Datasheet

LTC3883/LTC3883-1
75
3883fa
For more information www.linear.com/LTC3883
PMBus COMMAND DETAILS
This command has two data bytes and is formatted in 16-bit 2’s complement integer ppm. N = –32768 to 32767 •
10
–6
. Nominal temperature is 27°C. The IOUT_CAL_GAIN is multiplied by:
[1.0 + MFR_IOUT_CAL_GAIN_TC • (READ_TEMPERATURE_1-27)]. DCR sensing will have a typical value of 3900.
The IOUT_CAL_GAIN and MFR_IOUT_CAL_GAIN_TC impact all current parameters including: READ_IOUT, MFR_
READ_IIN_CHAN, IOUT_OC_FAULT_LIMIT and IOUT_OC_WARN_LIMIT.
MFR_T_SELF_HEAT, MFR_IOUT_CAL_GAIN_TAU_INV AND MFR_IOUT_CAL_GAIN_THETA
The LTC3883 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 LTC3883 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 = 1s
τ
INV
= MFR_IOUT_CAL_GAIN_TAU_INV
θ
IS
= MFR_IOUT_CAL_GAIN_THETA
Initially self heat is set to zero. After each temperature measurement self heat is updated to be the previous value of
self heat incremented or decremented by ∆MFR_T_SELF_HEAT.