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Linear Regulator Fundamentals 2.5 PMOS Linear-Regulator Operation • Voltage feedback samples the output R1 and R2 may be internal or external VIN VOUT PASS TRANSISTOR • Feedback controls pass transistor’s current to the load R1 CIN ERROR AMP COUT R LOAD VREF R2 2 P-FET-LDO Linear Regulators • Drop-out voltage set by by FET RDS-ON • Very low quiescent (ground pin) current • Ground-pin current independent of load 3 Driving PMOS LDO Pass Element 4 Gate Drive vs. Low Load Current 5 Gate Drive vs. High Load Current 6 Summary • The PMOS LDO has the following Characteristics: – Requires that the input voltage be higher than the output voltage based on the load current and the On Resistance of the pass element: • VIN > RDS(on) x IOUT – Requires that the output voltage be higher than the VGS requirement of the pass element – Requires careful selection of the output capacitor value and ESR ratings – To achieve similar RDS(on) performance a PMOS transistor will require a larger die area than NMOS transistors – The larger die area will affect pricing, and might affect performance 7 Thank you! 8