ISL6261 Datasheet, PDF (18/34 Pages) Intersil Corporation – Single-Phase Core Regulator for IMVP-6 Mobile CPUs

ISL6261
The NTC thermistor’s resistance is approximately given by
the following formula:
b⋅( ¹ − ¹ )
RNTC (T ) = RNTCTo ⋅ e ^{T + 273 To + 273}
(EQ. 6)
T is the temperature of the NTC thermistor and b is a
constant determined by the thermistor material. T_o is the
reference temperature at which the approximation is
derived. The most commonly used T_o is 25°C. For most
commercial NTC thermistors, there is b = 2750k, 2600k,
4500k or 4250k.
From the operation principle of VR_TT#, the NTC resistor
satisfies the following equation group:
R_NTC(T₁ ) + R_s
= ^1.20V
60 μA
=
20kΩ
(EQ. 7)
R_NTC(T₂ ) + R_s
=
1.23V
54 μA
=
22.78kΩ
(EQ. 8)
From Equation 7 and Equation 8, the following can be
derived:
R_NTC(T₂ ) − R_NTC(T₁ ) = 2.78kΩ
(EQ. 9)
Substitution of Equation 6 into Equation 9 yields the required
nominal NTC resistor value:
b⋅( ¹ )
R_NTCTo =
2.78kΩ ⋅ e ^{To +273}
b⋅( ¹ )
b⋅( ¹ )
e − e ^{T2 +273}
T₁ + 273
(EQ. 10)
In some cases, the constant b is not accurate enough to
approximate the resistor value; manufacturers provide the
resistor ratio information at different temperatures. The
nominal NTC resistor value may be expressed in another
way as follows:
R_NTCTo = _Λ
2.78kΩ
Λ
R NTC (T₂ ) − R NTC (T₁ )
(EQ. 11)
Λ
where R NTC (T) is the normalized NTC resistance to its
nominal value. The normalized resistor value on most NTC
thermistor datasheets is based on the value at 25°C.
Once the NTC thermistor resistor is determined, the series
resistor can be derived by:
R_s
=
1.20V
60 μA
−
R_NTC(T₁ ) =
20kΩ
− R_{NTC_T1}
(EQ. 12)
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 _T 2} = 2.78kΩ + R_{NTC _T 1}
(EQ. 13)
T_{2 _ actual}
=
1 _ln( ^RNTC _ T₂
b
R_NTCTo
1
− 273
) + 1 (273 + T_o )
(EQ. 14)
One example of using Equations 10, 11 and 12 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,
Equation 9 gives the required NTC nominal resistance as
R_{NTC_To} = 431kΩ
The NTC thermistor datasheet gives the resistance ratio as
0.03956 at 100°C and 0.03322 at 105°C. The b value of
4700k in Panasonic datasheet only covers up to 85°C;
therefore, using Equation 11 is more accurate for 100°C
design and the required NTC nominal resistance at 25°C is
438kΩ. The closest NTC resistor value from manufacturers
is 470kΩ. So Equation 12 gives the series resistance as
follows:
R_s = 20kΩ − R_{NTC _105C} = 20kΩ − 15.61kΩ = 4.39kΩ
The closest standard value is 4.42kΩ. Furthermore,
Equation 13 gives the NTC resistance at T₂:
R_{NTC _T 2} = 2.78kΩ + R_{NTC _T 1} = 18.39kΩ
The NTC branch is designed to have a 470k NTC and a
4.42k resistor in series. The part number of the NTC
thermistor is ERTJ0EV474J. It is a 0402 package. The NTC
thermistor should be placed in the spot that gives the best
indication of the temperature of the voltage regulator. The
actual temperature hysteretic window is approximately
105°C to 100°C.
18
FN9251.1
September 27, 2006