ISL6326 Datasheet, PDF (23/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision rDS ON or DCR Differential Current Sensing

ISL6326
TCOMP voltage can also be used to compensate for the
difference between the recommended TM voltage curve in
Figure 13 and that of the actual design. According to the
VCC voltage, ISL6326 converts the TCOMP pin voltage to a
4-bit TCOMP digital signal as TCOMP factor N.
The 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.
ISL6326 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 the TM pin to
match the TM voltage vs temperature curve with the
recommended curve in Figure 12.
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 using 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.
External Temperature Compensation
By pulling the TCOMP pin to GND, the integrated
temperature compensation function is disabled. And one
external temperature compensation network (shown in
Figure 15) can be used to cancel the temperature impact on
the droop (i.e. load line).
COMP
ISL6326
INTERNAL
CIRCUIT
IDROOP
°C
FB
VDIFF
FIGURE 15. EXTERNAL TEMPERATURE COMPENSATION
The sensed current will flow out of the IDROOP pin and
develop a droop voltage across the resistor equivalent (R_FB)
between the 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 the FB and VDIFF pins 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 ISL6326.
Therefore, this network cannot compensate for the
temperature impact on the overcurrent protection function.
Current Sense Output
The current from the IDROOP pin is the sensed average
current inside the ISL6326. In typical application, the
IDROOP pin is connected to the FB pin for the application
where load line is required.
When load line function is not needed, the IDROOP pin can
be used to obtain the load current information: with one
resistor from the IDROOP pin to GND, the voltage at the
IDROOP pin will be proportional to the load current in
Equation 23:
V_IDROOP
=
--R----^I--^D----^R-----^O----^O-----^P-
N
------R----^X-------
R_ISEN
I_L
O
A
D
(EQ. 23)
23
FN9262.1
May 5, 2008