ISL78211 Datasheet, PDF (19/35 Pages) Intersil Corporation – Automotive Single-Phase Core Regulator for IMVP-6™ CPUs

ISL78211
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
) +1
(273 + T_o )
− 273
(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 Equation 15.
R_{NTC_To} = 431kΩ
(EQ. 15)
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 Equation 16:
R_s
=
20kΩ −
R_{NTC _105C}
=
20kΩ −15.61kΩ
=
4.39kΩ
(EQ.
16)
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
(EQ. 17)
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
FN7578.0
March 8, 2010