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ADP3191

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INDUCTOR DCR TEMPERATURE CORRECTION

With the inductor’s DCR being used as the sense element and

copper wire being the source of the DCR, compensation is

needed for temperature changes of the inductor’s winding.

Fortunately, copper has a well-known temperature coefficient

(TC) of 0.39%/°C.

If RCS is designed to have an opposite and equal percentage

change in resistance to that of the wire, it cancels the tempera-

ture variation of the inductor’s DCR. Due to the nonlinear

nature of NTC thermistors, Resistor RCS1 and Resistor RCS2 are

needed. See Figure 10 to linearize the NTC and produce the

desired temperature tracking.

CSSUM

CSCOMP

PLACE AS CLOSE AS POSSIBLE

TO NEAREST INDUCTOR

OR LOW-SIDE MOSFET

CSREF

ADP3191/

ADP3191A

CCS1

CCS2

RCS1

RTH

RCS2

KEEP THIS PATH

AS SHORT AS POSSIBLE

AND WELL AWAY FROM

SWITCH NODE LINES

SWITCH

NODES

VOUT

SENSE

RPH1

RPH3

RPH2

Figure 10. Temperature Compensation Circuit Values

The following procedure and expressions yield values to use for

RCS1, RCS2, and RTH (the thermistor value at 25°C) for a given RCS

value.

Select an NTC based on type and value. Because there isn’t

a value yet, start with a thermistor with a value close to RCS.

The NTC should also have an initial tolerance of better

than 5%.

Based on the type of NTC, find its relative resistance value

at two temperatures. The temperatures that work well are

50°C and 90°C. These resistance values are called A

(RTH(50°C)/RTH(25°C)) and B (RTH(90°C)/RTH(25°C)). The NTC’s

relative value is always 1 at 25°C.

Find the relative values of RCS required for each of these

temperatures. This is based on the percentage change

needed, which in this example is initially 0.39%/°C. These

are called r1 (1/(1 + TC × (T1 − 25))) and r2 (1/(1 + TC ×

(T2 − 25))), where TC = 0.0039 for copper. T1 = 50°C and

T2 = 90°C are chosen. From this, calculate that r1 = 0.9112

and r2 = 0.7978.

Compute the relative values for RCS1, RCS2, and RTH using

(

)

()

(

)

()

(

)

CS2

−

(

)

CS2

CS1

−

CS1

CS2

−

(8)

Calculate RTH = rTH × RCS, then select the closest value of

thermistor available. Also, compute a scaling factor k

based on the ratio of the actual thermistor value used

relative to the computed one:

()

CALCULATED

ACTUAL

k =

(9)

Calculate values for RCS1 and RCS2 using Equation 10:

CS1

(

)

(

)

(

)

CS2

−

(10)

For this example, RCS has been calculated to be 110 kΩ.

Start with a thermistor value of 100 kΩ. Next, look

through the available 0603-size thermistors, and find

a Vishay NTHS0603N01N1003JR NTC thermistor

with A = 0.3602 and B = 0.09174. From these, compute

RCS1 = 0.3795, RCS2 = 0.7195, and RTH = 1.075. Solve for RTH,

which yields 118.28 kΩ. Then, choose 100 kΩ, which

makes k = 0.8455. Finally, RCS1 and RCS2 are 35.3 kΩ

and 83.9 kΩ. Choose the closest 1% resistor values,

which yields a choice of 35.7 kΩ or 84.5 kΩ.

OUTPUT OFFSET

The Intel specification requires that at no load should the

nominal output voltage of the regulator be offset to a value

lower than the nominal voltage corresponding to the VID code.

The offset is set by a constant current source flowing out of the

FB pin (IFB) and flowing through RB. The value of R

can be

found using Equation 11:

ONL

VID

−

Ω

μA

281

−

(11)

The closest standard 1% resistor value is 1.21 kΩ.

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