| 1JJ'^ | 0_01| |
| LOAD REGULATION (continued) | |
| Even when the circuit is configured optimally, parasitic resistance | can be u.sed, a.s long a.s it.s added contribution to thermal re.sistance |
| can be a significant source of error. A 100 mil (2.54 rrun) wide PC | is considered. Note that the ca.se of all device.s in thi.s series is |
| trace built from l oz. copper-clad circuit board material ha.s a | electrically connected to the output. |
| parasitic resi.stance of about 5 milliohm.s per inch of its length at |
| room temperature. If a 3-terminalregulator used to .supply 2.50 volts | Example |
| is connected by 2 inches of thi.s trace to a load which draw.s 5 amps | Given: V,k. = Sv - - ' Ih - |
| of current, a 50 millivolt drop will appear between the regulator and | Voin. = 2.8V, IOUT = 5.OA |
| the load. Even when the regulator output voltage is preci.sely | Ambient Temp., Ti\ = 500C |
| 2.50 volt.s, the load will only see 2.45 volts, which i.s a 2% error. It | Ren = 2.70C/W for T0-220 |
| is important to keep the connection between the regulator output | 300 ft/min airflow available |
| pin and the load a.s short as po.s.sible, and to u.se wide traces or i- - -- - - - - - - - - | Find: Proper Heat Sink to keep IC'.s junction |
| neavy-gauge wire. The minimum .specified output capacitance for the regulator | temperature below 1250C. |
| .should be located near the reglator package. If several capacitors | Solution: The junction temperature i.s: |
| are u.sed in parallel to con.struct the power .system output capaci- | T = P (ROr + Ro, + Ro..) + T J 1J \ 'qlT 'e)c:s 'esA' A |
| tance, any capacitors beyond the minimum needed to meet the .specified requirements of the regulator should be located near the section.s of the load that require rapidly-changing amount.s of current. Placing capacitors near the sources ofload tran.sients will help en.sure that power system transient response i.s not impaired by the effects of trace impedance. To maintain good load regulation, wide traces should be used on the input side of the regulator, e.specially between the input capacitor.s and the regulator. Input capacitor ESR must be small enough that the voltage at the input pin doe.s not drop below VIN (WIIN, during transient.s. | where: Pi) - Di.s.sipated power. 1) Ren. - Thermal re.sistance from the junction to the mounting tab of the package. Ro, - Thermal re.sistance through the interface HCS - between the IC and the .surface on which it is mounted. (l.OoC/W at 6 in-lbs mounting screw torque.) ReSA - Thermal resisrance from the mounting surface to ambient Cthermal re.sistance of the heat sink). T< - Heat .sink temperature. |
| Vm fMIN) = VOIJT + VDR01'OLJT (hIAX) IN (MI\I) OI | LwVr~/V TA |
| where: V . ', - the lowe.st allowable instantaneous - - IN (MIh) voltage at the input pin. | R jT R cs R SA Fir.st, find the maximum allowable thermal resistance of the 1.-1heat .sink: |
| Vour - the de.signed output voltage for the |
| power supply system. VuT (' ,X) - the .specified dropout voltage DROPOUT (MAX) | R=(R+R) |
| for the in.stalled regulator. | Pll = (VIN(MAX, - VOUT) IOI:T = (5.OV-2.8V) * 5.OA D \ IN(M, = 11.OW |
| THERMAL CONSIDERATIONS | R = 1250C - 500C _ r, 70C/W+ 1 00(-jWl |
| 'rhe LX8585/85A regulator.s have internal power and thermal | I\OSA - (5.OV-2.8V) * 5.OA - LL.I W T.L_] o w ] |
| limiting circuitry de.signed to protect each device under overload condition.s. For continuou.s normal load conditions, however, | = 3.ioC/w |
| maximum junction temperature rating.s mu.st not be exceeded. It i.s | Next, .select a suitable heat .sink. The selected heat sink mu.st have |
| important to give careful consideration to all sources of thermal resistance from junction to ambient. This includes junction to case, | R"^ " 3.10C/W. Thermalloy heatsink 6296B ha.s ROSA = 3.OoC/W with t)SA d -' 300ft/min air flow. |
| case to heat sink interface, and heat sink thermal resistance itself. | Finally, verify that junction temperature remains within speci- |
| Junction-to-case thermal re.sistance is .specified from the IC | fication u.sing the .selected heat .sink: |
| junction to the back .surface of the case directly opposite the die. |
| Thi.s is the lowest re.sistance path for heat flow. Proper mounting | T, = 11W (2.70C/W + 1.OOC/W + 3.OoC/WJ + 500C = 1240C |
| is required to en.sure the best possible thermal flow from this area |
| of the package to the heat sink. Thermal compound at the case-to- heat-.sink interface i.s strongly recommended. If the case of the device must be electrically isolated, a thermally conductive .spacer | Although the device can operate up t0 1500C junction, it is recom- mended for long term relialoility to keep the junction temperature below 1250C whenever po.ssible. |
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