LTM4677 datasheet(63/116 Pages) AD

63 / 116 page

LTC3883/LTC3883-1

Rev. F

For more information www.analog.com

APPLICATIONS INFORMATION

the inductor DC resistance at the reference temperature

TREF and a is the temperature coefficient of the resistance.

Since most inductors are made of copper, we can expect

a temperature coefficient close to aCU = 3900ppm/°C.

For a given a, the remaining parameters θIS and R0 can

be calibrated at a single temperature using only two load

currents:

RO =

R2 – R1

(

) P2 + P1

(

) – R2 + R1

(

) P2 – P1

(

)

a T2 – T1

(

) P2 + P1

(

) – P2 – P1

(

) 2 + a T1+ T2 – 2TREF

[

]

(

)

θIS =

aRO

•

a R1+ R2

(

) T2 – T1

(

) – R2 – R1

(

) 2 + a T1+ T2 – 2TREF

[

]

(

)

a T2 – T1

(

) P2 + P1

(

) – P2 – P1

(

) 2 + a T1+ T2 – 2TREF

[

]

(

)

The inductor resistance, RK = VDCR(K)/IOUT(K), power dis-

sipation PK = VDCR(K) IOUT(K) and the sensed temperature

TK,(K=1,2)arerecordedforeachloadcurrent.Toincrease

theaccuracyincalculatingθIS,thetwoloadcurrentsshould

be chosen around I1 = 10% and I2 = 90% of the current

range of the system.

Theinductorthermaltimeconstantτmodelsthefirstorder

thermal response of the inductor and allows accurate DCR

compensation during load transients. During a transition

from low-to-high load current, the inductor resistance

increases due to the self-heating. If we apply a single load

step from the low current I1 to the higher current I2, the

voltage across the inductor will change instantaneously

from I1R1 to I2R1 and then slowly approach I2R2. Here R1

is the steady-state resistance at the given temperature and

load current I1, and R2 is the slightly higher DC resistance

at I2, due to the inductor self-heating. Note that the electri-

cal time constant τEL = L/R is several orders of magnitude

shorter than the thermal one, and “instantaneous” is rela-

tive to the thermal time constant. The two settled regions

give us the data sets (I1, T1, R1, P1) and (I2, T2, R2, P2)

and the 2-point calibration technique (1.3-1.4) is used to

extract the steady-state parameters θIS and R0 (given a

previously characterized average a). The relative current

error calculated using the steady-state expression (1.2)

will peak immediately after the load step, and then decay

to zero with the inductor thermal time constant τ.

∆I

( ) = a θIS V2I2 – V1I1

(

– t/ τ

The time constant τ is calculated from the slope of the

best-fit line y = ln(∆I/I) = a1 + a2t:

τ = –

In summary, a single load current step is all that is needed

to calibrate the DCR current measurement. The stable por-

tionsoftheresponsegiveusthethermalresistanceθISand

nominal DC resistance R0, and the settling characteristic

is used to measure the inductor thermal time constant τ.

To get the best performance, the temperature sensor has

to be as close as possible to the inductor and away from

other significant heat sources. For example in Figure 30,

the bipolar sense transistor is close to the inductor and

away from the switcher. Connecting the collector of the

PNPtothelocalpowergroundplaneassuresgoodthermal

contact to the inductor, while the base and emitter should

be routed to the LTC3883 separately, and the base con-

nected to the signal ground close to LTC3883.

LTpowerPlay: AN INTERACTIVE GUI FOR DIGITAL

POWER

LTpowerPlay is a powerful Windows-based development

environment that supports Linear Technology digital

power ICs including the LTC3883. The software supports

a variety of different tasks. LTpowerPlay can be used to

evaluate Linear Technology ICs by connecting to a demo

board or the user application. LTpowerPlay can also be

used in an offline mode (with no hardware present) in

order to build multiple IC configuration files that can be

saved and re-loaded at a later time. LTpowerPlay provides

unprecedented diagnostic and debug features. It becomes

a valuable diagnostic tool during board bring-up to pro-

gram or tweak the power system or to diagnose power

issues when bring up rails. LTpowerPlay utilizes Linear

Technology’s USB-to-I2C/SMBus/PMBus controller to

communication with one of the many potential targets

including the DC1778A demo board, the DC1890A sock-

eted programming board, or a customer target system.

The software also provides an automatic update feature

to keep the revisions current with the latest set of device

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