L6711 datasheet(27/50 Pages) STMICROELECTRONICS

27 / 50 page

L6711

Voltage positioning

27/50

The ITS circuit subtracts from the IFB current a current proportional to the sensed

temperature as follow (see Figure 11, Only the IDROOP and ITC contributes to IFB have been

considered):

where

where A and B are positive constants depending on the value of the external resistor RTC

(see Figure 12), TJ is the device junction temperature and TMOS is the mosfet (or the used

sensing element) temperature.

The resistor RTC can be designed in order to zero the temperature influence on the output

voltage at a fixed current as follow:

obtaining the following relationship:

where RSENSE is the sensing element resistance value (at TMOS = 25ºC), B is the constant

obtainable from

Figure 12, kT is the Temperature Coupling Coefficient between the sensing element and the

Controller (it results KT = (TJ-25)/(TMOS-25)) and α is the Temperature Coefficient of the

sensing element. Since RTC depends from the constant B depending in turn from RTC, an

iterative process is required to properly design the RTC value. As a consequence of the

nature of the thermal sensor, a negative offset is needed to compensate the native offset

introduced by the ITS at a referenced temperature Tref and it is obtainable by connecting a

ROFFSET resistor between the OFFSET pin and SGND as follow:

To disable this function, short the pin to SGND.

Figure 12.

Integrated thermal sensor constant vs. external resistor RTC

OUT T,IOUT

()

VID

SEN SE TMOS

()

------------------------------------------ IOUT ITC TJ

()

–

⋅

–

TC TJ

()

-----------

–

()

⋅

[]

⋅

SENSE α

MOS

–

()

⋅⋅

------------------------------------------------------------------- IOUT

----------- BTJ 25

–

()

⋅⋅

–

SENSE

---------------------

⋅

α I

OUT

⋅

--------------------

⋅

OFFSET

TC Tref

()

–

re f

–

()

⋅

----------------------------------------------

450

475

500

525

550

575

600

625

650

1020 30

405060 7080 90 100

1.50

1.60

1.70

1.80

1.90

2.00

1020 3040 50 60 7080 90 100

A [mV] vs. RTC [KΩ]

B [mV/°C] vs. RTC [KΩ]

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