LTC3883 Datasheet, PDF (61/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Applications Information
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 resistance
at I₂, due to the inductor self-heating. Note that the electri-
cal time constant τ_EL = 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 θ_IS and R0 (given a
previously characterized average α). 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
I
2
–
V1I
1^)e
–
t/τ
The time constant τ is calculated from the slope of the
best-fit line y = ln(∆I/I) = a1 + a2t:
τ=– ¹
a2
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 30,
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 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
drivers and documentation. A great deal of context sen-
sitive help is available with LTpowerPlay along with sev-
eral tutorial demos. Complete information is available at
http://www.linear.com/ltpowerplay.
PMBus Communication and Command
Processing
The LTC3883/LTC3883-1 have a one deep buffer to hold
the last data written for each supported command prior
to processing as shown in Figure 32; Write Command
Data Processing. When the part receives a new command
from the bus, it copies the data into the Write Command
Data Buffer, indicates to the internal processor that this
command data needs to be fetched, and converts the
command to its internal format so that it can be executed.
Two distinct parallel blocks manage command buffering
and command processing (fetch, convert, and execute) to
ensure the last data written to any command is never lost.
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