LM3556

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SNVS796C – AUGUST 2011 – REVISED APRIL 2013

Torch Mode

In Torch Mode, the current source (LED) is programmed via the Current Control Register (0x09). Torch Mode is

activated by the Enable Register (0x0A) or by the hardware TORCH input. Once the Torch Mode is enabled the

current source will ramp up to the programmed Torch current level. The Ramp-Up and Ramp-Down times are

independently adjustable via the Torch Ramp Time Register (0x06). Torch Mode is not affected by Flash

Timeout.

Indicator Mode

This mode has two options: the Internal Indicator Mode and the External Indicator Mode. Both these modes are

activated by the Configuration Register (0x07) in addition to the Enable Register (0x0A).

In the Internal Indicator Mode, the current source (LED) can be programmed to 8 different intensity levels, with

current values being 1/8th the values in Current Control Register (0x09) bits [6:4]. The Ramp-Up, Ramp-Down,

the pulse time, number of Blanks and Periods of the desired output current can be independently controlled via

the Indicator Ramp Time Indicator (0x03), Indicator Blinking Register (0x04) and the Indicator Period Count

Register (0x05).

In the External Indicator Mode, the current source (LED) is controlled via the TORCH pin. An external PWM

signal can be input to the part via the TORCH pin to choose any one of the 8 available intensity settings (Bits

[6:4] of the Current Control Register (0x09)) for the current source (LED).

Power Amplifier Synchronization (TX)

The TX pin is a Power Amplifier Synchronization input. This is designed to reduce the flash LED current and thus

limit the battery current during high battery current conditions such as PA transmit events. When the LM3556 is

engaged in a Flash event, and the TX pin is pulled high, the LED current is forced into Torch Mode at the

programmed Torch current setting or shutdown. If the TX pin is then pulled low before the Flash pulse

terminates, the LED current will return to the previous Flash current level. At the end of the Flash time-out,

whether the TX pin is high or low, the LED current will turn off. The polarity of the TX input can be changed from

active high to active low through the Configuration Register (0x07) and can be disabled/enabled by setting the

TX Enable bit in the Enable Register (0x0A) to a ‘0’.

Input Voltage Flash Monitor (IVFM)

The LM3556 has the ability to adjust the flash current based upon the voltage level present at the IN pin utilizing

an Input Voltage Flash Monitor. Two adjustable thresholds (IVM-D and IVM-U) ranging from 2.9V to 3.6V in 100

mV steps, and four different usage modes (Report Mode, Stop and Hold, Adjust Down Only, Adjust Up and

Down), are provided. The Flags register has the fault flag set when the input voltage crosses the IVM-D value. In

the Report Mode, apart from the fault flag triggering, no action is taken on the LED current. Additionally, the IVM-

D threshold sets the input voltage boundary that forces the LM3556 to either stop ramping the flash current

during startup (Stop and Hold Mode) or to start decreasing the LED current during the flash (Adjust Down Only

and Adjust Down and Up). The IVM-U threshold sets the input voltage boundary that forces the LM3556 to start

ramping the flash current back up towards the target (Adjust Up and Down Mode). The IVM-U threshold is equal

to the IVM-D value plus the programmed hysteresis value also stored in the Input Voltage Flash Monitor (IVFM)

Mode Register (0x01).

To help prevent a premature current reduction, the LM3556 has four different filter timers that start once the input

voltage decreases below the IVM-D line. These filter times are set in the Silicon Revision and Filter Time

Register (0x00). For more information, please refer to the Input Voltage Flash Monitor (IVFM) Mode Register

(0x01) and Configuration Register (0x07) sections of this datasheet.

Fault Protections

Fault Operation

Upon entering a fault condition, the LM3556 will set the appropriate flag in the Flags Register (0x0B), placing the

part into standby by clearing and locking the Torch Enable bit (TEN), Pre-Charge bit and Mode Bits (M1, M0) in

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