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RT9177-30CB Datasheet(PDF) 8 Page - List of Unclassifed Manufacturers |
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RT9177-30CB Datasheet(HTML) 8 Page - List of Unclassifed Manufacturers |
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8 / 12 page  RT9177 Preliminary www.richtek-ic.com.tw DS9177-00 Feb. 2001 8 Fig. 2 Load-Transient Considerations The RT9177 load-transient response graphs (see Typical Operating Characteristics) show two components of the output response: a DC shift from the output impedance due to the load current change, and the transient response. The DC shift is quite small due to the excellent load regulation of the IC. Typical output voltage transient spike for a step change in the load current from 1mA to 100mA is tens mV, depending on the ESR of the output capacitor. Increasing the output capacitor’s value and decreasing the ESR attenuates the overshoot. Shutdown Input Operation The RT9177 is shutdown by pulling the SHDN input low, and turned on by driving the input high. If this feature is not to be used, the SHDN input should be tied to VIN to keep the regulator on at all times (the SHDN input must not be left floating). To ensure proper operation, the signal source used to drive the SHDN input must be able to swing above and below the specified turn-on/turn-off voltage thresholds which guarantee an ON or OFF state (see Electrical Characteristics). The ON/OFF signal may come from either CMOS output, or an open-collector output with pull-up resistor to the RT9177 input voltage or another logic supply. The high-level voltage may exceed the RT9177 input voltage, but must remain within the absolute maximum ratings for the SHDN pin. Internal P-Channel Pass Transistor The RT9177 features a typical 1.1 Ω P-channel MOSFET pass transistor. It provides several advantages over similar designs using PNP pass transistors, including longer battery life. The P-channel MOSFET requires no base drive, which reduces quiescent current considerably. PNP-based regulators waste considerable current in dropout when the pass transistor saturates. They also use high base-drive currents under large loads. The RT9177 does not suffer from these problems and consume only 80µA of quiescent current whether in dropout, light-load, or heavy-load applications. Input-Output (Dropout) Voltage A regulator’s minimum input-output voltage differential (or dropout voltage) determines the lowest usable supply voltage. In battery-powered systems, this will determine the useful end-of-life battery voltage. Because the RT9177 uses a P- channel MOSFET pass transistor, the dropout voltage is a function of drain-to-source on-resistance [Rds (on)] multiplied by the load current. Reverse Current Path The power transistor used in the RT9177 has an inherent diode connected between the regulator input and output (see Fig.3). If the output is forced above the input by more than a diode-drop, this diode will become forward biased and current will flow from the VOUT terminal to VIN. This diode will also be turned on by abruptly stepping the input voltage to a value below the output voltage. To prevent regulator mis-operation, a Schottky diode should be used in any applications where input/output voltage conditions can cause the internal diode to be turned on (see Fig.4). As shown, the Schottky diode is connected in parallel with the internal parasitic diode and prevents it from being turned on by limiting the voltage drop across it to about 0.3V. < 100 mA to prevent damage to the part. T T T 1 > 1 > 1 > 1 > 2 > 2 > 2 > 2 > CB = 470pF 10nF 2.2nF VIN = 3.6V, CIN = 1 µF, COUT = 4.7µF 500 µS/Div CH1: 2V/div; CH2 = VOUT, 500mV/div CH1 CH2 |
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