L6751 datasheet(42/59 Pages) STMICROELECTRONICS

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Single NTC thermal monitor and compensation

L6751

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Doc ID 023992 Rev 1

increases. Un-compensated systems show temperature dependencies on the regulated

voltage, overcurrent protection and current reporting.

The temperature information available on the TM/STM pin and used for thermal monitoring

may be used also for this purpose. By comparing the voltage on the TM/STM pin with the

voltage present on the TCOMP/STCOMP pin, the L6751 corrects the IDROOP/ISDROOP

current used for voltage positioning (see Section 6.3), so recovering the DCR temperature

deviation. Depending on NTC location and distance from the inductors and the available

airflow, the correlation between NTC temperature and DCR temperature may be different:

TCOMP/STCOMP adjustments allow the gain between the sensed temperature and the

correction made on the IDROOP/ISDROOP current to be modified.

Short TCOMP/STCOMP to GND to disable thermal compensation (no correction of

IDROOP/ISDROOP is made).

8.3

TM/STM and TCOMP/STCOMP design

This procedure applies to both single-phase and multi-phase sections.

Properly choose the resistive network to be connected to the TM pin. Recommended

values/network is reported in Figure 11.

Connect voltage generator to the TCOMP pin (default value 3.3 V).

Power on the converter and load the thermal design current (TDC) with the desired

cooling conditions. Record the output voltage regulated as soon as the load is applied.

Wait for thermal steady-state. Adjust down the voltage generator on the TCOMP pin in

order to get the same output voltage recorded at point #3.

Design the voltage divider connected to TCOMP (between VCC5 and GND) in order to

get the same voltage set to TCOMP at point #4.

Repeat the test with the TCOMP divider designed at point #5 and verify the thermal

drift is acceptable. In the case of positive drift (i.e. output voltage at thermal steady-

state is bigger than output voltage immediately after loading TDC current), change the

divider at the TCOMP pin in order to reduce the TCOMP voltage. In the case of

negative drift (i.e. output voltage at thermal steady-state is smaller than output voltage

immediately after loading TDC current), change the divider at the TCOMP pin in order

to increase the TCOMP voltage.

The same procedure can be implemented with a variable resistor in place of one of the

resistors of the divider. In this case, once the compensated configuration is found,

simply replace the variable resistor with a resistor with the same value.

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