L6718 datasheet(60/71 Pages) STMICROELECTRONICS

60 / 71 page

Single NTC thermal monitor and compensation

L6718

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DocID023399 Rev 3

10.2

Thermal compensation

The L6718 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: positive thermal coefficient specific from DCR must be compensated to get

stable behavior of the converter as the temperature increases. 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

also be used for this purpose. In single NTC thermal compensation, the L6718 corrects the

IDROOP and IMON current by comparing the voltage on the TM pin with the voltage present

on the TCOMP pin and recovering the DCR temperature deviation. Depending on the 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 the

gain between the sensed temperature and the correction made on the IDROOP and IMON

currents to be modified.

Shorting TCOMP to GND disables single NTC thermal compensation on the multi-phase

rail. In this case IDROOP and IMON can be still adjusted by adding one NTC on the

compensation network for IDROOP and another NTC for the current monitoring network for

IMON. Both NTCs must be positioned close to the inductor related to Phase1 as it is the only

phase working in all PS status.

If STCOMP/DDR is short to GND, the DDR mode is selected and the single NTC thermal

compensation is disabled on the single-phase rail. In this case the two currents can be

adjusted by adding an NTC close to the inductor on the compensation network for IDROOP

and the current monitoring network for IMON.

10.3

TM and TCOMP design

This procedure applies to both the single-phase and multi-phase section when using single

NTC thermal compensation:

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

recommended values/network is given in Figure 13.

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 the 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 the case of

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

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