LM3556_15 Datasheet, PDF (15/39 Pages) Texas Instruments – Synchronous Boost LED Flash Driver With High-Side Current Source

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LM3556
SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015
Feature Description (continued)
7.3.3.6 Undervoltage Lockout (UVLO)
The LM3556 has an internal comparator that monitors the voltage at IN and forces the LM3556 into shutdown if
the input voltage drops to 2.8 V. If the UVLO monitor threshold is tripped, the UVLO flag bit is set in the Flags
Register (0x0B). If the input voltage rises above 2.8 V, the device is available for operation until there is an I²C
read command initiated for the Flags Register (0x0B). Upon a read, the Flags Register is cleared, and normal
operation can resume. This feature can be disabled by writing a '0' to the UVLO EN bit in the Input Voltage Flash
Monitor (IVFM) Mode Register (0x01). The mode bits in the Enable Register (0x0A) are cleared upon a UVLO
event.
7.3.3.7 Thermal Shutdown (T_SD)
When the LM3556 device’s die temperature reaches 150°C, the boost converter shuts down, and the NFET and
PFET turn off, as does the current source (LED). When the thermal shutdown threshold is tripped, a '1' gets
written to the corresponding bit of the Flags Register (0x0B) (T_SD bit), and the device goes into standby. The
LM3556 can only restart after the Flags Register (0x0B) is read, clearing the fault flag. Upon restart, if the die
temperature is still above 150°C, the device resets the fault flag and re-enters standby. The mode bits in the
Enable Register (0x0A) are cleared upon a T_SD.
7.3.3.8 LED and/or V_OUT Fault
The LED fault flag in the Flags Register (0x0B) reads back a '1' if the part is active in Flash Mode or Torch Mode,
and the LED output or the V_OUT node experiences a short condition. The LM3556 determines an LED open
condition if the OVP threshold is crossed at the OUT pin while the device is in Flash or Torch Mode. An LED
short condition is determined if the voltage at LED goes below 500 mV (typical) while the device is in either
Torch or Flash Mode. There is a delay of 256-μs deglitch time before the LED flag is valid, and 2.048 ms before
the V_OUT flag is valid. This delay is the time between when the flash or torch current is triggered and when the
LED voltage and the output voltage are sampled. The LED flag can only be reset to '0' by removing power to the
LM3556, or by reading back the Flags Register (0x0B). The mode bits in the Enable Register (0x0A) are cleared
upon an LED and/or V_OUT fault.
7.4 Device Functional Modes
7.4.1 Start-Up (Enabling The Device)
Turnon of the LM3556 Torch and Flash Modes can be done through the Enable Register (0x0A). On start-up,
when V_OUT is less than V_IN the internal synchronous PFET turns on as a current source and delivers 200 mA
(typical) to the output capacitor. During this time the current source (LED) is off. When the voltage across the
output capacitor reaches 2.2 V (typical) the current source turns on. At turnon the current source steps through
each FLASH or TORCH level until the target LED current is reached. This gives the device a controlled turnon
and limits inrush current from the V_IN supply.
7.4.2 Pass Mode
The LM3556 starts up in Pass Mode and stays there until Boost Mode is needed to maintain regulation. If the
voltage difference between V_OUT and V_LED falls below V_HR, the device switches to Boost Mode. In Pass Mode the
boost converter does not switch, and the synchronous PFET fully turns on bringing V_OUT up to (V_IN − I_LED ×
R_PMOS). In Pass Mode the inductor current is not limited by the peak current limit. In this situation the output
current must be limited to 2 A.
7.4.3 Flash Mode
In Flash Mode, the LED current source (LED) provides 16 target current levels from 93.75 mA to 1500 mA. The
Flash currents are adjusted via the Current Control Register (0x09). Flash mode is activated by the Enable
Register (0x0A), or by pulling the STROBE pin HIGH. Once the Flash sequence is activated the current source
(LED) ramps up to the programmed Flash current by stepping through all current steps until the programmed
current is reached.
When the device is enabled in Flash Mode through the Enable Register, all mode bits in the Enable Register are
cleared after a flash time-out event.
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