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ADP3166

Selecting a Standard Inductor

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Output Droop Resistance

The design requires that the regulator output voltage measured

at the CPU pins drops when the output current increases. The

specified voltage drop corresponds to the static output droop

resistance (RO).

The output current is measured by summing together the voltage

across each inductor and then passing the signal through a low-

pass filter. This summer-filter is the CS amplifier configured with

resistors RPH(X) (summers) and RCS, and CCS (filter). The output

resistance of the regulator is set by the following equations, where

RL is the DCR of the output inductors:

PH(X)

(6)

LCS

(7)

One has the flexibility of choosing either RCS or RPH(X). It is best

to select RCS equal to 100 k

Ω, and then solve for RPH(X) by

rearranging Equation 6.

PH(X)

. k

PH(X)

100

145 5

Ω

ΩΩ

Next, use Equation 6 to solve for CCS:

600

100

375

ΩΩ

It is best to have a dual location for CCS in the layout so stan-

dard values can be used in parallel to get as close to the value

desired. For this example, choosing CCS to be a 1.5 nF and 2.2 nF

in parallel is a good choice. For best accuracy, CCS should be

a 10% capacitor. The closest standard 1% value for RPH(X) is

147 k

Ω .

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 will cancel the tem-

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

nature of NTC thermistors, resistors RCS1 and RCS2 (see Figure 2)

are needed to linearize the NTC and produce the desired tem-

perature tracking.

CSSUM

CSCOMP

PLACE AS CLOSE AS POSSIBLE

TO NEAREST INDUCTOR

OR LOW SIDE MOSFET

CSREF

ADP3166

CCS

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 2. Temperature Compensation Circuit Values

The following procedure and expressions will 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. Since we do not

have 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%.

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

two temperatures. The temperatures to use that work well are

°C and 90°C. We will call these resistance values A (A is

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

the NTC’s relative value is always 1 at 25

°C.

3. Next, find the relative value of RCS required for each of these

temperatures. This is based on the percentage change needed,

which we will initially make 0.39%/

°C. We will call these r1

(r1 is 1/(1 + TC

(T1 – 25))) and r2 (r2 is 1/(1 + TC

(T2 – 25))),

where TC = 0.0039, T1 = 50

°C and T2 = 90°C.

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