ISL6261CRZ-T Intersil, ISL6261CRZ-T Datasheet - Page 18

• Manufacturer Part Number: ISL6261CRZ-T
• Description: IC CTRLR BUCK 1PHASE 40-MLFP
• Manufacturer: Intersil

Specifications of ISL6261CRZ-T

• Applications: Converter, Intel IMVP-6
• Voltage - Input: 5 ~ 21 V
• Number Of Outputs: 1
• Voltage - Output: 0.3 ~ 1.5 V
• Operating Temperature: -10°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 40-VFQFN, 40-VFQFPN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The NTC thermistor’s resistance is approximately given by
the following formula:
T is the temperature of the NTC thermistor and b is a
constant determined by the thermistor material. T
reference temperature at which the approximation is
derived. The most commonly used T
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:
From Equation 7 and Equation 8, the following can be
derived:
Substitution of Equation 6 into Equation 9 yields the required
nominal NTC resistor value:
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:
where
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
R
R
R
R
R
R
NTC
s
NTC
NTC
NTC
NTCTo
NTCTo
=
(T
(T
(T
1
60
(
R
.
T
Λ
1
2
20
2
=
=
)
)
)
NTC
)
μA
+
+
V
=
−
R
e
Λ
R
R
R
2
R
( b
−
NTC
(
⋅
NTCTo
s
s
) T
.
NTC
78
T
R
=
=
2
+
NTC
(T
1
is the normalized NTC resistance to its
kΩ
1
1
(T
273
60
54
2
.
.
23
2
20
1
(T
.
78
μA
)
)
μA
⋅
)
⋅
V
V
e
−
−
1
=
e
b
)
kΩ
( b
e
R
⋅
=
Λ
=
=
2
⋅
(
( b
T
.
T
NTC
⋅
22
78
20
20
o
T
+
18
+
1
1
273
1
.
+
kΩ
kΩ
kΩ
78
1
273
(T
273
)
kΩ
1
)
−
)
−
o
R
To
is 25°C. For most
NTC_T
+
1
273
1
)
o
is the
(EQ. 10)
(EQ. 12)
(EQ. 11)
(EQ. 6)
(EQ. 7)
(EQ. 8)
(EQ. 9)
ISL6261
Once R
at T
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 T
and T
Equation 9 gives the required NTC nominal resistance as
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:
The closest standard value is 4.42kΩ. Furthermore,
Equation 13 gives the NTC resistance at T
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.
T
R
R
R
R
2
s
NTC
NTC_To
NTC
_
2
=
actual
and the actual T
_
2
20
_
T
T
= 100°C, if we use a Panasonic NTC with b = 4700,
NTCTo
2
k
2
=
=
=
Ω
=
431
1
b
. 2
−
. 2
ln(
78
and R
R
78
kΩ
NTC
k
R
Ω
R
k
NTC
_
Ω
NTCTo
s
105
+
is designed, the actual NTC resistance
2
R
+
_
C
temperature can be found in:
T
NTC
2
R
=
)
1
NTC
20
+
_
T
1
k
1
_
Ω
(
=
T
273
1
−
18
15
.
39
+
.
61
T
k
o
Ω
k
)
Ω
2
−
:
=
273
. 4
September 27, 2006
1
39
= 105°C
k
Ω
(EQ. 13)
(EQ. 14)
FN9251.1