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TPS54678 Datasheet(PDF) 10 Page - Texas Instruments |
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TPS54678 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 39 page 0 200 1200 1400 1800 0 40 80 100 180 200 RT Resistance (KΩ) G009 1000 160 400 600 800 1600 140 120 60 20 1.052 56183 R (kΩ) = T F (KHz) SW é ù ë û TPS54678 SLVSBF3 – JUNE 2012 www.ti.com The minimum controllable on time is typically 85 ns at 3 A current load and 100 ns at no load, and will limit the maximum operating input voltage or output voltage. (5) Figure 4. Switching Frequency vs RT Set Resistor Overcurrent Protection The TPS54678 implements current mode control which uses the COMP pin voltage to turn off the high side MOSFET and turn on the low side MOSFET on a cycle-by-cycle basis. Each cycle the switch current and the COMP pin voltage are compared, when the peak switch current intersects the COMP voltage the high side switch is turned off. High-Side Overcurrent Protection During overcurrent conditions that pull the output voltage low, the error amplifier will respond by driving the COMP pin high, increasing the switch current. The error amplifier output is clamped internally. This clamp functions as a high-side switch current limit. When the high-side switch current limit occurs consecutively for 512 CLK cycles, the converter enters hiccup mode in which no switching action happens for about 16000 cycles. This helps to reduce the power consumption during an overcurrent event. Low-Side Overcurrent Protection The low-side MOSFET’s conduction current is also monitored by TPS54678. During normal operation, the low- side sources current into the load. When the sourcing current reaches the internally set low-side sourcing (forward) current limit, the high-side is not turned on and skipped during the next clock cycle. Under this condition, the low-side is kept on until the sourcing current becomes less than the internally set current limit and then the high-side is turned on at the beginning of the following clock cycle. This ensures protection under an output short condition; thereby, preventing current run-away. The low-side can also sink current from the load. If the low-side sinking (reverse) current limit is exceeded, the low-side is turned off immediately for the rest of the clock cycle. Under this condition, both the high-side and low- side are off until the start of the next cycle. Safe Start-Up into Pre-Biased Outputs The TPS54678 allows monotonic startup into pre-biased output. The low side Fet turns on for a short time period every cycle before the output voltage reaches the pre-biased voltage. This ensures the boot cap has enough charge to turn on the top Fet when the output voltage reaches the pre-biased voltage. The TPS54678 also implements low side current protection by detecting the voltage over the low side MOSFET. When the converter sinks current through the low side FET and if the current exceeds 4 A, the control circuit turns the low side Fet off. Due to the implemented prebias function, the low side Fet reverse current protection should not be reached, but it provides another layer of protection. 10 Submit Documentation Feedback Copyright © 2012, Texas Instruments Incorporated Product Folder Link(s): TPS54678 |
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