Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

LM2940 Stability Problems

Status
Not open for further replies.
R

Rocketman46

Member level 3
Joined
Jan 9, 2014
Messages
57
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
1,859
Hi All,

I am having problems with the stability of a LM2940 5V 1A regulator. When this regulator is powered it oscillates on the output. When I put a DVM across either Vin & Gnd or Vout & Gnd the output stops oscillating.

I have checked my capacitor values:

* Vin Cap = 0.47uF

* Vout Cap = 22uF

Also attached below is the datasheet to the Vout 22uF cap. I am using the 35V version and I think the ESR meets the LM2940 requirements. Does the DVM test above mean my capacitor values or ESR or wrong?

Thanks,

Rocketman46
 

Attachments

  • 22uF 35V Radial SMT.pdf
    102.9 KB · Views: 57

From the datasheet you attached, the capacitor you used has a tan(delta)=0.12 @ 120 Hz. This means an ESR = 7 ohm that falls in the instability region of the component.
From the chapter "8.1 Application information" of the datasheet of the LM2940 the series resistance of the capacitor has to be in the range 0.1 < ESR < 1 ohm for a load of few mA. The range increase up to roughly 0.05 < ESR < 5 ohm (graph 20 in the datasheet, presuming the y-axis is logarithmic)

https://www.ti.com/lit/ds/symlink/lm2940c.pdf
 

Hi,

Thanks for your input. Am I reading the datasheet wrong, I thought the esr was 0.7 ohm for the 22uf 35v capacitor? What capacitor would you suggest?

Thanks,
 

Very good analysis albbg. Tantalum capacitors normally present low ESR, we can suggest appropriate based on voltage and ripple current rating and derating factors.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top