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TPS72733DSET Datasheet(PDF) 10 Page - Texas Instruments |
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TPS72733DSET Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 23 page  I (mA)=C ( F) 0.07(V/ s)+I (mA) m m LOAD SOFTSTART OUT ´ TPS727xx SBVS128C – JUNE 2009 – REVISED JANUARY 2011 www.ti.com APPLICATION INFORMATION ILIMIT × RLOAD. The PMOS pass transistor dissipates The TPS727xx family belongs to a family of new (VIN – VOUT) × ILIMIT until thermal shutdown is generation LDO regulators that consume extremely triggered and the device is turned off. As the device low quiescent current while simulatenously delivering cools down, it is turned on by the internal thermal excellent PSRR with very little headroom (VIN – VOUT shutdown circuit. If the fault condition continues, the differential voltage), and very good transient device cycles between current limit and thermal response. These features, combined with low noise shutdown. See the Thermal Information section for without a noise reduction pin in an ultrasmall more details. package, make this device ideal for portable applications. This family of regulators offers The PMOS pass element in the TPS727xx has a sub-bandgap output voltages, current limit and built-in body diode that conducts current when the thermal protection, and is fully specified from –40°C voltage at the OUT pin exceeds the voltage at the IN to +125°C. pin. This current is not limited, so if extended reverse voltage operation is anticipated, external limiting to 5% of rated output current is recommended. INPUT AND OUTPUT CAPACITOR REQUIREMENTS SOFT START Although an input capacitor is not required for stability, it is good analog design practice to connect The startup current is given by Equation 1: a 0.1mF to 1.0mF low equivalent series resistance (1) (ESR) capacitor across the IN pin and GND input of the regulator. This capacitor counteracts reactive This equation shows that soft-start current is directly input sources and improves transient response, noise proportional to COUT. rejection, and ripple rejection. A higher-value The output voltage ramp rate is independent of COUT capacitor may be necessary if large, fast rise-time and load current, and has a typical value of 0.07V/ms. load transients are anticipated, or if the device is not located close to the power source. If source The TPS727xx automatically adjusts the soft-start impedance is not sufficiently low, a 0.1mF input current to supply both the load current and the COUT capacitor may be necessary to ensure stability. charge current. For example, if ILOAD = 0mA upon enabling the LDO, ISOFT START = 1mF × 0.07 V/ms + The TPS727xx is designed to be stable with standard 0mA = 70mA, the current that charges the output ceramic capacitors with values of 1.0mF or larger at capacitor. the output. X5R- and X7R-type capacitors are best because they have minimal variation in value and If ILOAD = 200mA, ISOFT START = 1mF × 0.07V/ms + ESR over temperature. Maximum ESR should be less 200mA = 270mA, the current required for charging than 200m Ω. output capacitor and supplying the load current. If the output capacitor and load are increased such BOARD LAYOUT RECOMMENDATIONS TO that the soft-start current exceeds the output current IMPROVE PSRR AND NOISE PERFORMANCE limit, it is clamped at the typical current limit of To improve ac performance such as PSRR, output 400mA. For example, if COUT = 10mF and IOUT = noise, and transient response, it is recommended that 200mA, 10mF × 0.07V/ms + 200mA = 900mA is not the board be designed with separate ground planes supplied. Instead, it is clamped at 400mA. for VIN and VOUT, with the ground plane connected only at the GND pin of the device. In addition, the SHUTDOWN ground connection for the output capacitor should The enable pin (EN) is active high and is compatible connect directly to the GND pin of the device. High with standard and low voltage, TTL-CMOS levels. ESR capacitors may degrade PSRR. When shutdown capability is not required, EN can be connected to the IN pin. INTERNAL CURRENT LIMIT The TPS727xx internal current limit helps protect the regulator during fault conditions. During current limit, the output sources a fixed amount of current that is largely independent of output voltage. In such a case, the output voltage is not regulated, and is VOUT = 10 Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated |
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