', and will terminate upon a timeout event. The safety timer
duration can be set in 50 ms intervals ranging from 100 ms to 850 ms by writing the desired value to the FT3-
FT0 bits in the Indicator and Timer Register (Address 0x02).
The Strobe pin provides added system flexibility in that it allows an additional external device (Camera Module,
GPU etc.) to trigger a flash event. To initiate a Strobe event in I²C Control mode, the Strobe Signal Mode (SEN)
bit and the Output Enable (OEN) bits in the Control Register (Address 0x04) must first be set to 1's.
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