LTC2972 datasheet(57/112 Pages) LINER

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LTC2972

2972fa

For more information www.linear.com/LTC2972

The best way to understand the self-heating effect inside the inductor is to model the system using the circuit analogy

of Figure 19. The 1st order differential equation for the above model may be approximated by the following difference

equation:

PI – TI/θIS = Cτ ∆TI/∆t (Eq1) (when TS = 0)

from which:

∆TI = ∆t (PI θIS – TI)/(θIS Cτ) (Eq2) or

∆TI = (PI θIS – TI) • τINV (Eq3)

where

τINV = ∆t/(θIS Cτ) (Eq4)

and ∆t is the sample period of the external temperature ADC.

The LTC2972 implements the self-heating algorithm using Eq3 and Eq4 where:

∆TI =∆MFR_T_SELF_HEAT

PI = READ_IOUT • (VISENSEP – VISENSEM)

TS = READ_TEMPERATURE_1

TI = MFR_T_SELF_HEAT + TS

∆t = 4 • tCONV_SENSE. (One complete external temperature loop period)

τINV = MFR_IOUT_CAL_GAIN_TAU_INV

θIS = 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 Cτ is not required. The important quantity is the thermal time constant τINV = (θIS Cτ). For example,

if an inductor has a thermal time constant τINV = 5 seconds then:

MFR_IOUT_CAL_GAIN_TAU_INV = (4 • tCONV_SENSE)/5 = 4 • 66ms/5s = 0.0528

Refer to the application section for more information on calibrating θIS and τINV.

READ_TEMPERATURE_2 is substituted for READ_TEMPERATURE_1 if the associated TSENSE network fails to detect

a valid temperature. Under these conditions TS = READ_TEMPERATURE_2 and the self-heating correction is applied

using the internal die temperature. See READ_TEMPERATURE_1 for more information.

MFR_T_SELF_HEAT Data Content

Bit(s) Symbol

Operation

b[15:0] Mfr_t_self_heat

Values are limited to the range 0°C to 50°C.

MFR_IOUT_CAL_GAIN_THETA Data Content

Bit(s) Symbol

Operation

b[15:0] Mfr_iout_cal_gain_theta

Values ≤ 0 set MFR_T_SELF_HEAT to zero.

PMBus COMMAND DESCRIPTION

LTC2972 Datasheet(PDF) 57 Page - Linear Technology