ISL28025 Datasheet, PDF(45/47 Page) Intersil Corporation – Precision Digital Power Monitor with Real Time Alerts

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ISL28025 Datasheet, PDF (45/47 Pages) Intersil Corporation – Precision Digital Power Monitor with Real Time Alerts

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ISL28025

The length of the

trace between the

two sense lines

defines the sense

resistor value.

FIGURE 90. ILLUSTRATES A LAYOUT EXAMPLE OF A CURRENT

SENSE RESISTOR MADE FROM A PCB TRACE

Figure 90 illustrates a layout example of a current sense resistor

defined by a PCB trace. The serpentine pattern of the resistor

reduces current crowding as well as limiting the magnetic

interference caused by the current flowing through the trace.

For the example discussed, the width of the trace in Figure 90

illustration would equal 2.192in and the length between the

sense lines equals 1.832in.

The width of the resistor is long for some applications. A means

of shortening the trace width is to connect two traces in parallel.

For calculation ease, assume the resistive traces are routed on

the outside layers of a PCB. Using Equations 18 and 19, the

width of the trace is reduced from 2.192in to 1.096in.

When using multiple layers to create a trace resistor, use

multiple vias to keep the trace potentials between the two

conductors the same. Vias are highly resistive compared to a

copper trace. Multiple vias should be employed to lower the

voltage drop due to current flowing through resistive vias.

Figure 91 illustrates a layout technique for a multiple layered

trace sense resistor.

TOP

VIA

TRACE

PCB

BOTTOM

TRACE

VIA

(A) CROSS SECTION VIEW

VIA

TRACE

PCB

(B) TOP VIEW

FIGURE 91. ILLUSTRATES A LAYOUT EXAMPLE OF A MULTIPLE

LAYER TRACE RESISTOR

Lossless Current Sensing (DCR)

A DCR sense circuit is an alternative to a sense resistor. The DCR

circuit utilizes the parasitic resistance of an inductor to measure

the current to the load. A DCR circuit remotely measures the

current through an inductor. The lack of components in series

with the regulator to the load makes the circuit lossless.

BUCK

REGULATOR

PHASE

DCR CIRCUIT

RSEN

VINP

CSEN RSEN + RDCR

LO RDCR

VINM

ADC

16-BIT

FIGURE 92. SIMPLIFIED CIRCUIT EXAMPLE OF A DCR

A properly matched DCR circuit has an equivalent circuit seen by

the ADC equals to Rdcr in Figure 92. Before deriving the transfer

function between the inductor current and voltage seen by the

ISL28025, letâs review the definition of an inductor and capacitor

in the Laplacian domain.

Xc(f)

jïï·(f)ïC

XL(f) jïï·(f)ïL

(EQ. 20)

Xc is the impedance of a capacitor related to the frequency and

XL is the impedance of an inductor related to frequency. Ï equals

to 2ï°f. f is the chop frequency dictated by the regulator. Using

Ohms law, the voltage across the DCR circuit in terms of the

current flowing through the inductor is define in Equation 21.

(EQ. 21)

In Equation 21, Rdcr is the parasitic resistance of the inductor.

The voltage drop across the inductor (Lo) and the resistor (Rdcr)

circuit is the same as the voltage drop across the resistor (Rsen)

and the capacitor (Csen) circuit. Equation 22 defines the voltage

across the capacitor (Vcsen) in terms of the inductor current (IL).

Vc(f)

1 ï« jïï·(f)ïCsenïRsen

ï©ïª

dcrïïªïªï«1

ï©ïª1 ï«

ï«

R dcr

ïº

ï»

ï¹ïº

ïº

ï« jïï·(f)ïCsenïRsenïºï»

ïi

(EQ. 22)

The relationship between the inductor load current (IL) and the

voltage across capacitor simplifies if the following component

selection holds true;

R dcr

C senïR sen

(EQ. 23)

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FN8388.3

June 17, 2015

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