| 1JJ'-^ | 0_01| |
| LOAD REGULATION (continued) | |
| Even when the circuit is optimally configured, parasitic resistance | can be used, as long as it.s added contribution to thermal resistance |
| can be a significant source of error. A 20 mil wide PC trace built from | is considered. Note that the ca.se of all device.s in thi.s series is |
| 1 0z. copper-clad circuit board material ha.s a parasitic resistance of | electrically connected to the output. |
| about 25 milliohms per inch ofits length at room temperature. If |
| a 3-terminal regulator used to .supply 2.50 volt.s is connected by 2 | Example |
| inches of this trace to a load which draws i.s amps of current, a | Given: V,k. = Sv Ih - |
| 75 millivolt drop will appear between the regulator and the load. | VOIIT = 2.5v, IOLIT = 1.5A |
| Even when the regulator output voltage is preci.sely 2.50 volt.s, the | Ambient Temp., Ti\ = 500C |
| load will only see 2.43 volts, which is a 3% error. It is important to | RHn = 2.70C/W for T0-220 |
| keep the connection between the regulator output pin and the load as .short as po.ssible, and to use wide traces or heavy-gauge wire. The minimum .specified output capacitance for the regulator | Find: Proper Heat Sink to keep IC's junction temperature below 1250C. |
| should be located near the reglator package. If -several capacitors | Solution: The junction temperature i.s: |
| are used in parallel to construct the power system output capaci- | ] n 'errRo(.s 'OSA' 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 transient.s 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 does not drop below VIN m during transients. | where: Pi) - Dis.sipated power. - - 1) Ren - Thermal resistance from the junction to the mounting tab of the package. Recis - Thermal re.si.stance through the interface ec:s between the IC and the surface on which it is mounted. (l.OoC/w at 6 in-lbs mounting .screw torque.) ReSA - Thermal re.si.stance from the mounting surface to ambient (thermal resistance of the heat sink). Ts - Heat sink temperature. |
| Vm(MIN) = VOLJT + Vl)ROPOIJT (MAX) IN (MI\I) OL | L /VrLVVrwA |
| where: VIN (MIh) - the lowest allowable instantaneous voltage at the input pin. | ejT Recs ROSA Fir.st, find the maximum allowable thermal re.sistance of the 1.-1heat .sink: |
| VOIJT - the de.signed output voltage for the |
| power supply .system. V ,,, (--Ax) the .specified dropout voltage DROPOIiT (MAX) | T. - T ReSA = - (ROjT+RocS) 'HSA P.. 1) |
| for the in.stalled regulator. | P = (V -. -. - V)IIT) IOI.T = (5.OV-2.5V) * 1.5A D \ IN(MAX) OIIT' = 3.75W |
| THERMAL CONSIDERATIONS The LX8385/85A/85B regulators have intemal power and thermal | R "eSA 1250C - 500C _ (2.70C/W+ 1.OOC/Wl |
| 3.75w |
| limiting circuitry designed to protect each clevice under overload condition.s. For continuou.s normal load conditions, however, | = i6.30c/w |
| maximum junction temperature rating.s must not be exceeded. it is | 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, case to heat sink interface, and heat sink thermal resistance it.self. | R"A " i6.30c/w. Thermalloy heatsink 6230B has Rq< = 12.OoC/W. OSA - v - - eSA - -, Finally, verify that junction temperature remains within speci- fication using the .selected heat .sink: |
| Junction-to-case thermal re.sistance is .specified from the IC |
| junction to the back .surface of the case directly opposite the die. | T, = 3.75W (2.70C/W + 1.OOC/W + 12.OoC/W) + 500C = 1090C |
| This i.s the lowe.st resistance path for heat flow. Proper mounting |
| is required to ensure the best po.ssible thermal flow from this area of the package to the heat sink. Thermal compound at the ca.se-to- heat-sink interface i.s strongly recommended. If the ca.se of the | 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 possilole. |
| device must be electrically isolated, a thermally conductive .spacer |
| |
