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TPS40021-EP Datasheet(PDF) 9 Page - Texas Instruments |
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TPS40021-EP Datasheet(HTML) 9 Page - Texas Instruments |
9 / 39 page TPS40021-EP www.ti.com SLUSB58 – SEPTEMBER 2012 APPLICATION INFORMATION The TPS40021 is a low-input voltage, synchronous, voltage mode-buck controller. A typical application circuit is shown in Figure 1. These controllers are designed to allow construction of high-performance dc-to-dc converters with input voltages from 2.25 V to 5.5 V, and output voltages as low as 690 mV. Using a top side N-channel MOSFET for the primary buck switch results in lower switch resistance for a given gate charge. The device controls the delays from main switch off to rectifier turn on and from rectifier turn off to main switch turn on in a way that minimizes diode losses (both conduction and recovery) in the synchronous rectifier. The reduction in these losses is significant and can mean that for a given converter power level, smaller FETs can be used, or that heat sinking can be reduced or even eliminated. The TPS40021 is the controller of choice for most general purpose synchronous buck designs, operating in two quadrant mode (i.e. source or sink current) full time. This device provides the best performance for output voltage load transient response over the widest load current range. The controller provides for a coarse short circuit current-limit function that provides pulse-by-pulse current limiting, as well as integrates short circuit current pulses to determine the existence of a persistant fault state at the converter output. If a fault is detected, the converter shuts down for a period of time (determined by six soft- start cycles) and then restarts. The current-limit threshold is adjustable with a single resistor connected from VDD to the ILIM/SYNC pin. This overcurrent function is designed to protect against catastrophic faults only, and cannot be guaranteed to protect against all overcurrent conditions. The controller implements a closed-loop soft start function. Startup ramp time is set by a single external capacitor connected to the SS/SD pin. The SS/SD pin also doubles as a shutdown function. Voltage Reference The bandgap cell is designed with a trimmed, curvature corrected (< 1%) 0.69-V output, allowing output voltages as low as 690 mV to be obtained. Oscillator The ramp waveform is a saw-tooth form at the PWM frequency with a peak voltage of 1.25 V, and a valley of 0.3 V. The PWM duty cycle is limited to a maximum of 97%, allowing the bootstrap and charge pump capacitors to charge during every cycle. Bootstrap/Charge Pump The TPS40021 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. 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. Copyright © 2012, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: TPS40021-EP |
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