Block a load supplied by an LDO

[Hammy]

Is there a simple method to block a load supplied by an LDO until the
LDO reaches its regulated output? The rise time of the LDO's output is
850uS (22uf output cap). The load is only 10mA. I've tried using a
P-FET between the load and the LDO no luck. I've tried a few things. I
won't bore you with them.

If anybody has a suggestion Thanks.

 

[Rich Webb]

On Tue, 03 Jun 2008 16:48:54 GMT, Hammy <spamme@hotmail.com> wrote:

Is there a simple method to block a load supplied by an LDO until the
LDO reaches its regulated output? The rise time of the LDO's output is
850uS (22uf output cap). The load is only 10mA. I've tried using a
P-FET between the load and the LDO no luck. I've tried a few things. I
won't bore you with them.

If anybody has a suggestion Thanks.

What about using a supervisor chip (e.g., MCP130 or MCP100) as the
trigger? Use the *Reset out line to gate a logic-level FET with a low
Ron at the available Vcc.

--
Rich Webb Norfolk, VA

 

[Hammy]

On Tue, 03 Jun 2008 13:12:41 -0400, Rich Webb
<bbew.ar@mapson.nozirev.ten> wrote:

What about using a supervisor chip (e.g., MCP130 or MCP100) as the
trigger? Use the *Reset out line to gate a logic-level FET with a low
Ron at the available Vcc.

Thanks .I never heard of them before.Pretty cheap to.

 

[John Fields]

On Tue, 03 Jun 2008 16:48:54 GMT, Hammy <spamme@hotmail.com> wrote:

Is there a simple method to block a load supplied by an LDO until the
LDO reaches its regulated output? The rise time of the LDO's output is
850uS (22uf output cap). The load is only 10mA. I've tried using a
P-FET between the load and the LDO no luck. I've tried a few things. I
won't bore you with them.

If anybody has a suggestion Thanks.

Sample the input voltage to the regulator and use a comparator to hold
a P-channel MOSFET OFF until VIN has been stable long enough for you
to know that the output of the regulator is where it's supposed to be.

View in Courier:

PCH
VIN>--+-----+---+----+---[LDO]----S D---+
| | | | | G |
[Rt] [R1] | [R] | | |
| | | | | | |
+-----|--|-\ | | | [LOAD]
| | | >--+-----|--------+ |
| +--|+/ | |
| | | | |
[Ct] [R2] | | |
| | | | |
GND>--+-----+---+----------+--------------+

BTW, what are the LDO's input and output voltages?

JF

 

[Hammy]

On Tue, 03 Jun 2008 16:56:44 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Tue, 03 Jun 2008 16:48:54 GMT, Hammy <spamme@hotmail.com> wrote:

Is there a simple method to block a load supplied by an LDO until the
LDO reaches its regulated output? The rise time of the LDO's output is
850uS (22uf output cap). The load is only 10mA. I've tried using a
P-FET between the load and the LDO no luck. I've tried a few things. I
won't bore you with them.

If anybody has a suggestion Thanks.

Sample the input voltage to the regulator and use a comparator to hold
a P-channel MOSFET OFF until VIN has been stable long enough for you
to know that the output of the regulator is where it's supposed to be.

View in Courier:

PCH
VIN>--+-----+---+----+---[LDO]----S D---+
| | | | | G |
[Rt] [R1] | [R] | | |
| | | | | | |
+-----|--|-\ | | | [LOAD]
| | | >--+-----|--------+ |
| +--|+/ | |
| | | | |
[Ct] [R2] | | |
| | | | |
GND>--+-----+---+----------+--------------+

BTW, what are the LDO's input and output voltages?

JF

Hi John

It's a 9V battery so about 9.5-6.5V in; and 5.6Vdc out. I went with
the 5.6vdc out so I could just use diodes as cheap reverse polairity
protection on the loads.

I have everything I need to try your circuit tommorrow. I'll let you
know how it goes.

Thanks

 

[Hammy]

On Tue, 03 Jun 2008 16:56:44 -0500, John Fields
<jfields@austininstruments.com> wrote:

Sample the input voltage to the regulator and use a comparator to hold
a P-channel MOSFET OFF until VIN has been stable long enough for you
to know that the output of the regulator is where it's supposed to be.

View in Courier:

PCH
VIN>--+-----+---+----+---[LDO]----S D---+
| | | | | G |
[Rt] [R1] | [R] | | |
| | | | | | |
+-----|--|-\ | | | [LOAD]
| | | >--+-----|--------+ |
| +--|+/ | |
| | | | |
[Ct] [R2] | | |
| | | | |
GND>--+-----+---+----------+--------------+

BTW, what are the LDO's input and output voltages?

JF

Thanks John worked like a charm!

 

[John Fields]

On Wed, 04 Jun 2008 18:57:39 GMT, Hammy <spamme@hotmail.com> wrote:

On Tue, 03 Jun 2008 16:56:44 -0500, John Fields
jfields@austininstruments.com> wrote:



Sample the input voltage to the regulator and use a comparator to hold
a P-channel MOSFET OFF until VIN has been stable long enough for you
to know that the output of the regulator is where it's supposed to be.

View in Courier:

PCH
VIN>--+-----+---+----+---[LDO]----S D---+
| | | | | G |
[Rt] [R1] | [R] | | |
| | | | | | |
+-----|--|-\ | | | [LOAD]
| | | >--+-----|--------+ |
| +--|+/ | |
| | | | |
[Ct] [R2] | | |
| | | | |
GND>--+-----+---+----------+--------------+

BTW, what are the LDO's input and output voltages?

JF

Thanks John worked like a charm!

---


JF