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TPS5402 Datasheet(PDF) 10 Page - Texas Instruments |
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TPS5402 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 18 page TPS5402 SLVSBK4 – SEPTEMBER 2012 www.ti.com The TPS5402 provides robust protection during short circuits. There is potential for overcurrent runaway in the output inductor during a short circuit at the output. The TPS5402 solves this issue by increasing the off time during short circuit conditions by lowering the switching frequency. The switching frequency is divided by 8, 4, 2, and 1 as the voltage ramps from 0 V to 0.8 V on the VSENSE pin. The relationship between the switching frequency and the VSENSE pin voltage is shown in Table 1. Table 1. Switching Frequency Conditions SWITCHING FREQUENCY VSENSE PIN VOLTAGE fSW VSENSE ≥ 0.6 V fSW/2 0.6 V > VSENSE ≥ 0.4 V fSW/4 0.4 V > VSENSE ≥ 0.2 V fSW/8 0.2 V > VSENSE Spread Spectrum In order to reduce EMI, TPS5402 introduces frequency spread spectrum. The jittering span is ±6% of the switching frequency with 1/512 swing frequency. Overvoltage Transient Protection The TPS5402 incorporates an overvoltage transient protection (OVTP) circuit to minimize output voltage overshoot when recovering from output fault conditions or strong unload transients. The OVTP circuit includes an overvoltage comparator to compare the VSENSE pin voltage and internal thresholds. When the VSENSE pin voltage goes above 109% × Vref, the high-side MOSFET will be forced off. When the VSENSE pin voltage falls below 107% × Vref, the high-side MOSFET will be enabled again. Inductor Selection The higher operating frequency allows the use of smaller inductor and capacitor values. A higher frequency generally results in lower efficiency because of switching loss and MOSFET gate charge losses. In addition to this basic trade-off, the effect of the inductor value on ripple current and low current operation must also be considered. The ripple current depends on the inductor value. The inductor ripple current (iL) decreases with higher inductance or higher frequency and increases with higher input voltage (VIN). Accepting larger values of iL allows the use of low inductances, but results in higher output voltage ripple and greater core losses. To calculate the value of the output inductor, use Equation 3. LIR is a coefficient that represents inductor peak- to-peak ripple to DC load current. It is recommended to set LIR to 0.1 ~ 0.3 for most applications. Actual core loss of the inductor is independent of core size for a fixed inductor value, but it is very dependent on the inductance value selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core loss and are preferred for high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates hard, which means that inductance collapses abruptly when the peak design current is exceeded. It results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate. It is important that the RMS current and saturation current ratings are not exceeding the inductor specification. The RMS and peak inductor current can be calculated from Equation 5 and Equation 6. 10 Submit Documentation Feedback Copyright © 2012, Texas Instruments Incorporated Product Folder Links: TPS5402 |
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