参数资料
| 型号： | ISL78211ARZ |
| 厂商： | Intersil |
| 文件页数： | 19/35页 |
| 文件大小： | 0K |
| 描述： | IC REG 1PHASE BUCK 40QFN |
| 标准包装： | 50 |
| 应用： | 转换器，Intel IMVP-6 |
| 输入电压： | 5 V ~ 21 V |
| 输出数： | 1 |
| 输出电压： | 0.3 V ~ 1.5 V |
| 工作温度： | -40°C ~ 105°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 40-VFQFN 裸露焊盘 |
| 供应商设备封装： | 40-QFN（6x6） |
| 包装： | 管件 |
第1页第2页第3页第4页第5页第6页第7页第8页第9页第10页第11页第12页第13页第14页第15页第16页第17页第18页当前第19页第20页第21页第22页第23页第24页第25页第26页第27页第28页第29页第30页第31页第32页第33页第34页第35页
ISL78211
The NTC thermistor’s resistance is approximately given by the
following formula:
Once the NTC thermistor resistor is determined, the series
resistor can be derived by:
?
R
NTC
( T ) = R
NTCTo
b ? (
? e
1 1
T + 273 To + 273
)
(EQ. 6)
R s =
1 . 20 V
60 μ A
? R NTC (T 1 ) = 20 k ? ? R NTC_T 1
(EQ. 12)
R NTC (T 1 ) + R s =
= 20 k ?
1 . 20 V
R NTC _ T 2 = 2 . 78 k Ω + R NTC _ T 1
T 2 _ actual =
? 273
R NTC _ T 2
1
ln( ) + 1 ( 273 + T o )
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:
(EQ. 7)
60 μ A
Once R NTCTo and R s is designed, the actual NTC resistance at T 2
and the actual T 2 temperature can be found in:
(EQ. 13)
1
(EQ. 14)
b R NTCTo
One example of using Equations 10, 11 and 12 to design a
thermal throttling circuit with the temperature hysteresis +100°C
R NTC (T 2 ) + R s =
1 . 23 V
54 μ A
= 22 . 78 k ?
(EQ. 8)
to +105 ° C is illustrated as follows. Since T 1 = +105 ° C and
T 2 = +100 ° C, if we use a Panasonic NTC with b = 4700, Equation 9
gives the required NTC nominal resistance as Equation 15.
R NTC (T 2 ) ? R NTC (T 1 ) = 2 . 78 k ?
b ? (
)
2 . 78 k ? ? e
R NTCTo =
b ? (
b ? (
)
)
? e
e
R NTC_To = 431 k ?
R s = 20 k Ω ? R NTC _ 105 C = 20 k Ω ? 15 . 61 k Ω = 4 . 39 k Ω
R NTCTo =
(EQ. 11)
R NTC _ T 2 = 2 . 78 k Ω + R NTC _ T 1 = 18 . 39 k Ω
From Equation 7 and Equation 8, the following can be derived:
(EQ. 9)
Substitution of Equation 6 into Equation 9 yields the required
nominal NTC resistor value:
1
T o + 273
(EQ. 10)
1 1
T 2 + 273 T 1 + 273
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:
2 . 78 k ?
Λ Λ
R NTC (T 2 ) ? R NTC (T 1 )
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.
19
(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 438k Ω . The
closest NTC resistor value from manufacturers is 470k Ω . So
Equation 12 gives the series resistance as Equation 16:
(EQ. 16)
The closest standard value is 4.42k Ω . Furthermore, Equation 13
gives the NTC resistance at T 2 :
(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.
FN7578.1
December 4, 2013
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