VOUT

STROBE

ILED

13

LM3648, LM3648TT

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SNVSA68D – OCTOBER 2014 – REVISED NOVEMBER 2016

Product Folder Links: LM3648 LM3648TT

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Copyright © 2014–2016, Texas Instruments Incorporated

Feature Description (continued)

Figure 24. IR Mode Timeout

7.4 Device Functioning Modes

7.4.1 Start-Up (Enabling The Device)

Turnon of the LM3648 Torch and Flash modes can be done through the Enable Register. On start-up, when

VOUT is less than VIN the internal synchronous PFET turns on as a current source and delivers 200 mA (typ.) to

the output capacitor. During this time the current source (LED) is off. When the voltage across the output

capacitor reaches 2.2 V (typ.), 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 VIN supply.

7.4.2 Pass Mode

The LM3648 starts up in Pass Mode and stays there until Boost Mode is needed to maintain regulation. If the

voltage difference between VOUT and VLED falls below VHR, the device switches to Boost Mode. In Pass Mode the

boost converter does not switch, and the synchronous PFET turns fully on bringing VOUT up to VIN − ILED ×

RPMOS. In Pass Mode the inductor current is not limited by the peak current limit.

7.4.3 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 LM3648 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. If the TX pin is then pulled low before the Flash pulse terminates, the LED

current returns 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 turns off.

7.4.4 Input Voltage Flash Monitor (IVFM)

The LM3648 has the ability to adjust the flash current based upon the voltage level present at the IN pin utilizing

the Input Voltage Flash Monitor (IVFM). The adjustable threshold IVFM-D ranges from 2.9 V to 3.6 V in 100-mV

steps, with three different usage modes (Stop and Hold, Adjust Down Only, Adjust Up and Down). The Flags2

Register has the IVFM flag bit set when the input voltage crosses the IVFM-D value. Additionally, the IVFM-D

threshold sets the input voltage boundary that forces the LM3648 to either stop ramping the flash current during

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

and Down Adjust). In Adjust Up and Down mode, the IVFM-D value plus the hysteresis voltage threshold set the

input voltage boundary that forces the LM3648 to start ramping the flash current back up towards the target.