[SI-LIST] Re: DC/DC converter output dip
- From: Jean_Pierre.Bouthemy@xxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 7 Aug 2003 19:44:38 +0200
Wyland,
The non-isolated DC converters (buck) are classical blocks from Tyco, with
switch, inductor and PWM comparator integrated.
Can you help me?
Thanks.
JP
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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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A question: Are the non-isolated converters linear regulators or buck
converters? A buck converter has
a switch + inductor + comparator. A linear regulator just has an op amp
and a pass transistor, which can be a mosfet.
Jean_Pierre.Bouthemy@xxxxxxxxxx wrote:
>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
>--------------------------------------------------------------------
>
>
>|---------+------------------------------>
>| | Christopher |
>| | Jakubiec |
>| | <Christopher.Jakubi|
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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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