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TPS40051PWPG4 Fiches technique(PDF) 10 Page - Texas Instruments |
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TPS40051PWPG4 Fiches technique(HTML) 10 Page - Texas Instruments |
10 / 37 page TPS40050 TPS40051 TPS40053 SLUS540F − DECEMBER 2002 − REVISED JUNE 2004 www.ti.com 10 APPLICATION INFORMATION Once the soft-start is initiated, the UVLO cicruit must see a total count of seven cycles in which the ramp duration is longer than the clock cycle before an undervoltage condition is declared. (See Figure 4). 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 Figure 4. Undervoltage Lockout Operation 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 start-up 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) = 18 V, the value of RA is calculated by: R A + R KFF (8 * 3.5) 0.1 V IN(min) * 3.5 + 565 kW ^ 562 kW CA is chosen to maintain the peak voltage between switching cycles. To keep the capacitor charge from drooping 0.1-V, or from 8 V to 7.9 V. C A + (8 * 3.5) R A 7.9 f SW The value of CA imay calculate to less than 10 pF, but some standard value up to 470 pF works adequately. The diode can be a small signal switching diode or Schottky rated for more then 20 V. Figure 5 illustrates 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 start up voltage, the maximum duty cycle is reduced approximately 10% at the nominal start up voltage. (4) (5) |
Numéro de pièce similaire - TPS40051PWPG4 |
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