ADP3198 datasheet(21/31 Pages) ONSEMI

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ADP3198

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The user has the flexibility to choose either RCS or RPH(X).

However, it is best to select RCS equal to 100 kΩ, and then solve

for RPH(X) by rearranging Equation 6. Here, RCSA = RO= 1 mΩ

because this is equal to the design load line.

()

140

100

mΩ

CSA

Next, use Equation 7 to solve for CCS.

100

320

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

standard values can be used in parallel to get as close to the

desired value. For best accuracy, CCS should be a 5% or 10%

NPO capacitor. This example uses a 5% combination for CCS

of two 1 nF capacitors in parallel. Recalculating RCS and RPH(X)

using this capacitor combination yields 114 kΩ and 160 kΩ.

The closest standard 1% value for RPH(X) is 158 kΩ.

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

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

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

of NTC thermistors, Resistor RCS1 and Resistor RCS2 are needed.

See Figure 11 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

ADP3198

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 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.

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%.

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.

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

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.

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

()

(

)

CS2

−

(8)

()

CS2

CS1

−

(9)

CS1

CS2

−

(10)

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 =

(11)

Calculate values for RCS1 and RCS2 using Equation 12 and 13.

CS1

(12)

()

(

)

(

)

CS2

−

(13)

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

available 100 kΩ thermistor, 0603 size. 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, so 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.

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