AP3770ANK6TR-G1-2

Application Note 1067 Application Notes for AP3770 System Solution Prepared by Su Qing Hua System Engineering Dept. Regulation (PSR). AP3770 has the special technique to suppress the audio noise, internal line compensation to reduce the number of system components, fixed cable compensation to compensate the voltage drop on different output cable for achieving good CV regulation. 1. Introduction The AP3770 uses Pulse Frequency Modulation (PFM) method to realize Discontinuous Conduction Mode (DCM) operation for FLYBACK power supplies. The principle of PFM is different with that of Pulse Width Modulation (PWM), so the design of transformer is also different. The AP3770 can achieve low standby power less than 30mW. The AP3770 can provide accurate constant voltage, constant current (CV/CC) regulation by using Primary Side Figure 1. 5V/1A Output for Battery Charger of Mobile Phone Figure 1 is AP3770 typical application circuit, which is a FLYBACK converter controlled by AP3770 with a 3-winding transformer---Primary winding (Np), Secondary winding (Ns) and Auxiliary winding (Na). The AP3770 senses the auxiliary winding feedback voltage at FB pin and obtains power supply at VCC pin. Vdri---A simplified driving signal of primary transistor Ip---The primary side current Is ---The secondary side current VSEC---The voltage of secondary tSW---The period of switching frequency tONP ---The time of primary side “ON” tONS ---The time of secondary side “ON” tOFF ---The discontinuous time tOFFS --- The time of secondary side “Off” IPK---Peak current of primary side IPKS---Peak current of secondary side VS---the sum of Vo and forward voltage of rectification diode Figure 2 is the typical operation waveforms of PFM controller. In this figure, a series of relative idea operation waveforms are given to illustrate some parameters used in following design steps. And the nomenclature of the parameters in Figure 2 is illustrated. Sep. 2011 Rev. 1. 0 BCD Semiconductor Manufacturing Limited 1 Application Note 1067 tONP Vdri tSW IPK IP IPKS IS tONS VS VSEC tOFFS tOFF Figure 2. Operation Waveforms Zero-Startup-Current technique, the startup up resistors R3+R4 should be higher to 10M to 14MΩ to further lower the power loss. The recommended value of dummy load resistor R13 is 4.7KΩ to 10KΩ for an model with 5V output voltage. The selection of dummy load resistor is a tradeoff between standby power and I-V Curve. 2. Five Aspects for System Design 1. 2. 3. 4. 5. Low Standby Power Design Switching Frequency Design Transformer and Power Devices Design Feedback Resistors Design Line Compensation Design 2.2 Switching Frequency Design 2.1 Low Standby Power Design In order to achieve low standby power, AP3770 decreases the minimum operating voltage. And due to proprietary VH=0.5V VCS_REF 1.4V VCPC fSW fSW 47.6kHz 20kHz IO 42%IO Figure 3. Relationship Between VCPC, fSW and IO with Constant Peak Current Sep. 2011 Rev. 1. 0 BCD Semiconductor Manufacturing Limited 2 Application Note 1067 Where, the fSW is the switching frequency. When the peak current IPK is constant, the output power depends on the switching frequency fSW. fSW is linearly increased with load increasing. When the constant peak current is adopted, the voltage of CPC pin is increased linearly with load increasing. The maximum value of VCPC is equal to t ONS 4 ⋅ VDD = ⋅ 3.5V = 1.4V t SW 10 (1) In AP3770, two-segmented peak current is used to realize audio noise suppression. The peak current is about 0.5V when IO>42%IOmax, and the peak current is about 0.5V/1.5 when IO