L7986ta Regulation issue at high voltage and light load

[Harshwardhana]

Hi..

I have used L7986ta ic for 10.8V to 34V vin and Output 3.7Vdc, during light load conditions my 3.7v dips and ic skips the pulses. What i can do??

[Peter BENSCH]

The tendency can be roughly estimated for 32V: fig 25 shows that the efficiency curve shifts more and more towards higher load current with increasing input voltage and also tends towards approx. 30...40% at 100mA. At 400kHz instead of 250kHz, as is often the case with switching regulators, the entire curve is shifted slightly to the lower right, i.e. towards poorer efficiency values.

You can also simulate your circuit with the eDesignSuite to determine the effects of decreasing current or increasing switching frequency.

As already mentioned, this is not primarily to do with the compensation network, but with the control principle.

[Harshwardhana]

Okay Peter, I would check increasing the Pre load as per graph..

just to confirm i have used following values in the circuit : 

R4: 2k2

C4: 0.1uf

C5:100pf

R2: 5K6 parallel with 12k

R1: 20k

R3: 2K7

C3: 0.47uF

in the datasheet it is given to choose value of R1 less than 5kohm as shown in pictures, is it required to change these ba

 

[Richard Li]

@Harshwardhana ,

Sounds you select Type III compensation network?

Do you have equipment can measure Bode plot?

If you didn't check Bode plot, I think you better use type II compensation network.

The compensation network impact phase, it may impact system stability.

For your case, the voltage so big range, load I guess change a lot.

This is very difficult situation, system may crush.

Regards,

Richard

[Peter BENSCH]

I have checked your design with the eDesignSuite and with your parameters I have determined e.g. a phase margin of 31.48°, which is significantly smaller than the recommended 45°.

Please change your parameters to these values:

• R4: 2k2 --> 910
• C4: 0.1uf --> 82nF (but 100n is fine, too)
• C5:100pf --> 1.5nF
• R2: 5K6 parallel with 12k --> 910
• R1: 20k --> 4.7k
• R3: 2K7 --> 330
• C3: 0.47uF --> 15nF

Regards
/Peter

[Harshwardhana]

Hi Peter,

Thanks for your suggestions. I have made the following changes.

• C3 0.47uF -> 10nF
• R3 2K7 -> 330E

All other values are unchanged. This config is giving me the best results for the entire voltage range of 19VDC to 38VDC the issue regarding voltage dips is solved after making just these two changes. The system is very stable upto 30VDC i.e the ripple on 3.7V is about 60-70mV but as I increase the input voltage above 30VDC the ripple on output increases to about 140 -160mV any tips to resolve this issue will be helpful.

Thanks in advance.

[Peter BENSCH]

This ripple is also typical of the operating principle. It inevitably increases as the input voltage increases because the pulses for recharging the coil become shorter and shorter. However, you could increase the inductance slightly as a test.

Regards
/Peter

[Harshwardhana]

Hi peter,

Base on our testing we have finalised the following component values:

R4: 2k2

C4: 0.1uf

C5:100pf

R2: 5K6 parallel with 12k

R1: 20k

R3: 330ohms

C3: 100pF

Is it okay to use these values..?

[Harshwardhana]

Hi.. can you please reply..