FreeLists / si-list / [SI-LIST] Re: DC/DC converter output dip

[Jean_Pierre.Bouthemy@xxxxxxxxxx]

Hi Chris,

The design is a little bit complex. Let me try to clarify.
A +5V isolated converter is used as a "housekeeping" voltage that comes
first in order to supply the monitoring circuits.
I have a 3.3V isolated DC converter whose output feeds 3 non-isolated DC
converters: 2.5V, 1.8V and 1.5V.
The 4 voltages 3.3V, 2.5V, 1.8V and 1.5V are all monitored with the same
way. They each feed an op-amp with a resistor divider on the input+. The
input- is a common reference voltage of 1.2V. The feedback is made with a
1Mohm resistor between input+ and the output.
The output of the op-amp are pulled-up together to the "housekeeping" +5V
and feed a circuit to drive the MOS transistors at the output of the DC
converters.
When all four voltage have correct levels, the common outputs are at high
level, and a signal is activated to open the MOS so that the devices on the
board can be supplied.
Hope you better understand my design.

The noise I have on the +2.5V output is not like ripple. The signal is high
at 2.5V during 90ms and then a 2ms pulse down to 1.5V.
I have investigated and it seems like it is due to the 90ms ripple noise on
the +5V output. But the problem is still nearly the same: when I remove the
resistor of the the +5V monitoring circuit, I have no noise anymore.
Strange isn't it?

Could a simple RC filter at the output of the +5V DC converter do the job
of phase margin compensation?
Any help will be welcome.

Jean-Pierre


--------------------------------------------------------------------
Jean-Pierre BOUTHEMY
Hardware Design Engineer
ALCATEL - Mobile Networking Division (MCG)
Mobile Core R&D
+33 (0)2 99 26 08 18
mailto:jean_pierre.bouthemy@xxxxxxxxxx
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|---------+------------------------------>
|         |           Christopher        |
|         |           Jakubiec           |
|         |           <Christopher.Jakubi|
|         |           ec@xxxxxxx>        |
|         |           Sent by:           |
|         |           si-list-bounce@free|
|         |           lists.org          |
|         |                              |
|         |                              |
|         |           06/08/2003 17:52   |
|         |           Please respond to  |
|         |           si-list            |
|         |                              |
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  |        cc:                                                                  
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  |        Subject: [SI-LIST] Re: DC/DC converter output dip                    
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Jean-Pierre,

As I understand from your description you have a 2.5V output from the dc-dc

converter that feeds into an op-amp comparator by using a resistor divider
on
the + and - inputs to the op-amp.  Is this correct?  It sounds like a very
basic
comparator setup.  Are you using negative feedback with nothing connected
inbetween the output and input of the op-amp?

Chris Jakubiec
Sun Microsystems

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