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TPS22918 Datasheet(PDF) 14 Page - Texas Instruments |
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TPS22918 Datasheet(HTML) 14 Page - Texas Instruments |
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14 / 27 page  14 TPS22918 SLVSD76A – FEBRUARY 2016 – REVISED MARCH 2016 www.ti.com Product Folder Links: TPS22918 Submit Documentation Feedback Copyright © 2016, Texas Instruments Incorporated 8.3.2.1 QOD when System Power is Removed The adjustable QOD can be used to control the power down sequencing of a system even when the system power supply is removed. When the power is removed, the input capacitor discharges at VIN. Past a certain VIN level, the strength of the RPD will be reduced. If there is still remaining charge on the output capacitor, this will result in longer fall times. For further information regarding this condition, refer to Shutdown Sequencing During Unexpected System Power Loss. 8.3.2.2 Internal QOD Considerations Special considerations must be taken when using the internal RPD by shorting the QOD pin to the VOUT pin. The internal RPD is a pulldown resistance designed to quickly discharge a load after the switch has been disabled. Care must be used to ensure that excessive current does not flow through RPD during discharge so that the maximum TJ of 125°C is not exceeded. When using only the internal RPD to discharge a load, the total capacitive load must not exceed 200 µF. Otherwise, an external resistor, REXT, must be used to ensure the amount of current flowing through RPD is properly limited and the maximum TJ is not exceeded. To ensure the device is not damaged, the remaining charge from CL must decay naturally through the internal QOD resistance and should not be driven. 8.3.3 Adjustable Rise Time (CT) A capacitor to GND on the CT pin sets the slew rate of VOUT. The CT capacitor will charge up until shortly after the switch is turned on and VOUT becomes stable. Once VOUT become stable, the capactior will discharge to ground. An approximate formula for the relationship between CT and the slew rate is shown in Equation 3: SR = 0.55× CT + 30 where • SR = slew rate (in µs/V) • CT = the capacitance value on the CT pin (in pF) • The units for the constant 28 are µs/V. The units for the constant 0.52 are µs/(V × pF). (3) This equation accounts for 10% to 90% measurement on VOUT and does not apply for CT = 0 pF. Use Table 2 to determine rise times for when CT = 0 pF. Rise time can be calculated by multiplying the input voltage by the slew rate. Table 2 contains rise time values measured on a typical device. (1) Typical values at 25°C with a 25 V X7R 10% ceramic capacitor on CT. Table 2. Rise Time Table CT× (pF) RISE TIME (µs) 10% - 90%, CL = 0.1 µF, CIN = 1 µF, RL = 10 Ω (1) VIN = 5 V VIN = 3.3 V VIN = 2.5 V VIN = 1.8 V VIN = 1.5 V VIN = 1.2 V VIN = 1.0 V 0 135 95 75 60 50 45 40 220 650 455 350 260 220 185 160 470 1260 850 655 480 415 340 300 1000 2540 1680 1300 960 810 660 560 2200 5435 3580 2760 2020 1715 1390 1220 4700 12050 7980 6135 4485 3790 3120 2735 10000 26550 17505 13460 9790 8320 6815 5950 As the voltage across the capacitor approaches the capacitor rated voltage, the effective capacitance reduces. Depending on the dielectric material used, the voltage coefficient changes. See Table 3 for the recommended minimum voltage rating for the CT capacitor. If using VIN = 1.2 V or 4 V, it is recommended to use the higher of the two CT Voltage ratings specified. |
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