ADP3207 datasheet(21/29 Pages) ONSEMI

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ADP3207

Rev. 1 | Page 21 of 29 | www.onsemi.com

The following procedure and equations yield values for RCS1, RCS2,

and RTH (the thermistor value at 25°C) for a given RCS value:

Select an NTC to be used based on type and value. Because

there is no value yet, 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. Temperatures that work well are 50°C

and 90°C. These are called Resistance Value A (A is

RTH(50°C)/RTH(25°C)) and Resistance Value B (B is

RTH(90°C)/RTH(25°C)). Note that the relative value of NTC

is always 1 at 25°C.

Next, find the relative value of RCS that is required for each

of these temperatures. This is based on the percentage of

change needed, which is initially 0.39%/°C. These are

called r1 and r2.

()

−

(9)

where:

TC = 0.0039

T1 = 50°C

T2 = 90°C.

Compute the relative values for

rCS1, rCS2, and rTH using

()

(

)

()

−

(10)

Calculate

RTH = RTH × RCS, then select the closest value of

thermistor that is available. Also, compute a scaling factor

k based on the ratio of the actual thermistor value relative

to the computed one

)

(

)

(

CALCULATED

ACTUAL

k =

(11)

Finally, calculate values for

RCS1 and RCS2 using

()

−

(12)

This example starts with a thermistor value of 100 k

Ω and uses

a Vishay NTHS0603N04 NTC thermistor (a 0603 size thermistor)

with A = 0.3359 and B = 0.0771. From this data, rCS1 = 0.359,

rCS2 = 0.729 and rTH = 1.094. Solving for RTH yields 240 kΩ, so

220 kΩ is chosen, making k = 0.914. Finally, RCS1 and RCS2 are

72.3 kΩ and 166 kΩ. Choosing the closest 1% resistor values

yields a choice of 71.5 kΩ and 165 kΩ.

COUT SELECTION

The required output decoupling for processors and platforms is

typically recommended by Intel. The following guidelines can

also be used if both bulk and ceramic capacitors in the system:

•

Select the total amount of ceramic capacitance. This is based

on the number and type of capacitors to be used. The best

location for ceramics is inside the socket; 20 pieces of

Size 0805 being the physical limit. Additional capacitors

can be placed along the outer edge of the socket.

•

Select the number of ceramics and find the total ceramic

capacitance (CZ). Combined ceramic values of 200 μF to

300 μF are recommended and are usually made up of

multiple 10 μF or 22 μF capacitors.

•

Note that there is an upper limit imposed on the total

amount of bulk capacitance (CX) when considering the

VID on-the-fly output voltage stepping (voltage step VV in

time tV with error of VERR), and also a lower limit based on

meeting the critical capacitance for load release at a given

maximum load step ΔIO. For a step-off load current, the

current version of the IMVP-6 specification allows a

maximum VCORE overshoot (VOSMAX) of 10 mV, plus 1.5% of

the VID voltage. For example, if the VID is 1.150 V, then

the largest overshoot allowed is 27 mV.

()

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≥

VID

OSMAX

MIN

(13)

VID

MAX

nKR

−

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≤

)

(

(14)

where:

⎟

⎠

⎞

⎜

⎝

⎛

−

ERR

(15)

To meet the conditions of these equations and transient

response, the ESR of the bulk capacitor bank (RX) should be less

than two times the droop resistance, RO. If the CX(MIN) is larger

than CX(MAX), the system does not meet the VID on-the-fly

and/or deeper sleep exit specification and can require a smaller

inductor or more phases (the switching frequency can also have

to be increased to keep the output ripple the same).

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