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Lowest noise LDO ?

V

vasile

Jan 1, 1970
0
I'm designing a RF module using a transciever which need a very stable
supply for the VCO and PLL. The requirements are: availability (US),
smallest size (SOT23, SC70), low noise and high PSRR, around 2.8V on
output and less than 100mA load.
So far I found MAX8510 which theoretically has less than 60uVpp noise
and 78dB PSRR at 1Khz. However my previous experience with MAXIM LDO's
shows that the noise is much bigger on the board at the LDO output,
than specified in the datasheet. If there is a switching load on the
board (as an oscillator) supplied from another separate LDO, the noise
is bigger, even with all rejecting capacitors.
Anyone have some experience with MAX8510 or with other low noise LDO
regulators of the same type/category ?

thx,
Vasile
 
M

martin griffith

Jan 1, 1970
0
I'm designing a RF module using a transciever which need a very stable
supply for the VCO and PLL. The requirements are: availability (US),
smallest size (SOT23, SC70), low noise and high PSRR, around 2.8V on
output and less than 100mA load.
So far I found MAX8510 which theoretically has less than 60uVpp noise
and 78dB PSRR at 1Khz. However my previous experience with MAXIM LDO's
shows that the noise is much bigger on the board at the LDO output,
than specified in the datasheet. If there is a switching load on the
board (as an oscillator) supplied from another separate LDO, the noise
is bigger, even with all rejecting capacitors.
Anyone have some experience with MAX8510 or with other low noise LDO
regulators of the same type/category ?

thx,
Vasile
have a look at this shunt noise reducer
http://www.techlib.com/electronics/finesse.html


martin
 
A

Andrew Holme

Jan 1, 1970
0
vasile said:
I'm designing a RF module using a transciever which need a very stable
supply for the VCO and PLL. The requirements are: availability (US),
smallest size (SOT23, SC70), low noise and high PSRR, around 2.8V on
output and less than 100mA load.
So far I found MAX8510 which theoretically has less than 60uVpp noise
and 78dB PSRR at 1Khz. However my previous experience with MAXIM LDO's
shows that the noise is much bigger on the board at the LDO output,
than specified in the datasheet. If there is a switching load on the
board (as an oscillator) supplied from another separate LDO, the noise
is bigger, even with all rejecting capacitors.
Anyone have some experience with MAX8510 or with other low noise LDO
regulators of the same type/category ?

thx,
Vasile
How about following the regulator with an RC filter? You could use a large
cap. Or, use an emitter follower, with a capacitively-decoupled potential
divider feeding the base.
 
P

Patrick Bolton

Jan 1, 1970
0
vasile said:
I'm designing a RF module using a transciever which need a very stable
supply for the VCO and PLL. The requirements are: availability (US),
smallest size (SOT23, SC70), low noise and high PSRR, around 2.8V on
output and less than 100mA load.
So far I found MAX8510 which theoretically has less than 60uVpp noise
and 78dB PSRR at 1Khz. However my previous experience with MAXIM LDO's
shows that the noise is much bigger on the board at the LDO output,
than specified in the datasheet. If there is a switching load on the
board (as an oscillator) supplied from another separate LDO, the noise
is bigger, even with all rejecting capacitors.
Anyone have some experience with MAX8510 or with other low noise LDO
regulators of the same type/category ?

thx,
Vasile

Try looking on the Wenzel website : http://www.wenzel.com/ they have some
interesting power supply filtering designs that you might be able to use. I
ended up using some Micrel parts for a similar task: Micrel MIC2941. These
are adjustable but much higher current than you need. Let us know your
findings, I'm interested for future work.

Patrick
 
vasile said:
I'm designing a RF module using a transciever which need a very stable
supply for the VCO and PLL. The requirements are: availability (US),
smallest size (SOT23, SC70), low noise and high PSRR, around 2.8V on
output and less than 100mA load.
So far I found MAX8510 which theoretically has less than 60uVpp noise
and 78dB PSRR at 1Khz. However my previous experience with MAXIM LDO's
shows that the noise is much bigger on the board at the LDO output,
than specified in the datasheet. If there is a switching load on the
board (as an oscillator) supplied from another separate LDO, the noise
is bigger, even with all rejecting capacitors.
Anyone have some experience with MAX8510 or with other low noise LDO
regulators of the same type/category ?

thx,
Vasile

What you are seeing at the output is probably a combination of noise,
line and load regulation. The noise measurement is shown right in the
datasheet, so it isn't faked. When under test, the line voltage is
heavily filtered, and the load is just a resistor. If the load
variation changes your "noise", then you are seeing load regulation
errors, not noise. You can work on the output capacitor, i.e. lower ESR
and/or more capacitance.

When you design a high PSRR regulator, the trick is generally to run
the internal voltage reference off of a regulated supply. This can be
the output of the LDO, or my preferred technique which is to create an
additional shunt regulator inside the chip just to feed the voltage
reference. Now if you have the headroom and budget, you can cascode
LDOs. That is, use one LDO to provide the input voltage to another LDO.

Note that ultimately, the PSSR of the LDO at high frequencies is a
capacitor divider made up from the capacitance of the pass element in
the LDO and your output capacitor. Thus besides providing lower ESR at
the output capacitor, you need lower ESL as well. This means a
combination of different output capacitors.
 
D

douglas dwyer

Jan 1, 1970
0
Tbe bandwidth of the regulator may limit the regulation performance. Then
relies on decoupling.
Best may have high standing current.
 
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