ADP3193A datasheet(21/32 Pages) AD

Part #	ADP3193A

Description	8-Bit, Programmable, 2- to 3-Phase, Synchronous Buck Controller

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21 / 32 page

ADP3193A

Rev. 0 | Page 21 of 32

INDUCTOR DCR TEMPERATURE CORRECTION

When the inductor DCR is used as the sense element and

copper wire is used as the source of the DCR, the user needs to

compensate 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 of

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

ture variation of the inductor DCR. Due to the nonlinear nature of

negative temperature coefficient (NTC) thermistors, Resistors RCS1

and RCS2 are needed. See Figure 11 to linearize the NTC and

produce the desired temperature tracking.

CSSUM

13

CSCOMP

PLACE AS CLOSE AS POSSIBLE

TO NEAREST INDUCTOR

OR LOW-SIDE MOSFET

12

CSREF

11

ADP3193A

CCS1

CCS2

RCS1

RTH

RCS2

KEEP THIS PATH

AS SHORT AS POSSIBLE

AND WELL AWAY FROM

SWITCH NODE LINES

TO

SWITCH

NODES

TO

VOUT

SENSE

RPH1

RPH3

RPH2

Figure 11. Temperature-Compensation Circuit Values

The following procedure and equations yield values to use for

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

RCS value.

1.

Select an NTC based on type and value. Because the value

is unknown, use a thermistor with a value close to RCS. The

NTC should also have an initial tolerance of better than 5%.

2.

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 relative

value of the NTC is always 1 at 25°C.

3.

Find the relative value of RCS required for each of these

temperatures. This is based on the percentage of change

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

temperatures are called r1 (1/(1 + TC × (T1 − 25°C)))

and r2 (1/(1 + TC × (T2 − 25°C))), where TC = 0.0039 for

copper, T1 = 50°C, and T2 = 90°C. From this, r1 = 0.9112 and

r2 = 0.7978.

4.

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

(

)

()

(

)

()

(

)

B

A

r

A

B

r

B

A

r

A

B

r

B

A

r

B

A

r

2

1

2

1

CS2

−

×

−

×

−

×

−

×

−

×

+

×

−

×

−

×

−

=

1

(8)

(

)

CS2

1

CS2

CS1

r

A

r

A

r

−

=

1

(9)

CS1

CS2

TH

r

1

−

=

(10)

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

thermistor available. In addition, compute a scaling factor (k)

based on the ratio of the actual thermistor value used relative

to the computed one.

()

CALCULATED

TH

ACTUAL

TH

R

k =

(11)

5.

Calculate values for RCS1 and RCS2 using Equation 12 and

Equation 13.

CS1

CS

CS1

r

k

R

×

=

(12)

(

)

(

)

(

)

CS2

CS

CS2

r

k

R

×

+

−

×

=

1

(13)

In this example, RCS is calculated to be 114 kΩ. Look for an

available 100 kΩ, 0603-size thermistor. One such thermistor

is the Vishay NTHS0603N01N1003JR NTC thermistor with

A = 0.3602 and B = 0.09174. From these values, rCS1 = 0.3795,

rCS2 = 0.7195, and rTH = 1.075.

Solving for RTH yields 122.55 kΩ; therefore, 100 kΩ is chosen,

making k = 0.816. Next, find RCS1 and RCS2 to be 35.3 kΩ and

87.9 kΩ. Finally, choose the closest 1% resistor values, which

yields a choice of 35.7 kΩ and 88.7 kΩ.

OUTPUT OFFSET

The Intel specification requires that with no load 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

B

can be found using

Equation 14.

FB

ONL

VID

B

I

V

R

−

=

(14)

Ω

k

27

.

1

μA

15

V

381

.

1

V

4

.

1

=

−

=

B

R

The closest standard 1% resistor value is 1.27 kΩ.

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