ISL6326B Datasheet, PDF (23/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision DCR Differential Current Sensing

ISL6326B
Temperature Compensation
ISL6326B supports inductor DCR sensing, or resistive
sensing techniques. The inductor DCR has a positive
temperature coefficient, which is about +0.38%/°C. Since the
voltage across inductor is sensed for the output current
information, the sensed current has the same positive
temperature coefficient as the inductor DCR.
In order to obtain the correct current information, there
should be a way to correct the temperature impact on the
current sense component. ISL6326B provides two methods:
integrated temperature compensation and external
temperature compensation.
Integrated Temperature Compensation
When the TCOMP voltage is equal or greater than VCC/15,
ISL6326B will utilize the voltage at TM and TCOMP pins to
compensate the temperature impact on the sensed current.
The block diagram of this function is shown in Figure 14.
V_CC
R_TM1
TM
o_c
R_NTC
V_CC
R_TC1
TCOMP
R_TC2
Non-linear
A/D
Channel current
sense
I₄
I₃
I₂
I₁
I_sen4
I_sen3
I_sen2
I_sen1
D/A
k_i
4-bit
A/D
Droop &
Over current protection
FIGURE 14. BLOCK DIAGRAM OF INTEGRATED
TEMPERATURE COMPENSATION
When the TM NTC is placed close to the current sense
component (inductor), the temperature of the NTC will track
the temperature of the current sense component. Therefore
the TM voltage can be utilized to obtain the temperature of
the current sense component.
Based on VCC voltage, ISL6326B converts the TM pin
voltage to a 6-bit TM digital signal for temperature
compensation. With the non-linear A/D converter of
ISL6326B, the TM digital signal is linearly proportional to the
NTC temperature. For accurate temperature compensation,
the ratio of the TM voltage to the NTC temperature of the
practical design should be similar to that in Figure 12.
Depending on the location of the NTC and the airflow, the
NTC may be cooler or hotter than the current sense
component. The 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, the
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, ISL6326B 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.
ISL6326B 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 the following equation 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. 21)
5. Use the following equation to calculate the TCOMP
factor N:
N
=
2----0---9----x---(_{---T----C----S----C-----}–_{-----T----N----T---C-----})
3xT_NTC + 400
+
4
(EQ. 22)
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 the following equation:
R_TC2
=
-N----x----R----^T----^C----¹-
15 – N
(EQ. 23)
9. Run the actual board under full load again with the proper
resistors connected to the TCOMP pin.
23
FN9286.0
April 21, 2006