TPS84259 / Input to Output Coupling Capacitor , Output ripple Application Report

Ryuji

Guru 20090 points

Other Parts Discussed in Thread: TPS84259

Hello,

Could you please kindly let me know about TPS84259 ?

1.....Calculation of Input to Output Coupling Capacitor.

In the data sheet page 21, input to output coupling capacitor is 2.2uF as typical value.

But, there is not calculation.

Could you please kindly let me know calculation ?

I would like to set best capacitance for reduce Vout ripple voltage.

Current conditions is as below.

Vin=5V, Vout=-3.3V, Iout=500mA, Fsw=800kHz

If you have reccomend capacitance value for this condition , please let me know too.

2.....Reducing Output Ripple and Noise with the TPS84259 (Application Report )

Could you please let me know the Input to Output coupling capacitor value of this reports?

3.....LDO + 47uF output filter in Application Report.

In this report, using TPS7A3301 for reducing output ripple voltage.

And this result is 3mVpp as output voltage ripple.

I think that TPS7A3301 has very high PSRR.

If TPS84259 voltage ripple is 800kHz , PSRR will be 50dB.

So, I think that TPS7A3301 can more reduce Vout ripple.

In the report, Vout ripple reduced from 21mV to 3mV only.

Could you please kindly let me know the reason ?

4....Output voltage ripple 0.1mV

In my customer's output voltage ripple target is 0.1mV by below conditions.....

Vin=5V, Vout=-3.3V, Iout=500mA, Fsw=800kHz

Could you please kindly let me know the refference value of external components or method for achieve 0.1mV?

I expected TPS7A3301 achive this....

Best Regards,

Ryuji Asaka

over 12 years ago

0 J Arrigo over 12 years ago

TI__Mastermind 28020 points

The information provided in the Application Note, Reducing Output Ripple and Noise with the TPS84259 is based on actual test data. Adding a filter or TPS7A3301 at the output of the TPS84259 reduces the ripple and noise by 10-15dB. To achieve greater ripple reduction additional filtering may be required at the load.

0 Ryuji over 12 years ago in reply to J Arrigo

Guru 20090 points

Hello Arrigo san,

Thank you.

Concerning Input-Output coupling capacitor, Is 2.2uF best value ?

If there is any calculation documment, please let me know.

Best Regards,

Ryuji Asaka

0 J Arrigo over 12 years ago in reply to Ryuji

TI__Mastermind 28020 points

Yes, 2.2 uF is a good value. This is the value used during testing of the device.

0 Ryuji over 12 years ago in reply to J Arrigo

Guru 20090 points

Hrllo Arrigo san,

Thank you.

I understood.

Best Regards,

Ryuji Asaka

0 Ryuji over 12 years ago in reply to Ryuji

Guru 20090 points

Hello Arigo san,

Sorry for bothering you.

TPS84259 has Output Capacitor Limit as 430uF.

Is this reason inrush current only ?

Are there any other reason ?

Best Regards,

Ryuji Asaka

0 J Arrigo over 12 years ago in reply to Ryuji

TI__Mastermind 28020 points

Hi Asaka san,

Yes, the maximum output capacitor limit is due to inrush current only.

See table 7 of the datasheet for inrush current information.

0 Ryuji over 12 years ago in reply to J Arrigo

Guru 20090 points

Hello Arrigo san,

Thank you.

In case of Vin=5V/Vout=-3.3V, inrush current is lower than other case.

In this case , can I use more output capacitance for reducing ripple volthage ?

If yes, could you please let me know the maximum output capacitance of this conditions?

I think that 430uF is decided by worst case condisions of Vin/Vout.

Best Regards,

Ryuji Asaka

0 J Arrigo over 12 years ago in reply to Ryuji

TI__Mastermind 28020 points

The device is spec'd with a maximum Cout of 430uF.

We cannot guarantee performance when operated out of spec.

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TPS84259 / Input to Output Coupling Capacitor , Output ripple Application Report