Datasheet

LTC2974
49
2974fc
For more information www.linear.com/LTC2974
MFR_T_SELF_HEAT, MFR_IOUT_CAL_GAIN_TAU_INV and MFR_IOUT_CAL_GAIN_THETA
The LTC2974 uses an innovative (patent pending) algorithm to dynamically model the temperature rise from the external
temperature sensor to the inductor core. This temperature rise is called MFR_T_SELF_HEAT and is used to calculate the
final temperature correction required by IOUT_CAL_GAIN. The temperature rise is a function of the power dissipated
in the inductor DCR, the thermal resistance from the inductor core to the remote temperature sensor and the thermal
time constant of the inductor to board system. The algorithm simplifies the placement requirements for the external
temperature sensor and compensates for the significant steady state and transient temperature error from the inductor
core to the primary inductor heat sink.
The best way to understand the self-heating effect inside the inductor is to model the system using the circuit analogy
of Figure 21. The 1st order differential equation for the above model may be approximated by the following difference
equation:
P
I
– T
I
/θ
IS
= C
τ
∆T
I
/∆t (Eq1) (when T
S
= 0)
from which:
∆T
I
= ∆t (P
I
θ
IS
– T
I
)/(θ
IS
C
τ
) (Eq2) or
∆T
I
= (P
I
θ
IS
– T
I
) • τ
INV
(Eq3)
where
τ
INV
= ∆t/(θ
IS
C
τ
) (Eq4)
and ∆t is the sample period of the external temperature ADC.
The LTC2974 implements the self-heating algorithm using Eq3 and Eq4 where:
∆T
I
=∆MFR_T_SELF_HEAT
P
I
= READ_IOUT • (V
ISENSEP
– V
ISENSEM
)
T
S
= READ_TEMPERATURE_1
T
I
= MFR_T_SELF_HEAT + T
S
∆t = 4 • t
CONV_SENSE
. (One complete external temperature loop period)
τ
INV
= MFR_IOUT_CAL_GAIN_TAU_INV
θ
IS
= MFR_IOUT_CAL_GAIN_THETA
Figure 21. Electronic Analogy for Inductor Temperature Model
I = P
I
P
I
= CURRENT REPRESENTING THE POWER DISSIPATED BY THE INDUCTOR
(V
DCR
• READ_IOUT WHERE V
DCR
= (V
ISENSEP
– V
ISENSM
))
C
τ
= CAPACITANCE REPRESENTING THERMAL HEAT CAPACITY OF THE INDUCTOR
(INCLUDED IN MFR_IOUT_CAL_GAIN_TAU_INV)
T
I
= VOLTAGE REPRESENTING THE TEMPERATURE OF THE INDUCTOR
θ
IS
= RESISTANCE REPRESENTING THE THERMAL RESISTANCE FROM THE DCR
TO THE REMOTE TEMPERATURE SENSOR (MFR_IOUT_CAL_GAIN_THETA)
T
S
= VOLTAGE REPRESENTING THE TEMPERATURE AT THE REMOTE
TEMPERATURE SENSOR
2974 F21
C = C
τ
R = θ
IS
V
S
= T
S
V
I
= T
I
pMbus coMManD DescripTion