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I am designing a board where the datasheet suggests a power up sequence. Basically, the VDD_PLL should be turned on first, after 100us, the VAA should be turned on, followed by the internal supply and then the VDD_IO. Current I am using the AMS1117 as my voltage regulator. I know I would have to chnage this to some other power up sequencing IC if i need to meet these requirements. My question is what happens if I dont meet the power up sequence? Is it merely a safet mechanism or will the board not work. Can I ignore the specfications in the datasheet?

The chip is http://www.onsemi.com/pub_link/Collateral/AR0330CM-D.PDF

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  • \$\begingroup\$ What are the consequences of your board not starting up correctly every time? \$\endgroup\$ – whatsisname Jan 14 '16 at 2:47
  • \$\begingroup\$ Not much. This is a proto board. I can power cycle it \$\endgroup\$ – red car Jan 14 '16 at 3:26
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Do not ignore the power up sequences specified by chip manufacturers. They specify these because that is what it takes to ensure that the device powers up correctly and will run reliably.

If you ignore sequencing you can cause internal chip latch-up if voltage rails and reset/power good signals come in the wrong order. If you are lucky the latch-up can be cleared via a full power cycle. Unlucky and there could be a catastrophic amount of current flowing through a path on the chip that can cause fusing and permanent damage.

Improper sequencing of PLLs or OSC or CLK inputs with respect to power rails or other control signals can lead to on chip meta-stability problems that will prevent the internals of the chip from working.

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  • \$\begingroup\$ The only reason i didnt want to use the sequencer is the cost. I can buy AMS1117 for a really low price. Is there a good power up sequencer you could suggest(for 1.8V,2.8V, assume 200-500mA of current) \$\endgroup\$ – red car Jan 14 '16 at 2:41
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    \$\begingroup\$ Typical is just use the uC already in your system to drive the ENABLE pins on the different regulators. If AMS1117 doesn't even have an enable pin, you may need to look for an alternative, but you should be able to get a not-too-much-more-expensive part with ENABLE functionality. \$\endgroup\$ – The Photon Jan 14 '16 at 2:48
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    \$\begingroup\$ @redcar There are many LDOs with an enable input that are very inexpensive. Given that you are working with what appears to be a very expensive image sensor ($1125 on Mouser), it seems odd to be worried about regulator cost. \$\endgroup\$ – uint128_t Jan 14 '16 at 2:48
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    \$\begingroup\$ +1 for comment from @uint128_t. The expensive part is certainly the LAST place you would want to try cutting corners around the manufacturers sequencing specification!!! \$\endgroup\$ – Michael Karas Jan 14 '16 at 4:13
  • \$\begingroup\$ High side switches are dirt cheap (multiple vendors too) and offer soft start, output protection etc. Even full up programmable power managers are only a few dollars (ISPPAC-POWR series for example). This is definitely not the place to skimp on cost but these won't add much. \$\endgroup\$ – Peter Smith Jan 14 '16 at 7:57

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