LDO regulator with series Schottky diode
LDO regulator with series Schottky diode
(OP)
Hello
We are using the LM2937-3.3 linear LDO voltage regulator to get 3.3V from 5V.
Output current is 50mA maximum.
LM2937-3.3 DATASHEET:
http://www.national.com/ds/LM/LM2937-2.5.pdf
The previous engineer has placed a series Schottky diode (D12) following the LM2937 (i.e, the Schottky is immediately upstream of the output cap as in following diagram)
CIRCUIT DIAGRAM:
http://i54.tinypic.com/a2py7k.jpg
do you know why the Schottky is there?
.....the load has been represented as a resistor, but actually it's a MC13211 RF Transceiver which has a 2V to 3.4V operating voltage with on-chip voltage regulators....(3.6V max recommended input voltage as per page 46 of the datasheet)
MC13211 DATASHEET:
htt p://cache. freescale. com/files/ rf_...1x.p df?pspll=1
Anyway,
I am wondering why the previous engineer added the series Schottky diode?
I noticed nothing about putting a Schottky at the output in the datasheet.
I am wondering perhaps , is it a problem if the input side is at zero volts and yet the output has some hold-up voltage lingering on the output caps.?
Or, do you know why a Scottky (D12) would be placed here?
We are using the LM2937-3.3 linear LDO voltage regulator to get 3.3V from 5V.
Output current is 50mA maximum.
LM2937-3.3 DATASHEET:
http://www.national.com/ds/LM/LM2937-2.5.pdf
The previous engineer has placed a series Schottky diode (D12) following the LM2937 (i.e, the Schottky is immediately upstream of the output cap as in following diagram)
CIRCUIT DIAGRAM:
http://i54.tinypic.com/a2py7k.jpg
do you know why the Schottky is there?
.....the load has been represented as a resistor, but actually it's a MC13211 RF Transceiver which has a 2V to 3.4V operating voltage with on-chip voltage regulators....(3.6V max recommended input voltage as per page 46 of the datasheet)
MC13211 DATASHEET:
htt
Anyway,
I am wondering why the previous engineer added the series Schottky diode?
I noticed nothing about putting a Schottky at the output in the datasheet.
I am wondering perhaps , is it a problem if the input side is at zero volts and yet the output has some hold-up voltage lingering on the output caps.?
Or, do you know why a Scottky (D12) would be placed here?





RE: LDO regulator with series Schottky diode
Or perhaps he/she was worried about some combination of events where the voltage on the capacitor might damage the regulator on power-off. But this is not mentioned in the application notes for that regulator, so this is less likely than the other explanation.
RE: LDO regulator with series Schottky diode
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize
RE: LDO regulator with series Schottky diode
Dan - Owner

http://www.Hi-TecDesigns.com
RE: LDO regulator with series Schottky diode
About the only thing I could think of was that the output voltage somehow was exceeding the rated value of the load momentarily, and instead of buying an adjustable regulator chip, the designer just put in the shottky.
www.MaguffinMicrowave.com
Maguffin Microwave wireless design consulting
RE: LDO regulator with series Schottky diode
RE: LDO regulator with series Schottky diode
That's the one I was thinking about (my less likely explanation #2). But now I seem to recall the solution was a 'backwards' diode in parallel output to input. The diode would allow the output capacitor to discharge around the regulator.
So I'll put a dollar on 'shaving-off' some output voltage.
RE: LDO regulator with series Schottky diode
Seriously, you may be right, and a quick glance through the datasheets for the LM317 series failed to locate the circuit diagram...ah well. The problem with your solution would be the possible zapping of the diode if the reverse current could get high enough, the problem with a forward diode is the voltage drop, thus using a Schottky with its lower Vf...
RE: LDO regulator with series Schottky diode
RE: LDO regulator with series Schottky diode
RE: LDO regulator with series Schottky diode
I vote shaving voltage too.
Keith Cress
kcress - http://www.flaminsystems.com