LP8860-Q1: BOOST_SEL_IRAMP calculation

Christian Willschek90

Part Number: LP8860-Q1

Hi,

using the LP8860-Q1, I want to set the I_RAMP in EEPROM Address 0x71[6:3] – BOOST_SEL_IND, BOOST_SEL_IRAMP register. According to the App report http://www.ti.com/lit/an/snva789/snva789.pdf equation (1) I get a negative result for:

L=100µH

Vin_min=7V

Vout_max=32V

I_RAMP_GAIN=-5.352

What is wrong?

Thank you

Christian

over 6 years ago

0 Shirley He over 6 years ago

TI__Expert 5475 points

Hi Christian,

Sorry the equation in the application report is not right. One "-" sign in the equation should be "*". Please use the equation in the datasheet to calculate the Iramp_gain, which is shown below.

0 Shirley He over 6 years ago in reply to Shirley He

TI__Expert 5475 points

0 Christian Willschek90 over 6 years ago in reply to Shirley He

TI__Prodigy 715 points

Thank you Sherly!

Can you explain the equation so I can use a different sense resistor?

Or how would an equation with a customer changeable R_sense look like?

Christian

0 Shirley He over 6 years ago in reply to Christian Willschek90

TI__Expert 5475 points

Hi Christian,

The Iramp_gain depends on the sensed inductor current slope when the boost switch is turned off. It's proportional to the Rsense resistance. For example, if customer uses a 50mohm resistor, the required value is doubled.
But actually we do not recommend to change the Rsense resistor. Customer can change the SW current limit by configuring Boost_imax_sel register bits.

0 Christian Willschek90 over 6 years ago in reply to Shirley He

TI__Prodigy 715 points

Hi Sherly,

is there a specific reason why you don´t recommend to change the R_sense?

Thanks

Christian

0 Shirley He over 6 years ago in reply to Christian Willschek90

TI__Expert 5475 points

Hi Christian,

The Rsense resistance is related to the loop stability. So we don't recommend changing the value too much.

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LP8860-Q1: BOOST_SEL_IRAMP calculation