LTC3883 Datasheet, PDF (75/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
PMBus Command Details
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.
The actual value of C_τ is not required. The important quantity is the thermal time constant τ_INV = (θ_IS C_τ). For example,
if an inductor has a thermal time constant τ_THERMAL = 5 seconds then:
MFR_IOUT_CAL_GAIN_TAU_INV = ∆t / τ_THERMAL = 1/5 = 0.2
Refer to the application section for more information on calibrating θ_IS and τ_INV.
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