ISL6262A_14 Datasheet, PDF (22/28 Pages) Intersil Corporation – Two-Phase Core Controller(Santa Rosa, IMVP-6+)

ISL6262A
pin and 20mV voltage increase on the 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. When the NTC pin
voltage increases to 1.24V, the comparator output will then
be able to flip back. Such a temperature hysteresis feature of
VR_TT# is illustrated in Figure 39. 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 39. TEMPERATURE HYSTERESIS OF VR_TT#
Usually, the NTC thermistor's resistance can be
approximated by Equation 5.
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. The 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 Equation 6 and 8.
R_NTC(T₁) + R_S = ₆--¹--₀-^.--²-_μ--^V--_A-- = 20kΩ
(EQ. 6)
R_NTC(T₂) + R_S = ¹-₅---^.4--²--_μ-⁴---_A-^V-- = 22.96kΩ
(EQ. 7)
From Equation 6 and Equation 7, Equation 8 can be derived,
R_NTC(T₂) – R_NTC(T₁) = 2.96kΩ
(EQ. 8)
Using Equation 5 into Equation 8, the required nominal NTC
resistor value can be obtained by: Equation 9.
R_NTCTo
=
--------²---^.--⁹---⁶----^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 those cases where the constant b is not accurate
enough to approximate the resistor value, the manufacturer
provides the resistor ratio information at different
temperatures. The nominal NTC resistor value may be
expressed in another way shown in Equation 10.
R_NTCTo = ---------------------------²---^.--⁹---⁶----^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 data sheets of the NTC thermistor give
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: Equation 11.
R_S
=
--¹---^.--²---^V----
60 μ A
–
R_NTC(T1)
=
20kΩ – 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:
Equations 12, and 13.
R_{NTC_T2} = 2.96kΩ + 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)
For example, if using Equations 9, 10 and 11 to design a
thermal throttling circuit with the temperature hysteresis
+100°C to +105°C, since T₁ = +105°C and T₂ = +100°C,
and if we use a Panasonic NTC with b = 4700, Equation 9
gives the required NTC nominal resistance as
R_{NTC_To} = 459kΩ.
In fact, the data sheet gives the resistor ratio value at
+100°C to +105°C, which is 0.03956 and 0.03322
respectively. The b value 4700k in the Panasonic data sheet
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 467kΩ. The closest NTC resistor
value from the manufacturer is 467kΩ. So the series
resistance is given by Equation 14.
R_S = 20kΩ – R_{NTC_105°C} = 20kΩ – 15.65kΩ = 4.35kΩ _{(EQ. 14)}
The closest standard resistor is 4.42kΩ. Furthermore, the NTC
resistance at T₂ is given by Equation 15.
R_{NTC_T2} = 2.96kΩ + R_{NTC_T1} = 18.16kΩ
(EQ. 15)
Therefore, the NTC branch is designed to have a 470k NTC
and 4.42k resistor in series. The part number of the NTC
thermistor is ERTJ0EV474J. It is a 0402 package. The NTC
22
FN6343.1
December 23, 2008