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TDE1897RFP 数据表(PDF) 6 Page - STMicroelectronics |
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TDE1897RFP 数据表(HTML) 6 Page - STMicroelectronics |
6 / 12 page WORST CONDITION POWER DISSIPATION IN THE ON-STATE In IPS applications the maximum average power dissipation occurs when the device stays for a long time in the ON state. In such a situation the internal temperature depends on delivered cur- rent (and related power), thermal characteristics of the package and ambient temperature. At ambient temperature close to upper limit (+85 °C) and in the worst operating conditions, it is possible that the chip temperature could increase so much to make the thermal shutdown proce- dure untimely intervene. Our aim is to find the maximum current the IPS can withstand in the ON state without thermal shutdown intervention, related to ambient tem- perature. To this end, we should consider the fol- lowing points: 1) The ON resistance RDSON of the output NDMOS (the real switch) of the device in- creases with its temperature. Experimental results show that silicon resistiv- ity increases with temperature at a constant rate, rising of 60% from 25 °C to 125°C. The relationship between RDSON and tem- perature is therefore: R DSON = R DSON0 ( 1 + k ) ( T j ± 25 ) where: Tj is the silicon temperature in °C RDSON0 is RDSON at Tj=25 °C k is the constant rate (k = 4.711 ⋅ 10 ±3) (see fig. 4). 2) In the ON state the power dissipated in the device is due to three contributes: a) power lost in the switch: P out = I out 2 ⋅ R DSON (Iout is the output cur- rent); b) power due to quiescent current in the ON state Iq, sunk by the device in addition to Iout:P q = I q ⋅ V s (Vs is the supply voltage); c) an external LED could be used to visualize the switch state (OUTPUT STATUS pin). Such a LED is driven by an internal current source (delivering Ios) and therefore, if Vos is the voltage drop across the LED, the dissi- pated power is: P os = I os ⋅ ( V s ± V os ). Thus the total ON state power consumption is given by: P on = P out + P q + P os (1) In the right side of equation 1, the second and the third element are constant, while the first one increases with temperature because RDSON increases as well. 3) The chip temperature must not exceed ΘLim in order do not lose the control of the device. The heat dissipation path is represented by the thermal resistance of the system device- board-ambient (Rth). In steady state condi- tions, this parameter relates the power dissi- pated Pon to the silicon temperature Tj and the ambient temperature Tamb: T j ± T amb = P on ⋅ R th (2) From this relationship, the maximum power Pon which can be dissipated without exceeding ΘLim at a given ambient temperature Tamb is: P on = Θ Lim ± T amb R th Replacing the expression (1) in this equation and solving for Iout, we can find the maximum current versus ambient temperature relation- ship: I outx = √ ΘLim ± T amb R th ± P q ± P os R DSONx where RDSONxis RDSON at Tj= ΘLim. Of course, Ioutx values are top limited by the maximum operative current Ioutx (500mA nominal). From the expression (2) we can also find the maximum ambient temperature Tamb at which a given power Pon can be dissipated: T amb =ΘLim ± P on ⋅ Rth = =ΘLim ± ( I out 2 ⋅ R DSONx + P q + P os ) ⋅ R th In particular, this relation is useful to find the maximum ambient temperature Tambx at which Ioutx can be delivered: T ambx =ΘLim ±( I outx 2 ⋅ R DSONx + + P q + P os ) ⋅ R th (4) Referring to application circuit in fig. 5, let us con- sider the worst case: - The supply voltage is at maximum value of in- dustrial bus (30V instead of the 24V nominal value). This means also that Ioutx rises of 25% TDE1897R - TDE1898R 6/12 |
类似零件编号 - TDE1897RFP |
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类似说明 - TDE1897RFP |
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