ADP3189 datasheet(24/36 Pages) AD

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ADP3189

Rev. 0 | Page 24 of 36

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 ADP3189 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 of choosing 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

since this is equal to our design loadline.

()

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

value desired. 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

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

copper wire is 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’s DCR. Due to the nonlinear

nature of NTC thermistors, resistors RCS1 and RCS2 are needed.

See Figure 12 to linearize the NTC and produce the desired

temperature tracking.

CSREF

CSSUM

CSCOMP

CCS1

CCS2

RCS1

RCS2

RPH1

RTM

RPH2

RPH3

PLACE AS CLOSE AS POSSIBLE

TO THE NEAREST INDUCTOR

KEEP THIS PATH

AS SHORT AS

POSSIBLE AND

WELL AWAY FROM

SWITCH NODE LINES

ADP3189

Figure 12. 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. Since 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 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, calculate that r1 = 0.9112

and r2 = 0.7978.

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