Forums

DCDC buck converter output capacitor placement

Started by Unknown June 15, 2013
I have a microprocessor with built in DCDC converter. We need to provide in=
ductor and output capacitors externally at the respective pins of the proce=
ssor. The regulated o/p from the internal DCDC converter is for internal co=
re and IO as well as for external peripherals. The manufacturer recommends =
using 0.01uf, 0.1uf, 1uf and 33uf at the DCDC output pin. What should be th=
e order of placement of these o/p capacitors on the board? 0.01uf close to =
the pin and 33uf away or the other way? The current from the o/P pin is bot=
h ways. i.e goes internal to the IC as well as goes out of the pin to the o=
ther peripherals on the board.

-bjnk
bhav.jnk@gmail.com wrote:
> I have a microprocessor with built in DCDC converter. We need to > provide inductor and output capacitors externally at the respective > pins of the processor. The regulated o/p from the internal DCDC > converter is for internal core and IO as well as for external > peripherals. The manufacturer recommends using 0.01uf, 0.1uf, 1uf and > 33uf at the DCDC output pin. What should be the order of placement of > these o/p capacitors on the board? 0.01uf close to the pin and 33uf > away or the other way? The current from the o/P pin is both ways. i.e > goes internal to the IC as well as goes out of the pin to the other > peripherals on the board. >
Smallest capacitance closest to the pin. But nowadays a 0.1uF in 0603 package is good enough, there is usually no need for an extra 0.01uF. The 1uF is debatable, may not be needed if the 33uF is a ceramic. -- Regards, Joerg http://www.analogconsultants.com/
On Sat, 15 Jun 2013 11:27:08 -0700 (PDT), bhav.jnk@gmail.com wrote:

>I have a microprocessor with built in DCDC converter. We need to provide inductor and output capacitors externally at the respective pins of the processor. The regulated o/p from the internal DCDC converter is for internal core and IO as well as for external peripherals. The manufacturer recommends using 0.01uf, 0.1uf, 1uf and 33uf at the DCDC output pin. What should be the order of placement of these o/p capacitors on the board? 0.01uf close to the pin and 33uf away or the other way? The current from the o/P pin is both ways. i.e goes internal to the IC as well as goes out of the pin to the other peripherals on the board. > >-bjnk
That sounds like overkill. I'd go for 2x15uF ceramics. The node should be a big copper pour, adjacent to the ground plane. Another cap, 330 nF or some such, near each peripherial chip wouldn't hurt, but don't overdo it. What uP is it? Built-in buck converters are uncommon.
On Jun 15, 11:53=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Sat, 15 Jun 2013 11:27:08 -0700 (PDT), bhav....@gmail.com wrote: > >I have a microprocessor with built in DCDC converter. We need to provide=
inductor and output capacitors externally at the respective pins of the pr= ocessor. The regulated o/p from the internal DCDC converter is for internal= core and IO as well as for external peripherals. The manufacturer recommen= ds using 0.01uf, 0.1uf, 1uf and 33uf at the DCDC output pin. What should be= the order of placement of these o/p capacitors on the board? 0.01uf close = to the pin and 33uf away or the other way? The current from the o/P pin is = both ways. i.e goes internal to the IC as well as goes out of the pin to th= e other peripherals on the board.
> > >-bjnk > > That sounds like overkill. I'd go for 2x15uF ceramics. The node should > be a big copper pour, adjacent to the ground plane. Another cap, 330 > nF or some such, near each peripherial chip wouldn't hurt, but don't > overdo it. > > What uP is it? Built-in buck converters are uncommon.
I wonder why? I remember working a zigbee chip years ago, it had an on-chip LDO since it the chip was 1.8V and usually it would run on a ~3V battery everyone looked at me funny when I suggested adding a buck converter, it was really only a matter of adding a switch on chip and an external coil and diode it didn't have to be very accurate and in sleep mode where the chip only used uA the switch could just be permanently on since it would be feeding the LDO I know it got put in, but I don't know how well it worked -Lasse
Thanks. Its i.mx23 from freescale.
On Sat, 15 Jun 2013 11:27:08 -0700, bhav.jnk wrote:

> I have a microprocessor with built in DCDC converter. We need to provide > inductor and output capacitors externally at the respective pins of the > processor. The regulated o/p from the internal DCDC converter is for > internal core and IO as well as for external peripherals. The > manufacturer recommends using 0.01uf, 0.1uf, 1uf and 33uf at the DCDC > output pin. What should be the order of placement of these o/p > capacitors on the board? 0.01uf close to the pin and 33uf away or the > other way? The current from the o/P pin is both ways. i.e goes internal > to the IC as well as goes out of the pin to the other peripherals on the > board. > > -bjnk
Does the manufacturer not provide an app note with a recommended layout? Often (I want to say "usually" but can't) a microprocessor will provide layout notes for "analog-y" parts of a circuit, such as oscillator pins and (presumably) regulator pins. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com