TPS23754/TPS23756 SOFT START RESISTOR VALUE CALCULATION

WhiteHorse

Other Parts Discussed in Thread: TLV431A

e2e,

We have a customer that is developing a 25W PoE. The customer needs to know how to calculate the resistor values for RFBU and RFBL in the soft start circuit in figure 35 on page 34 of the data sheet.

Thanks for your help.

Regards,

John Wiemeyer

over 8 years ago

0 Darwin Fernandez over 8 years ago

TI__Mastermind 40100 points

Hi John,

For setting the output voltage, choose RFBU that minimizes current through the divider (to maintain efficiency) and minimize VOUT offsets.

Typically RFBU= 41.2k.

RFBL = (VREF*RFBU)/(VOUT-VREF)

VREF is the reference voltage of TLV431A (1.24V)
VOUT is the output voltage.

0 WhiteHorse over 8 years ago in reply to Darwin Fernandez

Genius 4430 points

Thanks Darwin. I'll get this to our customer.
Regards,
John Wiemeyer

0 WhiteHorse over 8 years ago in reply to WhiteHorse

Genius 4430 points

Hi Darwin,

The customer followed the instructions above and is still having some trouble.

Their output is at 5V so they calculated the value for RFBL around 13.7K Ohms. They end up setting RFBL to 42k and RFBU to 41.2 k in order to have 5 Volt output. If they use the calculated value of 13.7K the output if 7 Volts instead of 5V. Do you know the reason the calculated value does not work? I have their schematic and can email it to you if that would help.

Regards,
John Wiemeyer

0 Eric Wright over 8 years ago in reply to WhiteHorse

TI__Mastermind 42795 points

John,

Please send the schematic. Also, have them do a double check of the populated passive values (using meters) on the customer board vs. what is in the schematic. They may have a wrong value or two on their board.

You can attach the SCH using the paperclip via E2E.

0 WhiteHorse over 8 years ago in reply to Eric Wright

Genius 4430 points

HI Eric,

I can't post the customer's schematic to this public site. I sent the schematic to the Central Applications Team and asked them to forward it to you.

Thanks for your help.

Regards,
John Wiemeyer

0 Eric Wright over 8 years ago in reply to WhiteHorse

TI__Mastermind 42795 points

Thanks John,
I did get the SCH privately. Please let us know how the check of assembled passive values compares with the schematic. If the measured values match positively, then we will take a more in depth look at the schematic.

0 Eric Wright over 8 years ago in reply to Eric Wright

TI__Mastermind 42795 points

John,

According to the schematic, they are using a shunt error amplifier with a 2.49V internal reference (U42) instead of the recommended 1.25V internal reference. This makes sense as they had to set RFBL and RFBU to about the same value to achieve 5V output.

Please have them switch U42 over to the TLV41ACDBVR to allow a 13.7k resistor for R74.

By the way, you can find the design equations surrounding the error amplifier on page 11, section 2.8 of .

0 WhiteHorse over 8 years ago in reply to Eric Wright

Genius 4430 points

Thanks for catching that Eric. Much appreciated.
Regards,
John Wiemeyer

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TPS23754/TPS23756 SOFT START RESISTOR VALUE CALCULATION