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AP3790MTR-G1 Datasheet(PDF) 8 Page - Diodes Incorporated |
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AP3790MTR-G1 Datasheet(HTML) 8 Page - Diodes Incorporated |
8 / 17 page AP3790 Document number: DS37221 Rev. 3 - 2 8 of 17 www.diodes.com October 2014 © Diodes Incorporated AP3790 A Product Line of Diodes Incorporated Operation Principle Description (Cont.) 0V See equation 5 tONS tSAMPLE Figure 3. Auxiliary Voltage Waveform The voltage detection point is at a constant delay time of the D1 on-time. The constant delay time is changed with the different primary peak current. The CV loop control function of AP3790 then generates a D1 off-time to regulate the output voltage. Constant Current Operation The AP3790 can work in constant-current (CC) mode. Figure 2 shows the secondary current waveforms. In CC operation mode, the CC control loop of AP3790 will keep a fixed proportion between D1 on-time tONS and D1 off-time tOFFS. The fixed proportion is 4 4 OFFS ONS t t (6) The relationship between the output current and secondary peak current IPKS is given by: OFFS ONS ONS PKS OUT t t t I I 2 1 (7) As to tight coupled primary and secondary winding, the secondary peak current is PK S P PKS I N N I (8) Thus the output constant-current is given by: PK S P OFFS ONS ONS PK S P OUT I N N t t t I N N I 8 2 2 1 (9) Therefore, AP3790 can realize CC mode operation by constant primary peak current and fixed diode conduction duty cycle. Multiple Segment Constant Peak Current As to the original PFM PSR system, the switching frequency decreases with output current decreasing, which will encounter audible noise issue since switching frequency decreases to audio frequency range, about less than 20kHz. In order to avoid audible noise issue, AP3790 uses 3-segment constant primary peak current control method. At constant voltage mode, the current sense threshold voltage is multiple segment with different loading, as shown in Figure 4, which are VCS_H for high load, VCS_M for medium load and VCS_L for light load. At constant current mode, the peak current is still VCS_H. |
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