AC/DC regulator enables low standby power in USB chargers

4th December 2015

Jordan Mulcare

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The AP3785T AC-DC primary-side switched-mode power supply controller introduced by Diodes Incorporated is aimed at 8W to 15W 5V output USB chargers that necessitate ultra-low standby power. This market requirement is driven by smart phones and tablets with larger battery capacities and increasingly tight power supply efficiency regulations.

Various features contribute to the superior performance delivered by the AP3785T: a no-load input power rating of less than 10mW; higher efficiency and better EMI result from valley-on operation and a low VCS jitter specification; higher conversion efficiency is also achieved when the AP3785T is used in conjunction with a synchronous rectifier, such as the AP4341. Multiple PWM/PFM control modes further help improve audio noise and efficiency.

Over-voltage protection, output short-circuit protection and transformer saturation protection (via primary peak-current limitation) are all built into the AP3785T. Internal temperature protection is also provided along with a dedicated pin to support external temperature protection. When operating as a constant current regulator, a feedback pin enables compensation for changes in line voltage whereas in constant voltage mode the AP3785T can compensate for voltage drops due to different cable lengths or gauge.