TPS61288: Schematic review

Hi JITHU,

The parameters are OK, and the inductance can be smaller.

the device will work in PFM mode and the peak current is under control, the inductance actually doesn't matter much—— 1uH will be acceptable.

for your usage, the device is over designed as its current limit is 15A.

you can use TPS61376 or TPS61170.

TPS61376 has true load disconnect and with 2.5*2 package.

TPS61170 is 2*2 package but need a diode outside.

Best Regards

Fergus

I am using this as source to run a hart device that operates at 4-20mA. 

1) Can i use this system for that?

2) TPS61376 is of higher cost and has lower efficiency @ 4-20mA . TPS61170 datasheet doesn't precisely show efficiency under 50mA.

3) When i simulated in Webench it shows 22uH with 3A current limit inductors as shown

Could you help me with choosing inductor as the equations in datasheet shows higher values of current.

 I can understand that as i need very low current ripple i may need 22uH, but why is webench suggesting such high current rating? Can i use smaller current rating like LQH2HPZ220MGRL?

Hi JITHU,

Of course, you can use the system.

The webench is calculated for PWM mode, but this usage is PFM mode obviously, and its parameters are not suitable.

Actually, you do not need low current ripple as the peak current is limited by the device. that is why 3A current limit is recommended.

 LQH2HPZ220MGRL's max current is only 430mA, and the PFM peak current will be about 1.3A, so it cannot be used.

You can use LQH2HPZR47MGRL as its rated current is about 2A.

TPS61170 is not synchronous Boost, so efficiency will be a little lower, but it gets fewer outside circuits and smaller package.

Best Regards

Fergus

I understand that it will be working in PFM mode. But shouldn't we increase the inductance to reduce the ripple?

Can i use this one - VLS5045EX-220M?

Hi JITHU,

You do not need increase the inductance to reduce the ripple. In PFM mode the inductor ripple is fixed.

What you need to concern is the peak current, you need to choose inductor with rated current above 2A.

you cannot use  VLS5045EX-220M as its rated current is not enough.

Best Regards

Fergus

So will the ripple cause issues for my hart communication at 10mA?

So my only criteria for selecting inductor would be only its current rating(>2A). Can i go with TYS5040100M-10?

Hi JITHU，

Sorry, I do not understand about the word "hart communication", can you explain it?

If you are worried about the input and output current ripple, it will not be a concern because of the large input and output capacitors.

Yes, you can go with TYS5040100M-10. Actually, you can choose a smaller package with smaller inductance.

Best Regards

Fergus