AN1696 datasheet(6/11 Pages) STMICROELECTRONICS

6 / 11 page

AN1696 APPLICATION NOTE

6/11

Table 2. Part list board L6615

To measure the current carried by each section, it is possible to open the jumper JP1 (see schematic) and to

place a ring for inserting a current probe.

2.1 OR-ing FET and current sensing

High side current sensing is implemented by reading the voltage drop across the RDS(ON) of Q1 (during normal

operation the FET is maintained on by the charge pump); the voltage at L6615 CGA pin of the section #N is

proportional to its output current, in particular [1]:

(1)

The first parameter to consider when selecting the OR-ing FET is its RDS(ON) because, for a given output cur-

rent, it defines both the power dissipation and the drop useful for current sharing. Increasing RDS(ON) leads to

a wide sense signal available but also means higher power dissipation.

In the motherboard Q1 is a STS8DNF3LL [5] having (for each MOS):

STS8DNF3LL contains two internal MOSFETs that are paralleled leading to halve the RDS(ON).

The design must be done considering worst case conditions to avoid share bus saturation: only two sections

running and maximum value for RDS(ON). Considering also the temperature variation, we have:

RDS(ON)|@Tmax = 0.015Ω

then the maximum drop will be 75mV and (1) leads to VCGA(MAX) = VSH(MAX) of about 2.8V.

It can be useful to calculate the losses associated to the OR-ing FET and compare them with the losses in case

RESISTORS

R1, R10

6K8

R5, R8

R13

20K

330K

R14

7K5

47K

R15

100

3K3

R11, R12

200

CAPACITORS

C1, C2, C4

100nF

µF

C8, C9, C10

open

4.7

µF

C6, C7

10nF

IC’s

L6615D

(ST) SO8

LM293

SO8

STS8DNF3LL

(ST) SO8

DIODES

D1, D2, D3

1N4148

SOT23

RDS(ON) =

0.017 (nom.)

@ ID=4A, VGS=10V, TCASE=25°C

0.020

Ω (max.)

CG A # N

()

OU T # N

() R

DS ON

()

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