 |
Motore di ricerca datesheet componenti elettronici |
|
|
Motore di ricerca datesheet componenti elettronici |
|
LM196 Scheda tecnica(PDF) 4 Page - National Semiconductor (TI) |
|
|
||||||||||||||||||||||||||||||
LM196 Scheda tecnica(HTML) 4 Page - National Semiconductor (TI) |
|
|
4 / 14 page  Application Hints (Continued) The actual heat sink chosen for the LM196 will be deter- mined by the worst-case continuous full load current input voltage and maximum ambient temperature Overload or short circuit output conditions do not normally have to be considered when selecting a heat sink because the thermal shutdown built into the LM196 will protect it under these conditions An exception to this is in situations where the regulator must recover very quickly from overload The LM196 may take some time to recover to within specified output tolerance following an extended overload if the regu- lator is cooling from thermal shutdown temperature (approx- imately 175 ) to specified operating temperature (125 Cor 150 C) The procedure for heat sink selection is as follows Calculate worst-case continuous average power dissipa- tion in the regulator from P e (VIN b VOUT) c (IOUT) To do this you must know the raw power supply voltagecur- rent characteristics fairly accurately For example consid- er a 10V output with 15V nominal input voltage At full load of 10A the regulator will dissipate P e (15 b 10) c (10) e 50W If input voltage rises by 10% power dissipa- tion will increase to (165 b 10) c (10) e 65W a 30% increase It is strongly suggested that a raw supply be assembled and tested to determine its average DC output voltage under full load with maximum line voltage Donot over-design by using unloaded voltage as a worst-case since the regulator will not be dissipating any power under no load conditions Worst-case regulator dissipation nor- mally occurs under full load conditions except when the effective DC resistance of the raw supply (DV DI) is larg- er than (VIN b VOUT)2IfL where VIN is the lightly-load- ed raw supply voltage and IfL is full load current For (VIN b VOUT) e 5V b 8V and IfL e 5A–10A this gives a resistance of 025X to 08X If raw supply resistance is higher than this the regulator power dissipation may be less at full load current then at some intermediate cur- rent due to the large drop in input voltage Fortunately most well designed raw supplies have low enough output resistance that regulator dissipation does maximize at full load current or very close to it so tedious testing is not usually required to find worst-case power dissipation A very important consideration is the size of the filter capac- itor in the raw supply At these high current levels capacitor size is usually dictated by ripple current ratings rather than just obtaining a certain ripple voltage Capacitor ripple cur- rent (rms) is 2 – 3 times the DC output current of the filter If the capacitor has just 005X DC resistance this can cause 30W internal power dissipation at 10A output current Ca- pacitor life is very sensitive to operating temperature de- creasing by a factor of two for each 15 C rise in internal temperature Since capacitor life is not all that great to start with it is obvious that a small capacitor with a large internal temperature rise is inviting very short mean-time-to-failure A second consideration is the loss of usable input voltage to the regulator If the capacitor is small the large dips in the input voltage may cause the LM196 to drop out of regula- tion 2000 mF per ampere of load current is the minimum recommended value yielding about 2 Vp-p ripple of 120 Hz Larger values will have longer life and the reduced ripple will allow lower DC input voltage to the regulator with subse- quent cost savings in the transformer and heat sink Some- times several capacitors in parallel are better to decrease series resistance and increase heat dissipating area After the raw supply characteristics have been determined and worst-case power dissipation in the LM196 is known the heat sink thermal resistance can be found from the graphs titled Maximum Heat Sink Thermal Resistance These curves indicate the minimim size heat sink required as a function of ambient temperature They are derived from a case-to-control area thermal resistance of 05 CW and a case-to-power transistor thermal resistance of 12 CW 02 CW is assumed for interface resistance A maximum control area temperature of 150 C is used for the LM196 and 125 C for the LM396 Maximum power transistor tem- perature is 200 C for the LM196 and 175 C for the LM396 For conservative designs it is suggested that when using these curves you assume an ambient temperature 25 C– 50 C higher than is actually anticipated to avoid running the regulator right at its design limits of operating temperature A quick look at the curves show that heat sink resistance (iSA) will normally fall into the range of 02 CW–15 CW These are not small heat sinks A model 441 for instance which is sold by several manufacturers has a iSA of 06 CW with natural convection and is about five inches on a side Smaller sinks are more volumetrically efficient and larger sinks less so A rough formula for estimating the vol- ume of heat sink required is V e 50 iSA15 CU IN This holds for natural convection only If the heat sink is inside a small sealed enclosure iSA will increase substantially be- cause the air is not free to form natural convection currents Fan-forced convection can reduce iSA by a factor of two at 200 FPM air velocity and by four at 1000 FPM Ripple Rejection Ripple rejection at the normal ripple frequency of 120 Hz is a function of both electrical and thermal effects in the LM196 If the adjustment pin is not bypassed with a capaci- tor it is also dependent on output voltage A 25 mF capaci- tor from the adjustment pin to ground will make ripple rejec- tion independent of output voltage for frequencies above 100 Hz If lower ripple frequencies are encountered the ca- pacitor should be increased proportionally To keep in mind that the bypass capacitor on the adjust- ment pin will limit the turn-on time of the regulator A 25 mF capacitor combined with the output divider resistance will give an extended output voltage settling time following the application of input power Load Regulation (LM196LM396) Because the LM196 is a three-terminal device it is not pos- sible to provide true remote load sensing Load regulation will be limited by the resistance of the output pin and the wire connecting the regulator to the load For the data sheet specification regulation is measured 14 from the bottom of the package on the output pin Negative side sensing is a true Kelvin connection with the bottom of the output divider returned to the negative side of the load 4 |
Codice articolo simile - LM196 |
|
Descrizione simile - LM196 |
|
|
|
Link URL |
| Privacy Policy |
| ALLDATASHEETIT.COM |
| Lei ha avuto il aiuto da alldatasheet? [ DONATE ] |
Di alldatasheet | Richest di pubblicita | contatti | Privacy Policy | scambio Link | Ricerca produttore All Rights Reserved©Alldatasheet.com |
| Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
|
Family Site : ic2ic.com |
icmetro.com |
Italian ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
