Datasheet

LTC2974
93
2974fc
For more information www.linear.com/LTC2974
applicaTions inForMaTion
The inductor resistance, R
K
= V
DCR(K)
/I
OUT(K)
, power dis-
sipation P
K
= V
DCR(K)
I
OUT(K)
and the sensed temperature
T
K
, (K = 1, 2) are recorded for each load current. To increase
the accuracy in calculating θ
IS
, the two load currents should
be chosen around I1 = 10% and I2 = 90% of the current
range of the system.
The inductor thermal time constant τ models the first order
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 resis
-
tance at I2, due to the inductor self-heating. Note that the
electrical time constant τ
EL
= L/R is several orders of mag-
nitude shorter than the thermal one, and “instantaneous”
is relative 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 two-point calibration technique (1.3-1.4)
is used to extract the steady-state parameters θ
IS
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
I
(t)= α θ
IS
V2•I2– V1•I1
( )
e
– t/τ
(1.5)
The time constant τ is calculated from the slope of the
best-fit line y = ln(∆I/I) = a1 + a2t:
τ = –
1
a2
(1.6)
In summary, a single load current step is all that is needed
to calibrate the DCR current measurement. The stable por
-
tions of the response give us the thermal resistance θ
IS
and
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 40,
the bipolar sense transistor is close to the inductor and
away from the switcher. Connecting the collector of the
PNP to the local power ground plane assures good thermal
contact to the inductor, while the base and emitter should
be routed to the LTC2974 separately, and the base con
-
nected to the signal ground close to LTC2974.
LTpowerPlay: A
N INTERACTIVE GUI FOR POWER
MANAGERS
LTpowerPlay is a powerful Windows based development
environment that supports Linear Technology Power Sys
-
tem Manager ICs with EEPROM, including the LTC2974
4-channel PMBus Power System Manager
. The software
supports a variety of different tasks. You can use L
Tpow
-
erPlay to evaluate Linear Technology ICs by connecting
to a demo board system. LTpowerPlay can also be used
in an offline mode (with no hardware present) in order
to build a multi-chip configuration file that can be saved
and re-loaded at a later time. LTpowerPlay provides un
-
precedented diagnostic and debug features. It becomes a
valuable diagnostic tool during board bring-up to program
the
power management scheme
in a system. LTpowerPlay
utilizes Linear Technology’s DC1613 USB-to-I
2
C/SMBus/
PMBus Controller to communicate with one of many
potential targets, including the DC1809/DC1810 demo
board set, the DC1735 socketed programming board, or
a customer target system. The software also provides an
automatic update feature to keep the software current
with the latest set of device drivers and documentation.
A great deal of context sensitive help is available within
LTpowerPlay along with several tutorial demos. Complete
information is available at:
www.linear.com/ltpowerplay