LDO at full load goes out of regulation

[Chethan]

LDO at full load

Hi,
I have designed an LDO with output voltage of 5V. The regulation seems fine till i decrease the load resistance below 100 ohms. The LDO should be capable of driving 150mA. But with low resistance to drive that high current my LDO is going out of regulation. Can someone explain me the reason.

thanx in advance

 

[Arkady90]

Re: LDO at full load

Hi

You said that You have designed a LDO regulator, I'm guessing from discreete component. That could be tricky. There are few configuration. The most common one is with use of PNP transistor, in operation with common emiter. That means in order to control the emiter current your Base potencial will be lover than emiter. In LDO any of the intermadiate control voltages cannot be abowe the input voltage.
In your case could be that your gain of transistor is not enought, simple use a darlington transistor and see what happen.
The best would be if you can use monolitic LDO regulator, there are plenty of those on the market.

Arek

 

[alenhan]

LDO at full load

maybe you need to check the driver ability of pass transistor

 

[surianova]

Re: LDO at full load

Hi,
I have designed an LDO with output voltage of 5V. The regulation seems fine till i decrease the load resistance below 100 ohms. The LDO should be capable of driving 150mA. But with low resistance to drive that high current my LDO is going out of regulation. Can someone explain me the reason.

thanx in advance

If the driving load if more than the power mos can drive, the transistor in the
feedback opamp might go to linear regoin and reduce the gain, and cause the
regulator out of regulation.