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LM3642 Datasheet(PDF) 11 Page - Texas Instruments |
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LM3642 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 30 page LM3642 www.ti.com SNVS891H – SEPTEMBER 2012 – REVISED SEPTEMBER 2015 Feature Description (continued) 7.3.3.5 Undervoltage Lockout (UVLO) The LM3642 has an internal comparator that monitors the voltage at IN which will force the LM3642 into shutdown if the input voltage drops to 2.8 V. If the UVLO monitor threshold is tripped, the UVLO flag bit will be set in the Flags Register. If the input voltage rises above 2.8 V, the LM3642 will not be available for operation until there is an I2C read command initiated for the Flags Register. Upon a read, the flag register will be cleared, and normal operation can resume. This feature can be disabled by writing a ‘0’ to the UVLO EN bit in the Input Voltage Flash Monitor Register. The mode bits in the Enable Register are cleared upon a UVLO event. 7.3.3.6 Thermal Shutdown (TSD) When the LM3642 device’s die temperature reaches 150°C the boost converter shuts down, and the NFET and PFET turn off, as does the current source (LED). When the thermal shutdown threshold is tripped, a '1' gets written to the corresponding bit of the Flags Register (Thermal Shutdown bit), and the LM3642 will go into standby. The LM3642 will only be allowed to restart after the Flags Register is read, clearing the fault flag. Upon restart, if the die temperature is still above 150°C, the LM3642 will reset the fault flag and re-enter standby. The mode bits in the Enable Register are cleared upon a TSD. 7.3.3.7 LED and/or VOUT Fault The LED Fault flag in the Flags Register reads back a '1' if the part is active in Flash or Torch Mode and the LED output or the VOUT node experiences short condition. The LM3642 determines an LED open condition if the OVP threshold is crossed at the OUT pin while the device is in Flash or Torch Mode. An LED short condition is determined if the voltage at LED goes below 500 mV (typ.) while the device is in Torch or Flash Mode. There is a delay of 256- μs deglitch time before the LED flag is valid and 2.048 ms before the VOUT flag is valid. This delay is the time between when the Flash or Torch current is triggered and when the LED voltage and the output voltage is sampled. The LED flag can be reset by reading back the flags register. The mode bits in the Enable Register are cleared upon an LED and/or VOUT fault. 7.4 Device Functional Modes 7.4.1 Start-up (Enabling the Device) Turnon of the LM3642 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 350 mA (typ.) to the output capacitor. During this time the current source (LED) is off. When the voltage across the output capacitor reaches 2.2V (typ.), the current source will turn on. At turnon the current source will step 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 LM3642 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. In this situation the output current must be limited to 2 A. 7.4.3 Flash Mode In Flash Mode, the LED current source (LED) provides 16 target current levels from 93.75 mA to 1500 mA. The Flash currents are adjusted via the Current Control Register. Flash Mode is activated by the Enable Register, or by pulling the STROBE pin HIGH. Once the Flash sequence is activated the current source (LED) will ramp up to the programmed Flash current by stepping through all current steps until the programmed current is reached. When the part is enabled in the Flash Mode through the Enable Register, all mode bits in the Enable Register are cleared after a flash time-out event. Table 1 shows the I2C commands and the state of the mode bits, if the STROBE pin is used to enable the Flash Mode. Copyright © 2012–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3642 |
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