ISL6364 Datasheet, PDF (31/44 Pages) Intersil Corporation – Dual 4-Phase + 1-Phase PWM Controller for VR12/IMVP7 Applications

ISL6364
Temperature Compensation
The ISL6364 supports inductor DCR sensing, or resistive sensing
techniques. The inductor DCR has a positive temperature
coefficient, which is about +0.385%/°C. Since the voltage across
the 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. ISL6364 provides two methods: integrated
temperature compensation and external temperature
compensation.
The ISL6364 utilizes the voltage at the TM pin and “TCOMP”
register to compensate the temperature impact on the sensed
current. The block diagram of this function is shown in Figure 22.
V_CC
R _TM
TM
o_c
R
NT C
PLACE NTC
CLOSE TO
CHANNLE 1
ISL6364
NON-LINEAR
A/D
D/A
k_i
4-BIT
A/D
TCO MP
CHANNEL
CURRENT
SENSE
I₄
I₃
I₂
I_sen4
I_sen3
I_sen2
I_sen1
I₁
DROOP AND
OVERCURRENT
PROTECTION
FIGURE 22. BLOCK DIAGRAM OF INTEGRATED TEMPERATURE
COMPENSATION
When the 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. Since NTC could pick up noise from the phase node,
a 0.1µF ceramic decoupling capacitor is recommended on the
TM pin in close proximity to the controller.
Based on the VCC voltage, the ISL6364 converts the TM pin
voltage to a 6-bit TM digital signal for temperature
compensation. With the non-linear A/D converter of ISL6364, 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 20.
VOUT
OUTPUT
INDUCTOR
NTC
PHASE1
POWER
STAGE
FIGURE 23. RECOMMENDED PLACEMENT OF NTC
Since the NTC attaches to the PCB, but not directly to the current
sensing component, it inherits high thermal impedance between
the NTC and the current sensing element. The “TCOMP” register
values can be utilized to correct the temperature difference
between NTC and the current sense component. As shown in
Figure 23, the NTC should be placed in proximity to the PSI
channel and the output rail; DON’T place it close to the MOSFET
side, which generates much more heat.
ISL6364 multiplexes the “TCOMP” value 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.
TABLE 8. “TCOMP” AND “TCOMPS” VALUES
TCOMP/TCOMPS (°C)
TCOMP/TCOMPS (°C)
13
29.7
16
32.4
18.9
35.1
21.6
37.8
24.3
40.5
27
43.2
OFF
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 20 and that of the actual design. If the same type NTC
(β = 3477) but different value is used, the pull-up resistor needs to
be scaled as shown in Equation 22:
R_TM
=
1_----k_----Ω_------⋅_---R_{----N----T---C----_--N----E--W---}
6.8 k Ω
(EQ. 22)
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 20.
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.
31
FN6861.0
December 22, 2010