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Design >> Analog Design >> LDO Design ( Any Advantage in using feedback resistors?)
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Message started by Larry_80 on Feb 20^th, 2013, 4:57pm

Title: LDO Design ( Any Advantage in using feedback resistors?)
Post by Larry_80 on Feb 20^th, 2013, 4:57pm

Hello Guys,
Please see attached circuits. The first on the LHS utilizes resistive feedback and the one on the RHS does not. I am driving a CONSTANT current load and capacitance. Could someone please explain if there is any advantage in using the Resistive feedback version over not using a resistive feedback? PSRR is of primary concern to me!

[img][/img]

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by AnilReddy on Feb 21^st, 2013, 5:43am

Hi,

Those resistors may used to define a particular output voltage (by changing the divider ratio) with the given Vref. It is not always the case where we will use only one output voltage throughout, in the case of these power-management blocks.

i don't see any other issues. you have to use it.

Thanks
Anil

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by raja.cedt on Feb 21^st, 2013, 6:10am

in addition to anil i would like to say PSRR depends on UGB, so if you make sure same loop gain in both cases (by adjusting opamp gain) PSRR will be more less same in both cases...

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by Larry_80 on Feb 21^st, 2013, 10:36am

raja.cedt wrote on Feb 21^st, 2013, 6:10am:

in addition to anil i would like to say PSRR depends on UGB, so if you make sure same loop gain in both cases (by adjusting opamp gain) PSRR will be more less same in both cases...

So what if i have a large Cload, won't that affect my PSRR?

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by raja.cedt on Feb 21^st, 2013, 10:48am

hello,
it effect's but it will be similar to both cases you shown in the fig. Please refer introduction in this pap.

http://www-vlsi.stanford.edu/papers/ea_rep_reg_jssc_06.pdf

Thanks,
Raj.

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by Praveen K on Feb 22^nd, 2013, 4:17am

For the same Phase Margin, with resistive feedback the compensation cap can be much lesser, but in unity gain feedback structure compensation cap required will be much more. This might degrade the PSRR.

praveen

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by buddypoor on Feb 22^nd, 2013, 8:00am

More than that - for feedback without any voltage divider you must apply a Vref identical to Vout (very large opamp gain).
How would you manage this?

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by Lex on Feb 22^nd, 2013, 8:14am

Advantage: the one without resistive feedback has higher loopgain.
Disadvantage: you will need an extra block if you want a vout different from vref.

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by Aman on Feb 26^th, 2013, 5:17pm

I believe the most important role feedback resistor plays to eliminate the zero-load current condition, if that is important for you. There are other ways to maintain a minimum load current, but for a voltage source i.e. LDO, resistor divider is the simplest fixed current load.

-Aman

Title: Re: LDO Design ( Any Advantage in using feedback resistors?)
Post by smlogan on Feb 26^th, 2013, 7:07pm

Larry_80 wrote on Feb 20^th, 2013, 4:57pm:

If PSSR is a major concern, an item I have not seen mentioned is the manner in which the resistors are implemented and if they are adjustable using analog switches. Parasitic capacitance of either the resistors or the switches to the supply can significantly reduce the high frequency supply rejection. Hence, the use of the topology and substrate connections of resistors and any potential switches to adjust the resistor ratio, must be considered to optimize high frequency PSSR. It took me some time to track down this issue when it significantly reduced the PSSR of an LDO using a layout extracted view of the block when compared to its schematic view based simulation.