L6759D datasheet(33/51 Pages) STMICROELECTRONICS

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L6759D

Single NTC thermal monitor and compensation

Doc ID 023240 Rev 1

33/51

8.2

Thermal compensation

The L6759D supports DCR sensing for output voltage positioning: the same current

information used for voltage positioning is used to define the overcurrent protection and the

current reporting (Register 15h in SVI). Having imprecise and temperature-dependant

information leads to a violation of the specifications and misleading information returned to

the SVI master: a positive thermal coefficient specific to DCR needs to be compensated to

get stable behavior of the converter as the temperature increase. Un-compensated systems

show temperature dependencies on the regulated voltage, overcurrent protection and

current reporting (Reg 15h).

The temperature information available on the TM pin and used for the thermal monitor may

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

present on the TCOMP pin, the L6759D gives a correction to the IDROOP current used for

voltage positioning (see Section 6.3) therefore 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

adjustments allow modification of the gain between the sensed temperature and the

correction made upon the IDROOP current.

Short TCOMP to GND to disable thermal compensation (no correction is given to IDROOP).

8.3

TM and TCOMP design

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

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

recommended values/network is reported in Figure 11

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

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

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

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