ISL6306 Datasheet, PDF (26/33 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision rDS ON or DCR Differential Current Sensing

ISL6306
Depending on the location of the NTC and the airflow, the
NTC may be cooler or hotter than the current sense
component. TCOMP pin voltage can be utilized to correct
the temperature difference between NTC and the current
sense component. When a different NTC type or different
voltage divider is used for the TM function, TCOMP voltage
can also be used to compensate for the difference between
the recommended TM voltage curve in Figure 16 and that of
the actual design. According to the VCC voltage, ISL6306
converts the TCOMP pin voltage to a 4-bit TCOMP digital
signal as TCOMP factor N.
TCOMP factor N is an integer between 0 and 15. The
integrated temperature compensation function is disabled for
N = 0. For N = 4, the NTC temperature is equal to the
temperature of the current sense component. For N < 4, the
NTC is hotter than the current sense component. The NTC is
cooler than the current sense component for N > 4. When
N > 4, the larger TCOMP factor N, the larger the difference
between the NTC temperature and the temperature of the
current sense component.
ISL6306 multiplexes the TCOMP factor N with the TM digital
signal to obtain the adjustment gain to compensate the
temperature impact on the sensed channel current. The
compensated channel current signal is used for droop and
overcurrent protection functions.
Design Procedure
1. Properly choose the voltage divider for TM pin to match
the TM voltage V_S temperature curve with the
recommended curve in Figure 15.
2. Run the actual board under the full load and the desired
cooling condition.
3. After the board reaches the thermal steady state, record
the temperature (T_CSC) of the current sense component
(inductor or MOSFET) and the voltage at TM and VCC
pins.
4. Use Equation 20 to calculate the resistance of the TM
NTC, and find out the corresponding NTC temperature
T_NTC from the NTC datasheet.
R_NTC(TNTC)
=
V-----_{T---M-----x----R----T----M-----1-}
V_CC – V_TM
(EQ. 20)
5. Use Equation 21 to calculate the TCOMP factor N:
N
=
2----0---9----x---(_{---T----C----S----C-----}–_{-----T----N----T---C-----})
3xT_NTC + 400
+
4
(EQ. 21)
6. Choose an integral number close to the above result for
the TCOMP factor. If this factor is higher than 15, use
N = 15. If it is less than 1, use N = 1.
7. Choose the pull-up resistor R_TC1 (typical 10kΩ).
8. If N = 15, do not need the pull-down resistor R_TC2,
otherwise obtain R_TC2 by Equation 22:
R_TC2
=
-N----x----R----^T----^C----¹-
15 – N
(EQ. 22)
9. Run the actual board under full load again with the proper
resistors connected to the TCOMP pin.
10. Record the output voltage as V1 immediately after the
output voltage is stable with the full load. Record the
output voltage as V2 after the VR reaches the thermal
steady state.
11. If the output voltage increases over 2mV as the
temperature increases, i.e. V2 - V1 > 2mV, reduce N and
redesign R_TC2; if the output voltage decreases over 2mV
as the temperature increases, i.e. V1 - V2 > 2mV,
increase N and redesign R_TC2.
The design spreadsheet is available for those calculations.
External Temperature Compensation
By setting the voltage of TCOMP pin to 0, the integrated
temperature compensation function is disabled. And one
external temperature compensation network, shown in
Figure 18, can be used to cancel the temperature impact on
the droop (i.e. load line).
COMP
FB
IDROOP
o_c
VDIFF
FIGURE 18. VOLTAGE AT IDROOP PIN WITH A RESISTOR
PLACED FROM IDROOP PIN TO GND WHEN
LOAD CURRENT CHANGES
The sensed current will flow out of IDROOP pin and develop
the droop voltage across the resistor equivalent (R_FB)
between FB and VDIFF pins. If R_FB resistance reduces as
the temperature increases, the temperature impact on the
droop can be compensated. An NTC resistor can be placed
close to the power stage and used to form R_FB. Due to the
non-linear temperature characteristics of the NTC, a resistor
network is needed to make the equivalent resistance
between FB and VDIFF pin is reverse proportional to the
temperature.
The external temperature compensation network can only
compensate the temperature impact on the droop, while it
has no impact to the sensed current inside ISL6306.
Therefore this network cannot compensate for the
temperature impact on the overcurrent protection function.
Current Sense Output
The current from IDROOP pin is the sensed average current
inside ISL6306. In typical application, IDROOP pin is
connected to FB pin for the application where load line is
26
FN9226.1
May 5, 2008