LM3642

SNVS891H – SEPTEMBER 2012 – REVISED SEPTEMBER 2015

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7.3 Feature Description

7.3.1 Power Amplifier Synchronization (TX/TORCH)

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 LM3642 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.

7.3.2 Input Voltage Flash Monitor (IVFM)

The LM3642 has the ability to adjust the flash current based upon the voltage level present at the IN pin utilizing

an Input Voltage Flash Monitor. Upon an IVFM event, the set voltage threshold from the IVFM Mode Register

sets the input voltage boundary that forces the LM3642 to stop ramping the flash current during start-up (Stop

and Hold Mode).

7.3.3 Fault and Protections

7.3.3.1 Fault Operation

Upon entering a fault condition, the LM3642 will set the appropriate flag in the Flags Register.

7.3.3.2 Flash Time-Out

The Flash Time-Out period sets the amount of time that the Flash Current is being sourced from the current

source (LED). The LM3642 has 8 time-out levels ranging 100 ms to 800 ms in 100-ms steps. The Flash Time-

Out period is controlled in the Flash Features Register (0x08). Flash Time-Out only applies to the Flash Mode

operation. The mode bits in the Enable Register (0x0A) are cleared upon a Flash Time-out.

If an I2C command is used to terminate the flash event, TI recommends selecting a flash time-out level 100 ms

above the desired flash duration.

7.3.3.3 Overvoltage Protection (OVP)

The output voltage is limited to typically 5 V (see VOVP in Electrical Characteristics). In situations such as an open

LED, the LM3642 will raise the output voltage in order to keep the LED current at its target value. When VOUT

reaches 5 V (typ.) the overvoltage comparator will trip and turn off the internal NFET. When VOUT falls below the

VOVP Off Threshold, the LM3642 begins switching again. The mode bits in the Enable Register are not cleared

upon an OVP.

7.3.3.4 Current Limit

The LM3642 features selectable inductor current limits that are programmable through the Flash Feature

Register of the I2C-compatible interface. When the inductor current limit is reached, the LM3642 will terminate

the charging phase of the switching cycle.

Since the current limit is sensed in the NMOS switch, there is no mechanism to limit the current when the device

operates in Pass Mode. In Boost Mode or Pass Mode if VOUT falls below 2.3 V, the part stops switching, and the

PFET operates as a current source limiting the current to 300 mA. This prevents damage to the LM3642 and

excessive current draw from the battery during output short-circuit conditions. The mode bits in the Enable

Register (0x0A) are not cleared upon a Current Limit event.

NOTE

Pulling additional current from the VOUT node during normal operation is not

recommended.

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