Diode question

[Rajinder1268]

Hi,
I am looking to use a buck dcdc 12V to 5V, then use the 5V to power an LDO @3V3.
Do I need to connect a diode from the output of the buck converter and the input to the LDO?
I can use a Schottky to reduce the voltage drop.
Is the diode necessary?
Thanks in advance

 

[dick_freebird]

Doesn't matter to normal efficiency since both are dissipative
and current will be the same.

Will matter to just how soon in the 5V soft start ramp, the 3.3V
rail will start and stabilize. Will matter to how well the LDO
can hold up IOUT at low side tolerance of 5V rail (4.5V for low
line / transient sag, and 0.8V for low-temp Vf*If would leave 3.7V
VIN to make 3.3V out - what's your dropout voltage @ this
corner, and are there "not mentioned until the very back of
the app note" accuracy / stability / marginal UVLO issues with
the LDO that might surface when you push on it?

Need to check your Schottky carefully - Vf they spec, and Vf you
get at load, can be far apart as Schottky diodes have no
conductivity modulation to swing the Rs with forward current.
Vf@Imax can be a lot worse, die size for die size, voltage for
voltage, using a Schottky where recovery time is not the
point.

 

[KlausST]

Hi,

I see no need for a diode.
But in detail it depends what exactly you want to achieve and all the situations, schematic, voltage accuracy, current..

Klaus

 

[Rajinder1268]

Hi,

I see no need for a diode.
But in detail it depends what exactly you want to achieve and all the situations, schematic, voltage accuracy, current..

Klaus

Thev5V is used to power a voltage translator line, the 3V3 used to power a micro, Bluetooth device. Current will be around 500mA max.

 

[KlausST]

Hi,

still missing informations (see post#3) to clearly answer your question.

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Klaus