ISL6334DIRZ datasheet(13/28 Pages) RENESAS

13 / 28 page

ISL6334D

FN6802 Rev 4.00

Page 13 of 28

Apr 29, 2016

Current Sensing

The ISL6334D senses current continuously for fast response. The

ISL6334D supports inductor DCR sensing, or resistive sensing

techniques. The associated channel current sense amplifier uses

the ISEN inputs to reproduce a signal proportional to the inductor

current, IL. The sense current, ISEN, is proportional to the inductor

current. The sensed current is used for current balance and

overcurrent protection.

The internal circuitry, shown in Figures 7 and 8, represents one

channel of an N-channel converter. This circuitry is repeated for

each channel in the converter, but may not be active depending

on the status of the PWM2, PWM3 and PWM4 pins, as described

in “PWM and PSI# Operation” on page 12. The input bias current

of the current sensing amplifier is typically 60nA; less than 5kΩ

input impedance is preferred to minimized the offset error.

INDUCTOR DCR SENSING

An inductor’s winding is characteristic of a distributed resistance,

as measured by the DCR (Direct Current Resistance) parameter.

Consider the inductor DCR as a separate lumped quantity, as

shown in Figure 7. The channel current IL, flowing through the

inductor, will also pass through the DCR. Equation 4 shows the

S-domain equivalent voltage across the inductor VL.

A simple R-C network across the inductor extracts the DCR

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 5).

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

time constant (= R*C) matches the inductor time constant

(= L/DCR), the voltage across the capacitor VC is equal to the

voltage drop across the DCR, i.e., proportional to the channel

current.

With the internal low-offset current amplifier, the capacitor

voltage VC is replicated across the sense resistor RISEN.

Therefore, the current out of ISEN+ pin, ISEN, is proportional to

the inductor current.

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

needed to match the time delay between the ISEN- and ISEN+

signals. Select the proper CT to keep the time constant of RISEN

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

Equation 6 shows that the ratio of the channel current to the

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

and the DCR of the inductor.

FIGURE 6. SWITCHING FREQUENCY vs RT

250

200

150

100

100k 200k 300k 400k 500k 600k 700k 800k 900k 1M

SWITCHING FREQUENCY (Hz)

VL s



IL sL DCR







(EQ. 4)

FIGURE 7. DCR SENSING CONFIGURATION

ISEN IL

DCR

RISEN

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

ISEN-(n)

CURRENT

SENSE

VIN

ISEN+(n)

PWM(n)

ISL6596

RISEN(n)

DCR

INDUCTOR

VOUT

COUT

VC(s)

IL s



ISL6334D INTERNAL CIRCUIT

VC s



DCR

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







DCR IL







sRC 1





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

(EQ. 5)

ISEN

DCR

RISEN

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



(EQ. 6)

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