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TPS57140QDRCRQ1 Datasheet(PDF) 17 Page - Texas Instruments |
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TPS57140QDRCRQ1 Datasheet(HTML) 17 Page - Texas Instruments |
17 / 53 page 0 500 1000 1500 2000 2500 0 25 50 75 100 125 150 175 200 RT/CLK-ClockResistance-kW V =12V, T =25°C I J 0 100 200 300 400 500 200 300 400 500 600 700 800 900 1000 1100 RT/CLK Resistance (kW) 1200 C006 SW RT k kHz 1.0888 206033 ( W) = ¦ ( ) 17 TPS57140-Q1 www.ti.com SLVSAP3D – DECEMBER 2010 – REVISED FEBRUARY 2016 Product Folder Links: TPS57140-Q1 Submit Documentation Feedback Copyright © 2010–2016, Texas Instruments Incorporated Feature Description (continued) 7.3.10 Constant Switching Frequency and Timing Resistor (RT/CLK Pin) The switching frequency of the TPS57140-Q1 device is adjustable over a wide range from approximately 100- kHz to 2500-kHz by placing a resistor on the RT/CLK pin. The RT/CLK pin voltage is typically 0.5 V and must have a resistor to ground to set the switching frequency. To determine the timing resistance for a given switching frequency, use Equation 7 or the curves in Figure 31 or Figure 32. To reduce the solution size, the designer would typically set the switching frequency as high as possible, but consider tradeoffs of the supply efficiency, maximum input voltage, and minimum controllable on-time. The minimum controllable on time is typically 130 ns, which limits the maximum operating input voltage. The frequency-shift circuit also limits the maximum switching frequency. Use Equation 7 or the curves in Figure 31 or Figure 32 to calculate the timing resistor values for the required switching frequency. (7) Figure 31. High-Range RT Figure 32. Low-Range RT 7.3.11 Overcurrent Protection and Frequency Shift The TPS57140-Q1 device implements current-mode control, which uses the COMP pin voltage to turn off the high-side MOSFET on a cycle-by-cycle basis. During each cycle, the device compares the switch current and COMP pin voltage. When the peak switch current intersects the COMP voltage, the high side switch turns off. During overcurrent conditions that pull the output voltage low, the error amplifier responds by driving the COMP pin high, increasing the switch current. The internal clamping of the error amplifier output functions as a switch current limit. To increase the maximum operating switching frequency at high input voltages, the TPS57140-Q1 device implements a frequency shift. The divisor of the switching frequency goes to 8, 4, 2, and 1 as the voltage ramps from 0 to 0.8 V on the VSENSE pin. The device implements a digital frequency shift to enable synchronizing to an external clock during normal start- up and fault conditions. Because the device can only divide the switching frequency by 8, there is a maximum input voltage limit in which the device operates and still has frequency-shift protection. During short-circuit events (particularly with high-input-voltage applications), the control loop has a finite minimum controllable on-time and the output has a very-low voltage. During the switch on-time, the inductor current ramps to the peak current limit because of the high input voltage and minimum on-time. During the switch off time, the inductor would normally not have enough off-time and output voltage for the inductor to ramp down by the ramp- up amount. The frequency shift effectively increases the off-time, allowing the current to ramp down. |
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