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0.7V on SMPS output when input disconnected

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zxt50

Electrical
Feb 19, 2008
9
GB
Dear Readers,

I am using Microchip’s Synchronous Buck Demo Board with dsPIC30F2020. This comprises two buck converters on a PCB.

I am finding that I have 0.7V on the output of the Buck converters even when the input is disconnected.

………Please see page 43,44 of the user’s guide……..


When the board is powered i have 0.7V on Vout1 and Vout2 even when their inputs are disconnected. (i.e. when JP1 & JP3 are disconnected).

Also, I removed the socketed dsPIC30F2020 (the PWM controller) and all the socketed mosfet drivers (U2 and U11) and the 0.7V was still there.

One of my colleagues has the same problem with his board.

It thus appears that the 0.7V is possibly coming out of the MAX4173F high-side current sense amplifier...
……This device (U3 and U14 on the schematics) is soldered in and I cannot easily remove it.

This device (MAX4173) must (?) be providing the 0.7V from either pin 8 or pin 6…..(these are the RS+ and RS- pins.).

I wondered if any reader knows if this is normal for the MAX4173 ?……

….a colleague has the same problem on his board and is just ignoring it, saying he can still use the Buck converter.

The board appears to run ok when the input (J1) is connected , though the example PIDJ, PWM controller code provided by Microchip gave 300mV pkpk output ripple for 5V (Vout) when switching from max (5R) to min (2K) load at T=1ms (50% duty cycle)

I have posted this qu on the Microchip forum though it’s so big a forum that I doubt it will get a response, it hasn’t yet.
 
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