LM3646 Datasheet, PDF (16/31 Pages) Texas Instruments – 1.5A Synchronous Boost Converter with Dual High-Side Current Sources and I2CCompatible Interface

LM3646
SNVS962 – DECEMBER 2013
www.ti.com
If the LED1 Current Control Value is set to a level that is higher than the MAX Flash LED Current Control Value,
LED1 will receive the MAX Flash LED Current Control value, and LED2 will be disabled.
When the part is enabled in Flash Mode through the Enable Register or the STROBE pin, all mode bits in the
Enable Register are cleared after a flash time-out event.
Torch Mode
In Torch Mode, the LED current sources (LED1/2) provide 128 target current levels from 0 mA to 187.5 mA. The
Torch currents are adjusted via bits[6:4] of the Max LED Current Control Register (0x05) and bits[6:0] of the
LED1 Torch Current Control Register (0x07). Torch Mode is activated by the Enable Register (0x01), or by
pulling the TORCH pin HIGH. Once the TORCH sequence is activated the current source (LED) will ramp up to
the programmed Torch current by stepping through all current steps until the programmed current is reached.
LED1 will receive the current value stored in the LED1 Torch Current Control Register, and LED2 will receive the
difference of the value stored in the MAX LED Current Control Register and LED1 Torch Current Control
Register.
If LED1 and LED2 Active:
LED1 = LED1 Torch Current Control Value
LED2 = MAX Torch Current Control Value - LED1 Torch Current Control Value
If MAX Torch Current Control Value < LED1 Torch Current Control Value
LED1 = MAX Torch Current Control Value
LED2 = Off
If the LED1 Torch Current Control Value is set to a level that is higher than the MAX Torch LED Current Control
Value, LED1 will receive the MAX Torch LED Current Control value, and LED2 will be disabled. Torch Mode is
not affected by Flash Timeout.
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 LM3646 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 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 TX input can be disabled by setting bit[3] (TX Enable) to a ‘0’ in the Max
LED Current Control Register (0x05).
Input Voltage Flash Monitor (IVFM)
The LM3646 has the ability to adjust the flash current based upon the voltage level present at the IN pin utilizing
an Input Voltage Flash Monitor. The IVFM block has an adjustable threshold (IVM-D) ranging from 2.9V to 3.2V
in 100 mV steps as well as adjustable hysteresis. The IVFM threshold and hysteresis are controlled by bits[4:3]
and bits[2:1] respectively, in the IVFM Mode Register (0x02). Flags Register1 (0x08) has the IVFM flag (bit[3])
set when the input voltage crosses the IVFM value. The IVFM threshold sets the input voltage boundary that
forces the LM3646 to stop ramping the flash current during startup in Stop and Hold Mode, or to actively adjust
the LED current lower in Down Adjust Mode, or to continuously adjust the LED current up and down in Up &
Down mode.
Stop and Hold Mode (Figure 54): Stops Current Ramp and Holds the level for the remaining flash if V_IN crosses
IVM-D Line. Sets IVFM Flag (bit[3] in Flags Register1) upon crossing IVM-D Line.
Down Mode (Figure 55): Adjusts current down if V_IN crosses IVM-D Line and stops decreasing once V_IN rises
above the IVM-D line + the IVFM hysteresis setting. The LM3646 will decrease the current throughout the flash
pulse anytime the input voltage falls below the IVM-D line, not just once. The flash current will not increase again
until the next flash. Sets IVFM Flag (bit[3] in Flags Register1) upon crossing IVM-D Line.
Up and Down Mode (Figure 56): Adjusts current down if V_IN crosses IVM-D Line and adjusts current up if V_IN
rises above the IVM-D line + the IVFM hystersis setting. In this mode, the current will continually adjust with the
rising and falling of the input voltage throughout the entire flash pulse. Sets IVFM Flag (bit[3] in Flags Register1)
upon crossing IVM-D Line.
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