On 26/01/2015 16:44, Dwayne Reid wrote:
> At 05:17 AM 1/24/2015, Isaac Marino Bavaresco wrote:
>
>> I studied your suggestion and improved the design. The circuit is
>> attached as a PDF.
> I can't help thinking that is has gotten way complicated.  Maybe I'm=20
> just not seeing some of your constraints.
>
> Question: why can't you just use a diode from CN1 to feed the=20
> midpoint between T1 & T2?  If you do your switching right, T2 is OFF=20
> whenever you have input voltage to IC1.  That is: a diode from CN1 to=20
> the gate of T2.  Add appropriate transient suppression.
>
> Same with monitoring the input voltage for dropout - simply look at=20
> the voltage across IC1.  Use a voltage divider from B-E of that=20
> transistor to IC1 out to set the dropout voltage if 1 Vbe is too=20
> low.  The collector of that transistor also connects to the gate of=20
> T2 to turn it OFF whenever there is sufficient voltage across IC1.
>
> I haven't checked boundary conditions when the input voltage is near=20
> dropout.  You will have to check those to ensure that your desired=20
> switching occurs cleanly.
>
> dwayne

Dwayne,

Really it got too complicated, so I researched some more and found a
good solution using three ICs:

1) LTC4088: Switching lithium battery charger with synchronous
rectification. Also integrates most of the discretes needed for power
steering.
US$4.88 @ 1, US$2.59 @ 100

2) PAM2401: Switching step-up converter with synchronous rectification
for producing 5V.
US$1.08 @ 1, US$0.76 @ 100, US$0.47 @ 1000

3) RT6150: Switching buck-boost converter (also with synchronous
rectification) for producing 3.3V.
US$1.28 @ 1, US$0.89 @ 100, US$0,69 @ 500

This is the best balance between price, board area and performance I
could find.


Best regards,

Isaac



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