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ADP3168

Rev. B | Page 16 of 24

INDUCTOR DCR TEMPERATURE CORRECTION

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

copper wire being the source of the DCR, one needs to com-

pensate 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 temper-

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

nature of NTC thermistors, resistors RCS1 and RCS2 are needed

(see Figure 12) 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

ADP3168

CCS

1.8nF

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 12. 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 is no

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. We will call these resistance values

A (RTH(50°C)/RTH(25°C)) and B (RTH(90°C)/RTH(25°C)). Note that

the NTC’s relative value is always 1 at 25°C.

Find the relative value 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, T1 = 50°C,

and T2 = 90°C.

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

(

)

()

(

)

()

(

)

RCS

−

(

)

−

(8)

CS1

CS2

−

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)

Finally, calculate values for RCS1 and RCS2 using Equation 10:

(10)

() (

)

()

CS2

CS1

−

For this example, RCS has been chosen to be 100 kΩ , so we start

with a thermistor value of 100 kΩ. Looking through available

0603 size thermistors, we find a Vishay NTHS0603N01N1003JR

NTC thermistor with A = 0.3602 and B = 0.09174. From these

we compute RCS1 = 0.3796, RCS2 = 0.7195, and RTH = 1.0751.

Solving for RTH yields 107.51 kΩ, so we choose 100 kΩ, making

k = 0.9302. Finally, we find RCS1 and RCS2 to be 35.3 kΩ and

73.9 kΩ. Choosing the closest 1% resistor values yields a choice

of 35.7 kΩ and 73.2 kΩ.

OUTPUT OFFSET

Intel’s specification requires that at no load the nominal output

voltage of the regulator be offset to a lower value 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 RB can be found using

Equation 11:

ONL

VID

−

(11)

Ω

µA

480

−

The closest standard 1% resistor value is 1.33 kΩ.

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