At 10:21 AM 5/21/2012, you wrote:

>Hi,
>I have a IC which, in the datasheet, is said to need a powerup rise time n=
ot
>slower than 100uS in order to initialize correctly. Effectively it=20
>doesn't work
>if the power ramps up slow, and there's no reset input, power must=20
>be disconnected
>in order to attempt a new successfull turn on.
>
>How would you ensure fast powerup rise time? The power turns on /=20
>ramps up very
>slowly, and I can't help it.
>
>I am currently investigating two options:
>
>1) a PNP/NPN or PMOS/NPN pass circuit at the output, and using a=20
>voltage regulator
>with error output (LP2951): when the error is signaled, no power is=20
>passed to the IC.
>I would prefer to power the problematic IC directly via the error=20
>output, but it sinks
>too much current (even 50 mA), so the PNP/NPN power switch is=20
>necessary, but it will
>introduce a voltage drop, which is gonna cause errors (it's an analogue IC=
).
>
>2) using a voltage supervisor (e.g. NCP302HSN40T1G) BEFORE the=20
>voltage regulator, to
>control the shutdown pin of the latter (a LP2951). Seems like a good=20
>idea, however
>the NCP302HSN40T1G has a 10V voltage input limit which ruins the 30V=20
>needed and
>offered by the LP2951.
>
>How would you handle this problem?

So the chip has a lousy internal reset circuit, but no reset pin.

You could use a supervisor chip after the regulator (eg. MCP101T)=20
driving a MOSFET
high side switch such as a Si2301.

Ideally, run everything through the high-side switch- otherwise something e=
lse
in the circuit may partially power up the chip before Vdd turns on (through=
 the
internal protection networks), which could certainly screw up the=20
reset or even
cause latch-up and destruction of your magical mystery chip. Best to=20
not do that.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



--=20
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