ADP3186 datasheet(16/24 Pages) AD

16 / 24 page

ADP3186

Rev. A | Page 16 of 24

()

145

100

mΩ

Next, use Equation 6 to solve for CCS.

100

600

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 as possible

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

10% NPO capacitor. This example uses a 5% combination for

CCS of 1.5 nF and 2.2 nF in parallel. Recalculating RPH(X) using

this capacitor combination yields a 1% value of 147 kΩ.

INDUCTOR DCR TEMPERATURE CORRECTION

When the inductor’s DCR is used as the sense element and

copper wire is the source of the DCR, one 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’s DCR. Due to the nonlinear

nature of NTC thermistors, resistors RCS1 and 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

ADP3186

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 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 the value

has not yet been found, 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)). 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 for copper. T1 = 50°C and

T2 = 90°C are chosen. From this, one can 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 100 kΩ, so

start with a thermistor value of 100 kΩ. Looking through

available 0603 size thermistors, one finds a Vishay

NTHS0603N01N1003JR NTC thermistor with A = 0.3602

and B = 0.09174. From these, one can compute rCS1 =

0.3796, rCS2 = 0.7195, and rTH = 1.0751. Solving for RTH

yields 107.51 kΩ, so 100 kΩ is chosen, making k = 0.9302.

Finally, one finds that RCS1 and RCS2 are 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

The AMD specification requires that at no load the nominal

output voltage of the regulator be offset to a value higher than

the nominal voltage corresponding to the VID code.

ADP3186 Datasheet(PDF) 16 Page - Analog Devices

ADP3186 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL