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LM3642 Datasheet(PDF) 10 Page - Texas Instruments |
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LM3642 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 30 page LM3642 SNVS891H – SEPTEMBER 2012 – REVISED SEPTEMBER 2015 www.ti.com 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. 10 Submit Documentation Feedback Copyright © 2012–2015, Texas Instruments Incorporated Product Folder Links: LM3642 |
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