LP8732-Q1: about LDO

nnn sato

Part Number: LP8732-Q1

Tool/software:

For the values listed in the Electrical Characteristics of an LDO, what capacitance do the input and output capacitors have?

Is there a formula or graph for determining the output capacitor for an LDO's output ripple and load fluctuation(Transient load step response)?
Also, is there a formula for determining the input capacitor?

over 1 year ago

0 Matt Sunna over 1 year ago

TI__Guru* 81034 points

Hi Sato,

If you use the recommended LDO input and output capacitors in the datasheet, you can expect to see results aligned with the datasheet typical values. We do not have a calculator or graph to determine load step response as a function of capacitance values. There is a PSpice model available, but I am not sure if this covers the LDOs or only the buck regulators.

Best regards,

Matt

0 nnn sato over 1 year ago in reply to Matt Sunna

Prodigy 230 points

Is the typical value only the load step response?
Are the other electrical characteristic values also values when the recommended capacitance capacitor is used?

0 Matt Sunna over 1 year ago in reply to nnn sato

TI__Guru* 81034 points

Hi Sato,

Yes, the typical value measurements are collected using the recommended typical capacitor values.

Regards,

Matt

0 nnn sato over 1 year ago in reply to Matt Sunna

Prodigy 230 points

Do you have a graph of PSRR at 2.2MHz or above?

0 Sarah Lao over 1 year ago in reply to nnn sato

TI__Genius 9867 points

Hi Sato,

Matt is currently out of office so I will be replying instead.

Unfortunately, we do not have a graph of the PSRR.
The only information is that listed in the datasheet, a PSSR of 53 dB was found under test conditions of 10 kHz.

Best,
Sarah

0 nnn sato over 1 year ago in reply to Sarah Lao

Prodigy 230 points

There are two recommended input and output capacitor values listed in the data sheet,

but which value does the data (electrical characteristic values) refer to?

0 nnn sato over 1 year ago in reply to nnn sato

Prodigy 230 points

according to the Toshiba PSRR document below, it is written that it can be calculated if you know the output impedance(ZOFB) and the output resistance (rDS ) of the output PMOS(in the LDO), so could you please tell me these two?

Simple Guide to Improving Ripple Rejection Ratio of LDO Regulators
https://toshiba.semicon-storage.com/info/application_note_en_20210326_AKX00309.pdf?did=67686

0 nnn sato over 1 year ago in reply to nnn sato

Prodigy 230 points

The above impedance is for 2.2MHz.

0 James Steenbock over 1 year ago in reply to nnn sato

TI__Mastermind 22060 points

Hi sato,

The typical COUT for the LDOs is listed as 1uF so this should have been used for official spec measurement.

As for the LDO PSRR, we're checking to see if there's any more info we can share about this. The PSRR is already reported in the datasheet so this should be what you use as a reference.

If we get additional information back internally that we can share, we will post it here but for now the datasheet PSRR it what we have available.

Regards,

James

0 nnn sato over 1 year ago in reply to James Steenbock

Prodigy 230 points

What was the capacitance of the input capacitor when you measured the electrical characteristic values?

Also, when it is not stated in the data sheet (even for other ICs), should I assume that the measurement was performed with a capacitor with the lowest recommended capacitance?

0 James Steenbock over 1 year ago in reply to nnn sato

TI__Mastermind 22060 points

Hi sato,

Unless otherwise stated in the datasheet, all measurements were done with the typical values.

Typical input capacitor for BUCKs is 10uF

Typical input capacitor for LDOs is 2.2uF

Regards,

James

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LP8732-Q1: about LDO