MP2303A dynamic load output voltage drop problem analysis

Hi


I have used MP2303A to design a 15Vinput to 12V output buck circuit, the maixmum output current is 1.5A. recently I encountered a strange output voltage drop problem.this buck circuit have two load. one is camera (work voltage:6V-15V, maixmun current 1A). another is fan(maixmun current 500mA).when the load connect camera alone. some batches of MP2303A chips showed the output voltage dropped to 3.5V(the chip have problem with the PIN 1 is groove,and the chip have no problem with the PIN 1 is dots),the output voltage drop rate of MP2303A is about 50%. I used large current static load to test the chip MP2303A alone, Did not find any problems。
I have tryed three ways to improve the voltage dropped problem

  1. I refer to the datasheet “External Bootstrap Diode”,when the dute cycle D >65% (D=12V/15V=80%).the circuit need add a 1N4148 diode form the output voltage to the BS pin. this way can enhance the enficiency of the regulator. so the output voltage drop problem is disappeared.the circuit work fine. but later I found only 5V or 3.3V output voltage can add this diodes. I guess it might be afraid of 12V voltage damaging the BS PIN. I Know this way Strengthen the driving ability of the upper MOSFET inside the chip.
  2. add a 470PF feed forward capacitor in paralled with the upper feedback resistor R183 in the circuit, the output voltage drop problem is disappeared.the circuit work fine. I know this way can increase crossover frequency(also called bandwidth ) of the regulater ,Increase the dynamic effect of the load response. but I used the online design tool to simulate the regulater after adding the 470PF capacitor .the bandwidth of the reulater increased a lot ,but the phase margin and gain margin are getting worse.
  3. removed the common mode inductor L10 of the camera laod line . the output voltage drop problem is disappeared,the circuit work fine. This situation may be caused by the interference of L10 in the load line to the feedback loop of MP2303A.

As described above I think the MP2303A with output voltage drop problem should be OK .Mainly due to some batch differences in performance。 I have some problem about this circuit. this output voltage drop problem Is it caused by bad loop design? such as crossover frequency is Low.
What is the main effect of common mode inductor to the MP2303A ? Please give some suggestions for circuit improvement
Thank you!

Hello,
Please note the MP2303A isn’t advised for new designs, so if another device can be used in this design that is recommended. To address this design, there are many components different from recommended.

  1. BST cap is 10nF, should be 100nF to support bias at switching frequency and high duty cycle.
  2. Inductor is large value at 33uH, can be 10uH to 15uH with no problem.
  3. Output cap is 100uF+10uF, which combined with large inductor, reduces bandwidth unnecessarily.
  4. Compensation components are off the data sheet values as well, can be adjusted with inductor, output cap for best performance.
  5. Soft start cap is very large at 10uF which is about 1.3 seconds startup delay. If there is no reason to have the long delay then a smaller value is advised.

Of the above items that can cause issues, the BST cap and the bandwidth would be critical. Normally a common mode choke on the output will not be an issue, depending on the impedance. It can help to test by using a power supply to bypass the DCDC converter and note what happens on turnon and with load change after the choke.
Regards,

Hi James

Thank you so much for your response,Based on your circuit suggestions and on-line simulations。
I plan to redesign the circuit. Please check it。I have a doubt that the maximum ESR value of this output tantalum capacitor is 500mR, but Actual measurement

value is 100mR(at 100 Khz) , I found that this ESR value has a greater impact on the bandwidth frequency and loop margin and gain margin.
Please give me some advice. Thank you