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4.7.7 V

OUT

4.5/5.0

The V

OUT

4.5/5.0 button programs bit 1 in Configuration Register 1. This changes the output voltage in

Voltage Output Mode to 5V when pushed or 4.5V when not pushed.

4.7.8 DisLL

The DisLL button gives the option of disabling the light load comparator of the LM3554 when pushed or

enabling the light load comparator when not pushed. When the light load comparator is disabled, the

LM3554 will not switch over to pulse skip mode at light load currents (typically I

OUT

= 100 mA with VIN =

3.6V). The light load comparator only applies when the LM3554 is operating in Voltage Out mode. In LED

mode the light load comparator is always disabled.

4.8 Shutdown Configuration Buttons

The following buttons correspond to Configuration Register 2, address (0xF0):

4.8.1 VIN Monitor SD

The VIN MONITOR SD button programs bit 3 of Configuration Register 2 with a ‘1’ when pushed, or a ‘0’

when not pushed. When this bit is a ‘1’, the LM3554 will be forced into shutdown when the VIN Monitor

feature is enabled (VIN Monitor Register bit [0] = 1), and the input voltage falls below the programmed VIN

Threshold. When this bit is a ‘0’, and the VIN Monitor feature is enabled, the LEDs are forced into Torch

mode when the input voltage falls below the programmed VIN Threshold.

4.8.2 Alt Ext Torch

When the Alt Ext Torch (Alternative External Torch) button is pushed, Configuration Register 2 bit 2 is

programmed with ‘1’. This sets the LM3554 up for Alternative External Torch mode (AET mode). In AET

mode the timeout counter is disabled when TX1 goes high before STROBE goes high. For example, in

AET mode a high on TX1, followed by a high on STROBE, forces the LEDs into Torch mode without a

timeout. If TX1 is then brought low with STROBE still high, the LEDs are forced into Flash mode, and the

timeout counter is re-enabled. When STROBE is brought low, or when the timeout counter expires, the

LEDs are turned off. When the Alt Ext Torch button is left un-pushed, this bit is programmed with a ‘0’,

and the LM3554’s timeout function operates normally.

4.8.3 NTC SD

When the NTC SD button is pushed, bit 1 of Configuration Register 2 is programmed with a ‘1’, and the

LM3554 is placed in NTC Shutdown mode. When the button is not pushed, bit 1 of Configuration Register

1 is programmed with ‘0’, and the LM3554 is placed into NTC force Torch mode. When the LM3554 is

placed in NTC mode (Configuration Register 1 bit 3 = 1), and NTC force shutdown mode, the LM3554 will

go into shutdown when the NTC voltage falls below 1.05V. When the LM3554 is placed in NTC mode

(Configuration Register 1 bit 3 = 1) and NTC force Torch mode, the LM3554 will reduce the active Flash

current to the programmed Torch level if the NTC voltage falls below 1.05V.

4.8.4 TX2 SD

When the TX2 SD button is pushed, Configuration Register 2 bit 0 is programmed with a ‘1’, and the

LM3554 is placed in TX2 Force Shutdown mode. If the LM3554 is placed into TX2 Force Shutdown mode,

and the ENVM/TX2 pin is configured as a TX2 input (Configuration Register 1, bit 5 = 1), a TX2 event

causes the LM3554’s LED current to turn off. If the LM3554 is placed into TX2 force Torch mode, and the

ENVM/TX2 pin is configured as a TX2 input (Configuration Register 1, bit 5 = 1), a TX2 event causes the

LM3554’s LED current to go from Flash mode to Torch mode.

A TX2 event is considered as a low-to-high transition at TX2 when the TX2 polarity bit is set for active

high (Configuration Register 1 bit 5 = 1) or a high-to-low transition on TX2 when the TX2 polarity bit is set

for active low (Configuration Register 1 bit 5 = 0).

AN-1742 LM3554 Evaluation Board SNVA646B–January 2009–Revised July 2013

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