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TPS40055 Datasheet(PDF) 10 Page - Texas Instruments |
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TPS40055 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 39 page ( ) ( ) f A A SW 8 3.48 C R 7.9 - = ´ ´ ( ) ( ) ( ) KFF A IN min R 8 3.48 R 495k 499k 0.1 V 3.48 ´ - = = W = W ´ - UDG-02132 Clock PWM RAMP PowerGood VIN UVLO Threshold 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 TPS40054, TPS40055, TPS40057 SLUS593I – DECEMBER 2003 – REVISED DECEMBER 2014 www.ti.com Feature Description (continued) Figure 7. Undervoltage Lockout Operation The tolerance on the UVLO set point also affects the maximum duty cycle achievable. If the UVLO starts the device at 10% below the nominal startup voltage, the maximum duty cycle is reduced approximately 10% at the nominal startup voltage. The impedance of the input voltage can cause the input voltage, at the controller, to sag when the converter starts to operate and draw current from the input source. Therefore, there is voltage hysteresis that prevents nuisance shutdowns at the UVLO point. With RT chosen to select the operating frequency and RKFF chosen to select the startup voltage, the approximate amount of hysteresis voltage is shown in Figure 3. Some applications may require an additional circuit to prevent false restarts at the UVLO voltage level. This applies to applications which have high impedance on the input voltage line or which have excessive ringing on the VIN line. The input voltage impedance can cause the input voltage to sag enough at startup to cause a UVLO shutdown and subsequent restart. Excessive ringing can also affect the voltage seen by the device and cause a UVLO shutdown and restart. A simple external circuit provides a selectable amount of hysteresis to prevent the nuisance UVLO shutdown. Assuming a hysteresis current of 10% IKFF, and the peak detector charges to 8 V and VIN(min) = 10 V, the value of RA is calculated by Equation 3 using a RKFF = 71.5 kΩ. (3) CA is chosen to maintain the peak voltage between switching cycles to keep the capacitor charge from drooping 0.1 V (from 8 V to 7.9 V). (4) The value of CA may calculate to less than 10 pF, but some standard value up to 47 pF works adequately. The diode can be a small-signal switching diode or Schottky rated for more then 20 V. Figure 8 shows a typical implementation using a small switching diode. The tolerance on the UVLO set point also affects the maximum duty cycle achievable. If the UVLO starts the device at 10% below the nominal startup voltage, the maximum duty cycle is reduced approximately 10% at the nominal startup voltage. 10 Submit Documentation Feedback Copyright © 2003–2014, Texas Instruments Incorporated Product Folder Links: TPS40054 TPS40055 TPS40057 TPS40054: Not Recommended For New Designs |
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