[SI-LIST] Re: DC/DC converter output dip
- From: "Pratt, Gary" <gary_pratt@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 6 Aug 2003 14:15:36 -0400
Jean-Pierre,
How is the common 1.2 reference voltage generated? And, what supply is
used for the op-amps? From your first description it sounded like a
control loop problem (Veneble being an excellent suggesting for solving
that type of problem). But, now, it sounds like you may have a
relaxation loop related to the switching of the MOS transistor and its
affect on the 3.3 supply.
Can you provide a JPEG of the schematic containing the supplies,
op-amps, MOS transistors and associated circuitry?
Gary
-----Original Message-----
From: Jean_Pierre.Bouthemy@xxxxxxxxxx
[ mailto:Jean_Pierre.Bouthemy@xxxxxxxxxx
<mailto:Jean_Pierre.Bouthemy@xxxxxxxxxx> ]
Sent: Wednesday, August 06, 2003 12:21 PM
To: si-list@xxxxxxxxxxxxx
Cc: Tom.Currelly@xxxxxxxxxxx
Subject: [SI-LIST] Re: DC/DC converter output dip
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
<mailto:jean_pierre.bouthemy@xxxxxxxxxx>
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|---------+------------------------------>
| | Christopher |
| | Jakubiec |
| | <Christopher.Jakubi|
| | ec@xxxxxxx> |
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| | 06/08/2003 17:52 |
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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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