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]

Welcome @Harshwardhana, to the community!

This is the normal behavior of a voltage-more PWM regulator. To prevent pulse skipping, you can only increase the base load.

Does it answer your question?

Regards
/Peter

[Harshwardhana]

Thanks peter..

what does base load means..?

do you mean Iout?

also is it possible to resolve this by tuning compensation network..?

[Peter BENSCH]

Increasing the base load means increasing the minimum load current flowing. As already mentioned, this is typical behaviour of voltage mode regulators, but COT (constant on-time) regulators also require some minimum load. You cannot change this inherent behaviour by changing the compensation.

Linear regulators do not have this problem, but they burn up the entire voltage difference.

 

[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..