Linear LDO Regulator 03/07/2011

 Application circuit Linear Regulator (LDO)  Block digram

Why use LDO and concerns Easy design Simple structure and low cost Ripple-free and less noisy Fast transient response  Why use:  Concerns: Poor efficiency Output voltage needs to be lower than input voltage

Lower Dropout Voltage(Vdo) Lower Quiescent Current(Ground Current) Line and Load Regulation Transient Response PSRR(Power Supply Rejection Ratio) Current Limiting and Thermal Protection Output Noise Voltage Key Specifications (LDO)

The minimum input-to-output different voltage is required to maintain the output voltage within specified voltage. e.g., Vin_min = 2.8V(typical Vo)+ 0.3V(Vdo) =3.1V higher efficiency for smaller Vin-Vo longer battery life. Battery discharges down to Vbat=Vo + Vdo Dropout voltage (LDO)

Quiescent Current (LDO)  Quiescent current (Iq): Lower Iq, higher efficiency. Especially important under standby mode

Load/Line Regulation (LDO)  Load regulation:  Line regulation:

Transient Response (LDO)  Transient load response: Load Vin Vo Cout esr Cb LDO INOUT GND Cin

Specified in db under 100, 1k, or 10k Hz. PSRR (LDO)  Power rejection ratio: -66dB at 10kHz

Stability LDO require proper Cout esr value to create a zero for enough phase margin to maintain control loop stability. Usually stable region of ESR is provided on the datasheets. RL Vin Vo Cout esr Cb LDO INOUT GND Cin Stable Range Unstable Range V IN =4.3V C IN =C OUT =1μF

Mainly caused by internal bandgap reference voltage. RF(Wifi, Wimax, MP) applications require LDO with low noise voltage <100uVrms to avoid noise voltage interfere with RF signals.. Low output noise (LDO)  Output noise voltage: V IN =3.3V V OUT =1.8V/10mA θnθn Output noise voltage =28.4uVrms

Efficiency and thermal issue (LDO)  Efficiency:  Power dissipation(Pd): device power dissipation should below package power rating Pd = (Vi –Vo) * Io  Thermal concern: typical package power rating below is for reference (please refer to datasheets  JA, data to calculate for each LDO) SOT23/25/26: Pw = 0.35W SOT89: Pw = 0.5W SOT223: Pw = 0.8W TO252: Pw = 1.2W TO263: Pw = 2W SOP8: Pw = 0.6W  JA : thermal resistance from junction to ambient  JC = thermal resistance from junction(die temperature) to case

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