Copyright © 2016

Microsemi

Page 21

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

Setting Soft-Start

A capacitor is connected between SS pin and IC ground. The current charging the capacitor is



SS_CHG

1.2V

I

(in seconds)

RFREQ

For example, if RFREQ=49.9k, we get







 



5

SS_CHG

3

1.2V

I

(in Amperes) = 2.4 10

24 A

49.9 10

So, charging a 0.1µF ceramic cap on the soft-start pin from 0 to 1.2V will take



 











3

SS

SS_CHG

C

V

0.1

1.2

0.12

t

(in seconds) =

(in seconds) =

(in seconds) = 5 10 (in seconds)

5ms

24

24

I

This is the soft-start time in this case.

Setting Pulse-skip Mode threshold

If a programming resistor RCLP is placed between RCLP pin and IC ground, the clamping voltage level is given by





CLP

0.3 RCLP

V

(in Volts)

RFREQ

For example, if RCLP = RFREQ, say both are 49.9k, then the converter will enter pulse skipping when the output

of the current sense amplifier drops to 0.3V. Note that the gain with this current amplifier is 5, so in terms of the

voltage on the sense resistor (input of the current amp), we get 0.3V/5 = 0.06V. Since we usually design the

converter so that its peak is around 0.2V (the peak of Rsense voltage before it starts to current limit), we are

getting a ratio of 0.06V/0.2V = 0.3. In other words, the converter will enter pulse-skipping when the output

current is 30% of the max designed output current.

Setting UVLO/Hysteresis thresholds

Note: A 470k resistor from PG pin to VINS pin is required for guaranteeing proper termination of Gate drive pulse

during UVLO.

Suppose we have a divider connected to input at the VINS pin. Suppose we call the resistors RUPPER and RLOWER.

We also have a hysteresis resistor, RHYST, from the output of the UVLO comparator, which provides positive

feedback on to the VINS pin, as explained in the Pin Description section. So, when the input voltage is rising, in