ISL6262 Datasheet, PDF (21/27 Pages) Intersil Corporation – Two-Phase Core Regulator for IMVP-6 Mobile CPUs

ISL6262
When temperature increases, the NTC resistor value on
NTC pin decreases. Thus, the voltage on NTC pin
decreases to a level lower than 1.18V. The comparator
output changes polarity and turns SW1 off and connects
SW2 to 1.20V. This pulls VR_TT# low and sends the signal
to start thermal throttle. There is a 6µA current reduction on
NTC pin and 20mV voltage increase on threshold voltage of
the comparator in this state. The VR_TT# signal will be used
to change the CPU operation and decrease the power
consumption. When the temperature goes down, the NTC
thermistor voltage will eventually go up. The NTC pin voltage
increases to 1.20V, the comparator output will then be able
to flip back. Such a temperature hysteresis feature of
VR_TT# is illustrated in Figure 33. T₁ represents the higher
temperature point at which the VR_TT# goes from low to
high due to the system temperature rise. T₂ represents the
lower temperature point at which the VR_TT# goes high
from low because the system temperature decreases to the
normal level.
VR_TT#
Logic_1
Logic_0
T₂
T₁
T (°C)
FIGURE 33. TEMPERATURE HYSTERESIS OF VR_TT#
Usually, the NTC thermistor's resistance can be
approximated by the following formula:
R_NTC(T)
=
R_N
T
C
T
o
•
e
b
•
⎛
⎝
---------¹----------
T + 273
–
T^-----o^-----+-1---2^----7^---3^{--⎠⎞}
(EQ. 5)
T is the temperature of the NTC thermistor and b is a
parameter constant depending on the thermistor material.
T_o is the reference temperature in which the approximation
is derived. Most common temperature for T_o is 25°C. For
example, there are commercial NTC thermistor products
with b = 2750k, b = 2600k, b = 4500k or b = 4250k.
From the operation principle of the VR_TT# circuit
explained, the NTC resistor satisfies the following equation
group.
R_NTC(T₁) + R_S = ¹-₆---^.0--¹--_μ-⁸---_A-^V-- = 19.67kΩ
(EQ. 6)
R_NTC(T₂) + R_S
=
--¹---^.--²---^V----
54 μ A
=
22.22 k Ω
(EQ. 7)
From Equation 6 and Equation 7, the following can be
derived,
R_NTC(T₂) – R_NTC(T₁) = 2.55kΩ
(EQ. 8)
Using Equation 5 into Equation 8, the required nominal NTC
resistor value can be obtained by:
R_NTCTo
=
--------²---^.--⁵---⁵----^k----^Ω------^•----^e----^b----^•----^⎝⎛--^T--^--^--^-o--^--^--^--^--^+--^-1-^---^--^2--^--^--^7--^--^---^3--^---^⎠⎞--------
b
e
•
⎛
⎝
T^----₂^-----+--1---2^---7^----3^{--⎠⎞}
b
–e
•
⎛
⎝
T^----₁^-----+--1---2^---7^----3^{--⎠⎞}
(EQ. 9)
For some cases, the constant b is not accurate enough to
approximate the NTC resistor value, the manufacturer
provides the resistor ratio information at different
temperature. The nominal NTC resistor value may be
expressed in another way as follows:
R_NTCTo = --------------------------²---^.--⁵---⁵----^k----^Ω----------------------------
Λ
–^Λ
R_{NTC – T2} R_{NTC – T1}
(EQ. 10)
Λ
where R_{NTC – T} is the normalized NTC resistance to its
nominal value. Most datasheet of the NTC thermistor gives
the normalized resistor value based on its value at 25°C.
Once the NTC thermistor resistor is determined, the series
resistor can be derived by:
R_S = ¹-₆---^.0--¹--_μ-⁸---_A-^V-- – R_NTC(T1) = 19.67kΩ – R_{NTC_T1}
(EQ. 11)
Once R_NTCTo and R_s is designed, the actual NTC resistance
at T₂ and the actual T₂ temperature can be found in:
R_{NTC_T2} = 2.55kΩ + R_{NTC_T1}
(EQ. 12)
T_{2 _actual}
=
-----------------------------------------¹------------------------------------------
1_--
b
ln
⎛
⎜
⎝
-^R-_R---^N-_N---^T--_T--^C-_C---^_-_T--^T--_o-²--_{⎠⎟⎞}
+
1
⁄
( 273
+
To)
–
273
(EQ. 13)
One example of using Equations 9, 10 and 11 to design a
thermal throttling circuit with the temperature hysteresis
100°C to 105°C is illustrated as follows. Since T₁ = 105°C
and T₂ = 100°C, if we use a Panasonic NTC with B = 4700,
the Equation 9 gives the required NTC nominal resistance as
R_{NTC_To} = 396kΩ
In fact, the datasheet gives the resistor ratio value at 100°C
to 105°C, which is 0.03956 and 0.03322 respectively. The b
value 4700K in Panasonic datasheet only covers to 85°C.
Therefore, using Equation 10 is more accurate for 100°C
design, the required NTC nominal resistance at 25°C is
402kΩ. The closest NTC resistor value from manufacturer is
470kΩ. So the series resistance is given by Equation 11 as
follows,
R_S = 19.67kΩ – R_{NTC_105°C} = 19.67kΩ – 15.65kΩ = 4.067kΩ
Furthermore, the NTC resistance at T₂ is given by Equation 12.
R_{NTC_T2} = 2.55kΩ + R_{NTC_T1} = 18.16kΩ
From the NTC datasheet, it can be concluded that the actual
temperature T₂ is about 97°C. If using the Equation 13, T₂ is
calculated to be 97.7°C. Check the NTC datasheet to decide
21
FN9199.2
May 15, 2006