ISL6277HRZ datasheet(28/37 Pages) RENESAS

28 / 37 page

ISL6277

FN8270 Rev 1.00

Page 28 of 37

Mar 8, 2012

Key Component Selection

Inductor DCR Current-Sensing Network

Figure 22 shows the inductor DCR current-sensing network for a

3-phase solution. An inductor current flows through the DCR and

creates a voltage drop. Each inductor has two resistors in Rsum

and Ro connected to the pads to accurately sense the inductor

current by sensing the DCR voltage drop. The Rsum and Ro

resistors are connected in a summing network as shown, and feed

the total current information to the NTC network (consisting of

Rntcs, Rntc and Rp) and capacitor Cn. Rntc is a negative

temperature coefficient (NTC) thermistor, used to temperature

compensate the inductor DCR change.

The inductor output side pads are electrically shorted in the

schematic but have some parasitic impedance in actual board

layout, which is why one cannot simply short them together for the

current-sensing summing network. It is recommended to use

1Ω~10Ω

Ro to create quality signals. Since Ro value is much smaller

than the rest of the current sensing circuit, the following analysis

ignores it.

The summed inductor current information is presented to the

capacitor Cn. Equations 17 thru 21 describe the frequency

domain relationship between inductor total current Io(s) and Cn

voltage VCn(s):

where N is the number of phases.

Transfer function Acs(s) always has unity gain at DC. The inductor

DCR value increases as the winding temperature increases,

giving higher reading of the inductor DC current. The NTC Rntc

value decrease as its temperature decreases. Proper selection of

Rsum, Rntcs, Rp and Rntc parameters ensures that VCn

represents the inductor total DC current over the temperature

range of interest.

There are many sets of parameters that can properly

temperature-compensate the DCR change. Since the NTC

network and the Rsum resistors form a voltage divider, Vcn is

always a fraction of the inductor DCR voltage. It is recommended

to have a higher ratio of Vcn to the inductor DCR voltage so the

droop circuit has a higher signal level to work with.

A typical set of parameters that provide good temperature

compensation are: Rsum = 3.65kΩ, Rp =11kΩ, Rntcs = 2.61kΩ

and Rntc = 10kΩ (ERT-J1VR103J). The NTC network parameters

may need to be fine tuned on actual boards. One can apply full

load DC current and record the output voltage reading

immediately; then record the output voltage reading again when

the board has reached the thermal steady state. A good NTC

network can limit the output voltage drift to within 2mV. It is

recommended to follow the Intersil evaluation board layout and

current sensing network parameters to minimize engineering

time.

VCn(s) also needs to represent real-time Io(s) for the controller to

achieve good transient response. Transfer function Acs(s) has a

pole wsns and a zero wL. One needs to match wL and wsns so

Acs(s) is unity gain at all frequencies. By forcing wL equal to wsns

and solving for the solution, Equation 22 gives Cn value.

For example, given N = 3, Rsum = 3.65kΩ, Rp = 11kΩ,

Rntcs =2.61kΩ, Rntc = 10kΩ, DCR = 0.88mΩ and L = 0.36µH,

Equation 22 gives Cn = 0.406µF.

Assuming the compensator design is correct, Figure 23 shows the

expected load transient response waveforms if Cn is correctly

selected. When the load current Icore has a square change, the

output voltage Vcore also has a square response.

If Cn value is too large or too small, VCn(s) does not accurately

represent real-time Io(s) and worsens the transient response.

Figure 24 shows the load transient response when Cn is too

small. Vcore sags excessively upon load insertion and may create

a system failure. Figure 25 shows the transient response when

RSUM

RNTCS

RNTC

DCR

RSUM

PHASE2 PHASE3

DCR

PHASE1

RSUM

ISUM+

ISUM-

VCN

FIGURE 22. DCR CURRENT-SENSING NETWORK

VCn s



Rntcnet

Rsum

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

------------------------------------------ DCR

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









Io s





Acs





(EQ. 17)

Rntcnet

Rntcs Rntc

 R



Rntcs Rntc Rp

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

(EQ. 18)

Acs s





-------



sns

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

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

(EQ. 19)



DCR

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

(EQ. 20)



sns

Rntcnet

Rsum

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



Rntcnet

Rsum

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

------------------------------------------ Cn



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

(EQ. 21)

Rntcnet

Rsum

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



Rntcnet

Rsum

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

------------------------------------------ DCR



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

(EQ. 22)

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