ISL6366 datasheet(18/44 Pages) INTERSIL

18 / 44 page

ISL6366

FN6964.0

January 3, 2011

A simple R-C network across the current sense resistor extracts

the RSEN voltage, as shown in Figure 7.

The voltage on the capacitor VC, can be shown to be proportional

to the channel current IL. See Equation 9.

If the R-C network components are selected such that the RC time

constant matches the ESL-RSEN time constant (R*C = ESL/RSEN),

the voltage across the capacitor VC is equal to the voltage drop

across the RSEN, i.e., proportional to the channel current. As an

example, a typical 1mΩ sense resistor can use R = 348 and

C = 820pF for the matching. Figures 7 and 8 show the sensed

waveforms without and with matching RC when using resistive

sense.

Because of the internal filter at the ISENS- pin, one capacitor, CT,

is needed to match the time delay between the ISENS- and

ISENS+ signals. Select the proper CT to keep the time constant of

RISEN and CT (RISEN x CT) close to 27ns.

Equation 10 shows that the ratio of the channel current to the

sensed current, ISEN, is driven by the value of the sense resistor

and the RISEN.

Figures 5 and 6 configurations apply for VR1 output, while the

RISEN should include the internal metal impedance of 10.5Ω for

accurate current sense.

For VR0 output, the RISEN resistor of each channel is integrated,

while its value is determined by the RSET resistor. The RSET resistor

value can be from 3.84k

Ω to 115.2kΩ and is 64x of the required

ISEN resistor value. Therefore, the current sense gain resistor

(Integrated RISEN) value can be effectively set at 60Ω to 1.8kΩ.

Figures 9 and 10 show the configurations for inductor DCR sensing

and resistive sensing of VR0, respectively; their sensing current is

represented by Equations 11 and 12, respectively.

The inductor DCR value will increase as the temperature increases.

Therefore, the sensed current will increase as the temperature of

the current sense element increases. In order to compensate the

temperature effect on the sensed current signal, a Negative

Temperature Coefficient (NTC) resistor can be used for thermal

compensation, or the integrated temperature compensation

function of ISL6366 should be utilized. The integrated temperature

compensation function is described in “Temperature

Compensation” on page 30.

Decoupling capacitor (CT) on ISEN[6:1]- pins are optional and

might be required for long sense traces and a poor layout.

VC s

()

ESL

RSEN

--------------

⋅

⎝⎠

⎛⎞

RSEN IL

⋅

()

⋅

sRC 1

⋅

()

---------------------------------------------------------------------

(EQ. 9)

FIGURE 7. VOLTAGE ACROSS R WITHOUT RC

FIGURE 8. VOLTAGE ACROSS C WITH MATCHING RC

ISEN

RSEN

RISEN

---------------

⋅

(EQ. 10)

ISEN

DCR 64

⋅

RSET

-----------------------

⋅

(EQ. 11)

ISEN

RSEN 64

⋅

RSET

-------------------------

⋅

(EQ. 12)

FIGURE 9. DCR SENSING CONFIGURATION FOR VR0

ISEN

DCR 64

⋅

RSET

-----------------------

ISEN-(n)

CURRENT

SENSE

ISEN+(n)

DCR

INDUCTOR

VOUT

COUT

VC(s)

IL s

()

ISL6366

RSET

RISEN(n)

CT, Optional

FIGURE 10. SENSE RESISTOR IN SERIES WITH INDUCTORS FOR

VR0

ISEN

RSEN 64

⋅

RSET

-------------------------

ISEN-(n)

CURRENT

SENSE

ISEN+(n)

ISL6366

RSET

RISEN(n)

RSEN

VOUT

COUT

VC(s)

ESL

RSENSE

CT, Optional

ISL6366 Datasheet(PDF) 18 Page - Intersil Corporation

ISL6366 Datasheet(HTML) 18 Page - Intersil Corporation

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