ADP3208C datasheet(32/41 Pages) ONSEMI

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ADP3208C

Rev. 1 | Page 32 of 41 | www.onsemi.com

current. Equation 4 can be used to determine the minimum

inductance based on a given output ripple voltage.

MIN

VID

−

)

(

(3)

RIPPLE

MIN

VID

−

≥

))

(

(4)

Solving Equation 4 for a 16 mV peak-to-peak output ripple

voltage yields

()

533

kHz

076

mΩ

−

≥

If the resultant ripple voltage is less than the initially selected

value, the inductor can be changed to a smaller value until the

ripple value is met. This iteration allows optimal transient

response and minimum output decoupling.

The smallest possible inductor should be used to minimize the

number of output capacitors. Choosing a 490 nH inductor is a

good choice for a starting point, and it provides a calculated

ripple current of 9.0 A. The inductor should not saturate at the

peak current of 24.5 A, and it should be able to handle the sum

of the power dissipation caused by the winding’s average current

(20 A) plus the ac core loss. In this example, 330 nH is used.

Another important factor in the inductor design is the DCR,

which is used for measuring the phase currents. Too large of a

DCR causes excessive power losses, whereas too small of a value

leads to increased measurement error. For this example, an

inductor with a DCR of 0.8 mΩ is used.

Selecting a Standard Inductor

After the inductance and DCR are known, select a standard

inductor that best meets the overall design goals. It is also

important to specify the inductance and DCR tolerance to

maintain the accuracy of the system. Using 20% tolerance for

the inductance and 15% for the DCR at room temperature are

reasonable values that most manufacturers can meet.

Power Inductor Manufacturers

The following companies provide surface-mount power inductors

optimized for high power applications upon request:

•

Vishay Dale Electronics, Inc.

(605) 665-9301

•

Panasonic

(714) 373-7334

•

Sumida Electric Company

(847) 545-6700

•

NEC Tokin Corporation

(510) 324-4110

Output Droop Resistance

The design requires that the regulator output voltage measured

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

specified voltage drop corresponds to the droop resistance (RO).

The output current is measured by summing the currents of the

resistors monitoring the voltage across each inductor and by

passing the signal through a low-pass filter. The summing is

implemented by the CS amplifier that is configured with resistor

RPH(x) (summer) and resistors RCS and CCS (filters). The output

resistance of the regulator is set by the following equations:

SENSE

)

(

(5)

SENSE

(6)

where RSENSE is the DCR of the output inductors.

Either RCS or RPH(x) can be chosen for added flexibility. Due to

the current drive ability of the CSCOMP pin, the RCS resistance

should be greater than 100 kΩ. For example, initially select RCS

to be equal to 200 kΩ, and then use Equation 6 to solve for CCS:

kΩ

200

mΩ

330

If CCS is not a standard capacitance, RCS can be tuned. For

example, if the optimal CCS capacitance is 1.5 nF, adjust RCS to

280 kΩ. For best accuracy, CCS should be a 5% NPO capacitor.

In this example, a 220 kΩ is used for RCS to achieve optimal results.

Next, solve for RPH(x) by rearranging Equation 5 as follows:

kΩ

220

mΩ

)

(

≥

The standard 1% resistor for RPH(x) is 86.6 kΩ.

Inductor DCR Temperature Correction

If the DCR of the inductor is used as a sense element and

copper wire is the source of the DCR, the temperature changes

associated with the inductor’s winding must be compensated

for. Fortunately, copper has a well-known temperature

coefficient (TC) of 0.39%/°C.

If RCS is designed to have an opposite but equal percentage of

change in resistance, it cancels the temperature variation of the

inductor’s DCR. Due to the nonlinear nature of NTC thermistors,

series resistors RCS1 and RCS2 (see Figure 36) are needed to linearize

the NTC and produce the desired temperature coefficient tracking.

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