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# how to adjust output voltage of LTC3421

Dear experts,

I have some questions about LTC3421 boost up DC/DC.

1. How to adjust output voltage
I think I should calculate with two resistors connected FB pin to adjust output voltage, but I couldn't find formula to calculate output voltage in the datasheet of LTC3421.

2. Absolute maximum value of NMOS current and PD
I also would like to know absolute maximum of NMOS current and power dissipation.
How can I know the limit of switch current in my application?

Here is specification of my application.
VIN min : 1.2V
VIN max : 3.0V
VOUT : 4.5V(or 5V)
IOUT max : 500mA

I've confirmed by simulation of LT Spice that 5V/500mA output from 1.2V Input will steady. but in that situation, Input current is about 3.2A.

HATTORI, Tsutomu

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• Hello,

1. The formula for Vout adjustment via two resistors at FB is always the same:

VFB/VOUT=Rbot/(Rbot+Rtop)

2. Current Limit is stated in the electrical characteristics:

LTSpice simulation model will be modelled with typical values (4.2A), hence no issue here.

BUT, as 3A might be valid for parameter/process variations during volume production your 1,2V Input voltage might not be enough for the desired 5V@500mA.

The current limit is a peak value, for the (DC) input current you have to substract current ripple from peak valus.

AND you have to reduce internal losses from the possible max input power..

Hence the 3.2A are correct and therefore not all of your produced boards might deliver 5V@500mA. You have to allow for output power derating at low VIN. 4.5V as Vout  should be ok for all Vin values.

• Hello,

1. The formula for Vout adjustment via two resistors at FB is always the same:

VFB/VOUT=Rbot/(Rbot+Rtop)

2. Current Limit is stated in the electrical characteristics:

LTSpice simulation model will be modelled with typical values (4.2A), hence no issue here.

BUT, as 3A might be valid for parameter/process variations during volume production your 1,2V Input voltage might not be enough for the desired 5V@500mA.

The current limit is a peak value, for the (DC) input current you have to substract current ripple from peak valus.

AND you have to reduce internal losses from the possible max input power..

Hence the 3.2A are correct and therefore not all of your produced boards might deliver 5V@500mA. You have to allow for output power derating at low VIN. 4.5V as Vout  should be ok for all Vin values.

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