LM3554TMXSLASHNOPB datasheet(21/47 Pages) TI1

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LM3554

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SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016

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Feature Description (continued)

7.3.18 LED Thermal Fault

The NTC flag (bit [5] of the Flags Register) reads back a 1 if the LM3554 is active in flash or torch mode, the

device is in NTC mode, and the voltage at LEDI/NTC has fallen below VTRIP (1.05 V typical). When this has

happened and the LM3554 has been forced into torch or LED shutdown (depending on the state of Configuration

Thermal Comparator Mode (NTC) for more details.)

7.3.19 Input Voltage Monitor Fault

The VIN Monitor Flag (bit [6] of the Flag Register) reads back a 1 when the Input Voltage Monitor is enabled and

VIN falls below the programmed VIN Monitor threshold. This flag must be read back in order to resume normal

operation after the LED current has been forced to Torch mode or turned off due to a VIN Monitor event.

7.3.20 TX1 And TX2 Interrupt Flags

The TX1 and TX2 interrupt flags (bits [3] and [4]) indicate a TX event on the TX1/TORCH and ENVM/TX2 pins.

Bit 3 is read back a 1 if TX1/TORCH is in TX1 mode and the pin has changed from low to high since the last

read of the Flags Register. Bit 4 reads back a 1 if ENVM/TX2 is in TX2 mode and the pin has had a TX event

since the last read of the Flags Register. A read of the Flags Register automatically resets these bits.

The ENVM/TX2/GPIO2 pin, when configured in TX2 mode, has a TX event that can be either a high-to-low

transition or a low-to-high transition depending on the setting of the TX2 polarity bit (see Table 6).

7.3.21 Light Load Disable

Configuration Register 1 bit [0] = 1 disables the light load comparator. With this bit set to 0 (default) the light load

comparator is enabled. Light load mode only applies when the LM3554 is active in voltage-output mode. In LED

mode the light load comparator is always disabled. When the light load comparator is disabled the LM3554

operates at a constant frequency down to ILOAD = 0. Disabling light load can be useful when a more predictable

switching frequency across the entire load current range is desired.

7.4 Device Functional Modes

7.4.1 Flash Mode

In flash mode the LED current sources (LED1 and LED2) each provide 16 different current levels from typically

34 mA to approximately 600 mA. The flash currents are set by writing to bits [6:3] of the Flash Brightness

Resister. Flash mode is activated by either writing a (1, 1) to bits [1:0] of the Torch Brightness Register, writing a

(1, 1) to bit [1:0] of the Flash Brightness Register, or by pulling the STROBE pin high. Once the Flash sequence

is activated, both current sinks (LED1 and LED2) ramps up to the programmed Flash current by stepping through

all Flash levels (16 µs/step) until the programmed current is reached.

7.4.2 Pass Mode

Once the output voltage charges up to VIN – 150 mV the the device operates either in pass mode or boost mode.

If the voltage difference between VOUT and VLED is less than 300 mV, the device transitions in boost mode. If the

difference between VOUT and VLED is greater than 300 mV, the device operates in pass mode. In pass mode the

boost converter stops switching, and the synchronous PFET turns fully on bringing VOUT up to VIN – IIN × RPMOS

(RPMOS = 150 mΩ). In pass mode the inductor current is not limited by the peak current limit. In this situation the

output current must be limited to 2.5A.

7.4.3 Voltage-Output Mode

Bit 2 (VM) of the Torch Brightness Register, bit 2 (VM) of the Flash Brightness Register, or the ENVM input

enables or disables the voltage-output mode. In voltage-output mode the device operates as a simple boost

converter with two selectable voltage levels (4.5 V and 5 V). Write a 1 to bit 1 (OV) of Configuration Register 1 to

set VOUT to 5 V. Write a 0 to this bit to set VOUT to 4.5 V. In voltage-output mode the LED current sources can

continue to operate; however, the difference between VOUT and VLED is dropped across the current sources. (See

Maximum Output Power.) In voltage-output mode when VIN is greater than VOUT the LM3554 device operates in

pass mode (see Pass Mode).

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