ISL6261A Datasheet, PDF (19/34 Pages) Intersil Corporation – Single-Phase Core Regulator for IMVP-6 Mobile CPUs

ISL6261A
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.
19
FN6354.3
November 5, 2009