MP2451 Under voltage lockout (UVLO)

DC-DC Power Converters
1

Hello everyone,

I am currently working on a power supply circuit using the MP2451 with following parameters:

Vin_range: 4,6 - 27V
Vin_optimal: 12V
Iout_max: 150mA
Iout_min: 3mA

The schematic proposed by MPS DCDC Designer is presented below:
see Pic.1

The schematic used is presented below:
see Pic.2

Everything works fine except the under voltage protection. I have a voltage divider at the Enable input, therefore, according to the documentation, the IC should operate from a voltage of 4.6V (1/34.6 = 1.53V). The trailing threshold is around 0.7A, because the converter shuts down at 3.9V. (1/33.9 = 1.3V). In practice, however, it is different, when the input voltage is within the trailing threshold range (4.6-3.9 V), the converter randomly turns off and on. See oscillograms below:

Channels

  • Green - input Voltage (DC)
  • Yellow - output voltage ripples (AC)
    Conditions
  • I_load = 150mA

see Pic.3, Pic.4, Pic.5

Channels

  • Purple - SW node
  • Yellow - output voltage ripples
    Conditions
  • Vin = 4.27V
  • I_load = 150mA

see Pic.6

I have already tried many things:

  • changing the values of the feedback resistors to those proposed by the MPS Designer by adding compensation.
  • changing the input / output capacity.
  • adding small value MLCC capacitor to EN pin.

Unfortunately, the result is still the same. I only improved Vout when I decreased the value of the feedback resistors, now I have a stable 3.3V across the entire input voltage range. Unfortunately this did not solve my UVP problem. Does anyone have a suggestion?

2

Pic1

The schematic proposed by MPS DCDC Designer

Pic1
Pic1864×608 49.8 KB

3

Pic2

The schematic used

Pic2
Pic21222×624 17.6 KB

4

Pic3

  • Green - input Voltage (DC)
  • Yellow - output voltage ripples (AC)
    Conditions
  • I_load = 150mA

Pic3
Pic3800×480 63.3 KB

5

Pic 4

  • Green - input Voltage (DC)
  • Yellow - output voltage ripples (AC)
    Conditions
  • I_load = 150mA

JPG2
JPG2800×480 87.9 KB

6

Pic 5

  • Green - input Voltage (DC)
  • Yellow - output voltage ripples (AC)
    Conditions
  • I_load = 150mA

JPG3
JPG3800×480 59.6 KB

7
  • Purple - SW node
  • Yellow - output voltage ripples
    Conditions
  • Vin = 4.27V
  • I_load = 150mA

JPG4
JPG4800×480 77 KB

8

Hi m.kirejczyk,

Thanks for posting on the MPSNow Forum.
Your enable voltage might be falling below the Enable Up Threshold minimum. Take a look at the EC Table on page 4 of the datasheet.
It states the minimum EN voltage is 1.4V.

image
image1092×103 13.3 KB

From your scope captures, it looks like your Vin is ~4.2V. With your selected voltage dividers, the EN voltage is probably just below the minimum EN threshold ~1.39V. You can verify this with a DMM.
4.2V (49.9/49.9+100) = 1.39V
This might be the reason your output is turning on and off.
The schematic from the DC DC Design Tool has an input voltage of 12V.

image
image1076×291 82.6 KB

Since you’re using a 4.2Vin, you’ll need to adjust your resistor divider values to be above the EN threshold min.

Thanks,
Cindy