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TPS40020PWPR датащи(PDF) 8 Page - Texas Instruments |
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TPS40020PWPR датащи(HTML) 8 Page - Texas Instruments |
8 / 32 page TPS40020 TPS40021 SLUS535C − MARCH 2003 − REVISED SEPTEMBER 2004 www.ti.com 8 APPLICATION INFORMATION Bootstrap/Charge Pump The TPS4002X series includes a charge pump to boost the drive voltage to the power MOSFET’s to higher levels when the input supply is low. A capacitor connected from PVDD to PGND is the storage cap for the pump. A capacitor connected from SW to BOOT2 gets charged every switching cycle while LDRV is high and its charge is dumped on the PVDD capacitor when HDRV goes high. An internal switch disables the charge pump when the voltage on PVDD reaches approximately 4.8 V and enables pumping when PVDD falls to approximately 4.6 V. The high-side driver uses the capacitor from SW to BOOT1 as its power supply. When SW is low, this capacitor charges from the PVDD capacitor. When the SW pin goes high, this capacitor provides above-rail drive for the high-side N-channel FET. PVDD, BOOT1 and BOOT2 are pre-charged to the VDD voltage during a shutdown condition. For low-input voltage converters, utilizing higher gate threshold voltage MOSFETs, it may be necessary to add an Schottky diode from VDD (anode) to BOOT1 to guarantee sufficient voltage for initial start up. Once switching starts the charge pump reverses bias on the Schottky diode. When operating the TPS40020 under no load or extremely light-load conditions the controller will be operating in discontinuous Mode (DCM); reverse current is prevented from flowing in the synchronous rectifier. In DCM the on times for both the HDRV and LDRV pulses can become too narrow to provide adequate charging of PVDD and BOOT1 outputs, causing their voltages to collapse. Insufficient PVDD and BOOT1 voltages prevent the external MOSFETS from becomming fully enhanced, causing loss of converter output regulation. Schottky diodes from VIN (anode) to PVDD, and VIN (anode) to BOOT1, as well as a pre-load can be added to maintain PVDD and BOOT1 at voltage levels sufficient enough to fully enhance the external MOSFETs. The amount of pre-load typically ranges from 50 mA to 100 mA depending on operating conditions and external MOSFET selection. Drivers The HDRV and LDRV MOSFET drivers are capable of driving gate-to-source voltages up to 5.0 V. Using appropriate MOSFETs, a 25-A converter can be achieved. The LDRV driver switches between VDD and ground, while the HDRV driver is referenced to SW and switches between BOOT1 and SW. The maximum voltage between BOOT1 and SW is 5.0 V when PVDD is in regulation. |
Аналогичный номер детали - TPS40020PWPR |
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Аналогичное описание - TPS40020PWPR |
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