Copyright © 2016

Microsemi

Page 24

Rev. 1.4, July 2017

CPG

– PoE BU

One Enterprise Aliso Viejo, CA 92656 USA

PD70211

PD Controller with Switching Regulator for

AF/AT/UPOE/HDBaseT/4-pair PoE Applications

For example, if we have a Buck application for 5A output, irrespective of the input and output voltage conditions

(as long as they are not violating the min and max duty cycle limits of the converter), and assuming we have

selected inductance appropriately, we should pick a sense resistor of



SENSE

0.138

R

=0.028

5A

We may need to put an adjust resistor in parallel (such as the “22Ω” placeholder) we have shown in all the

typical application schematics.

For a Forward converter (Buck with a transformer), instead of the load current IOR in the above equation, use the

reflected load current of IO/n, where n is the turns ratio (number of Primary-side turns divided by number of

Secondary-side turns). You will also need to lower the sense resistance further (by means of the adjust resistor),

to account for the magnetization current component on the switch side. So roughly:

 P

SENSE

OS

N

0.138

R

(Forward)

IN

For a Boost or Buck-Boost, we have to account for the fact that the peak current is not just 1.3 times max load

current, but is actually





O

PEAK

I

I

1.3

(where D can be as high as 44%)

1D

So we should use the following equation for sense resistor















SENSE

O

O

O

0.18 1 D

0.101

1

R

=

=

1.3 I

1.3 I

13 I



SENSE

O

0.077

R

(Boost, Buck-Boost)

I

For example, if the max load current is 5A, the sense resistor value to use is



SENSE

0.077

R

=0.015

5A

As we can see, this is roughly half of what we got for the Buck (same load current).

For a Flyback topology (Buck-Boost with a transformer), we have to use the reflected output current. So we get:

 P

SENSE

OS

N

0.077

R

(Flyback)

IN