[christianw.]

Maxim schrieb mir folgendes:

Zitat:| Hello Christian,
|
| I am looking at your start up waveform and it doesn't look like a
| smooth start up. Is this a 12V supply ramping or a battery
| through a switch, and where is the voltage for those waveforms
| measure with respect to the supply and the MAX9704? It looks like
| the part maybe jumps in and out of shutdown as the supply ramps
| up from 0V to 12V, and that the supply dips at each transient
| (the spike to droop to flat waveform to rise again waveforms
| shown).
|
| This is potentially a result of tying /SHDN to the supply through
| a 10nF cap and to the REG pin through a 10kΩ resistor. This is
| NOT a recommended configuration for how to connect /SHDN and REG
| should not be used to enable / disable the device. Please drive
| the /SHDN pin with an independent control signal or (for always
| on operation) use a divider to the supply that will place the
| voltage at /SHDN above the turn on threshold when the high supply
| is fully ramped (VIH is 2.5V, and the supply is 12V - so design
| the divider to place the voltage at /SHDN above 2.5V when the
| supply ramps).
|
| It may be helpful in your current configuration to monitor the
| voltage at REG and /SHDN as the 12V supply ramps (as shown in the
| supplied images) as seeing all three together during sequencing
| would reveal if their is a problem.
|
| I can describe what is likely happening based on the description
| from the data sheet for REG (page 10):
|
| "VREG is not available as a logic voltage high in shutdown mode.
| Do not apply VREG as a 6V potential to surrounding system
| components."
|
| I suspect as the 12V supply ramps, the cap (C13) pulls /SHDN high
| and drags VREG along with it. The the chip briefly enables once
| the supply ramps high enough, turning REG on (which is a low
| impedance output at GND before it ramps up). This (since
| capacitor voltage cannot change instantaneously, and is current
| limited to REG through a 10k resistor from the cap) pulls down on
| the supply which then causes the "droop" and the device to shut
| back down which turns REG back off. This appears to happen 3
| times as the supply sequences in each case shown before the
| supply finally reaches 12V and the system equalizes.
|
| The question I have is where are the waveforms provided measured
| (as I am unsure)? Is it a battery through a switch that you are
| connecting, or a 12V regulated supply? Is the supply itself
| dipping (and if so can it handle the surge current)? Have you
| tried powering up the supply fully with shutdown held low, and
| then taking the device out of shutdown (and does the supply dip
| then)?
|
| Bottom line, I cannot help with further debug until you stop
| using REG (which is disabled when the device is in shutdown) to
| control the /SHDN pin (which is required to exit shutdown and
| turn REG on to take the device out of shutdown). This is
| circular! REG is only intended to be used on G1/G2 and FS1/2 -
| and should not be used for any other externally functionality at
| all - including /SHDN. Once we correct this problem, we can
| determine if there are any other potential system design issues
| (such as those mention in the previous paragraph).
|
| Thank you for your patience,
|
| Matthew Mowdy
| Audio Applications Engineer