ADP3198 datasheet(22/32 Pages) AD

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ADP3198

Rev. A | Page 22 of 32

CURRENT SENSE AMPLIFIER

Most designs require the regulator output voltage, measured at

the CPU pins, to drop when the output current increases. The

specified voltage drop corresponds to a dc output resistance (RO),

also referred to as a load line. The ADP3198 has the flexibility of

adjusting RO, independent of current-limit or compensation

components, and it can also support CPUs that do not require

a load line.

For designs requiring a load line, the impedance gain of the

CS amplifier (RCSA) must be to be greater than or equal to the load

line. All designs, whether they have a load line or not, should

keep RCSA ≥ 1 mΩ.

The output current is measured by summing the voltage across

each inductor and passing the signal through a low-pass filter.

This summer filter is the CS amplifier configured with resistors

RPH(X) (summers), and RCS and CCS (filter). The impedance gain

of the regulator is set by the following equations, where RL is the

DCR of the output inductors:

()

CSA

(6)

(7)

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 ASCLOSE 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

= 0.7978.

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