ADP3189 datasheet(25/36 Pages) AD

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ADP3189

Rev. 0 | Page 25 of 36

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

Equation 13:

CS1

(12)

(

) (

)

(

)

CS2

−

(13)

In this example, RCS was 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, compute

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.

LOAD LINE SETTING

For load line values greater than 1 mΩ, RCSA can be set equal

to RO, and the LLSET pin can be directly connected to the

CSCOMP pin. When the load line value needs to be less than

1 mΩ, two additional resistors are required. Figure 13 shows

the placement of these resistors.

CSREF

LLSET

CSSUM

CSCOMP

ADP3189

RLL1

RLL2

QLL

OPTIONAL LOAD LINE

SELECT SWITCH

Figure 13. Load Line Setting Resistors

The two resistors RLL1 and RLL2 set up a divider between the

CSCOMP pin and CSREF pin. This resistor divider is input into

the LLSET pin to set the load line slope RO of the VR according

to the following equation:

CSA

(14)

For best results, start with a 1% resistor of 20.0 kΩ for RLL2.

Then, solve for the required value of RLL1 by rearranging

Equation 14 as follows:

CSA

Another useful feature for some VR applications is the ability to

select different load lines. Figure 13 shows an optional MOSFET

switch that allows this. Here, design for RCSA = RO(MAX) (selected

with QLL on) and then use Equation 14 to set RO = RO(MIN)

(selected with QLL off).

For this design, RCSA = RO = 1 mΩ, so connect LLSET directly to

CSCOMP, and the resistors RLL1 and RLL2 are not needed.

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