LDO DCDC Charge Pump的原理比较与不同之处.ppt

* Voltage regulator Simcom Hardware Dept.2  wangtao Agenda Voltage regulator presentation: AC-AC DC-AC DC-DC Agenda DC-DC Voltage regulator presentation: LDO Charge pump (inductor less DC-DC) DC-DC (inductor) LDO LDO ( Low Dropout) LDO is a linear regulator Dropout voltage output voltage within 100mV, (Vin – Vout) min LDO LDO Working principle: The voltage divided by resistors R1 & R2 is compared with the internal reference voltage by the error amplifier The MOSFET, which is connected to the Vout pin, is then driven by the subsequent output signal. The output voltage at the Vout pin is controlled & stabilized by a system of negative feedback. LDO parameters ? Input Voltage The minimum Vin must be larger than Vout + VDO, independent from the minimum value given in the selection table. ? Efficiency By neglecting the quiescent current (Iq) of the LDO, efficiency can be calculated as Vout/Vin. LDO parameters ? Power Dissipation PD = (Vin – Vout) x Iout; PD is limited by package. Compare with step down buck DC-DC, for higher power dissipation or requirements for higher efficiency, recommend buck. ? Capacitor Requirements The output capacitor and especially Equivalent Series Resistance (ESR) are critical for stability. ? Noise and PSRR Select an LDO with high power supply rejection ratio (PSRR) for noise immunity from the input supply and low output noise. Some LDO have a bypass (BP) pin for adding capacitance to lower the output noise. LDO parameters Ceramic Capacitor Equivalent Circuit Equivalent Series Resistance (ESR) is a critical factor in circuit performance Capacitor Impedance is a function of: Cap Value, ESR and Frequency LDO parameters Things to know about Ceramic Caps: ESR is a function of: Physical size Larger case size caps have lower ESR Material Type X7R – Best (lowest ESR) X5R – Good Y5V – Low cost (highest ESR) Capacitance vs. Frequency: Capacitance value becomes smaller as frequency increases (impedance drops