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RT8008-15QB Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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RT8008-15QB Datasheet(HTML) 11 Page - Richtek Technology Corporation |
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11 / 14 page  RT8008 11 DS8008-04 March 2007 www.richtek.com curves. Thus, to obtain I 2R losses, simply add RSW to RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% of the total loss. Thermal Considerations The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature difference between junction to ambient. The maximum power dissipation can be calculated by following formula : PD(MAX) = ( TJ(MAX) - TA ) / θJA Where TJ(MAX) is the maximum operation junction temperature 125 °C, TAis the ambient temperature and the θJAis the junction to ambient thermal resistance. For recommended operating conditions specification of RT8008 DC/DC converter, where TJ (MAX) is the maximum junction temperature of the die (125 °C) and TA is the maximum ambient temperature. The junction to ambient thermal resistance θJA is layout dependent. For SOT-23-5/TSOT-23-5 packages, the thermal resistance θJA is 250 °C/W on the standard JEDEC 51-3 single-layer thermal test board. The maximum power dissipation at TA= 25 °C can be calculated by following formula : PD(MAX) = ( 125 °C - 25°C ) / 250 = 0.4 W for SOT-23-5/ TSOT-23-5 packages The maximum power dissipation depends on operating ambient temperature for fixed TJ(MAX) and thermal resistance θJA. For RT8008 packages, the Figure 5 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. The value of junction to case thermal resistance θJC is popular for users. This thermal parameter is convenient for users to estimate the internal junction operated temperature of packages while IC operating. It's independent of PCB layout, the surroundings airflow effects and temperature difference between junction to ambient. The operated junction temperature can be calculated by following formula : TJ = TC + PD x θJC Where TC is the package case (Pin 2 of package leads) temperature measured by thermal sensor, PD is the power dissipation defined by user's function and the θJC is the junction to case thermal resistance provided by IC manufacturer. Therefore it's easy to estimate the junction temperature by any condition. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ∆ILOAD (ESR), where ESR is the effective series resistance of COUT. ∆ILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. Layout Considerations Follow the PCB layout guidelines for optimal performance of RT8008. } For the main current paths as indicated in bold lines in Figure 6, keep their traces short and wide. } Put the input capacitor as close as possible to the device pins (VIN and GND). } LX node is with high frequency voltage swing and should be kept small area. Keep analog components away from LX node to prevent stray capacitive noise pick-up. Figure 5. Derating Curves for RT8008 Package 0 50 100 150 200 250 300 350 400 450 0 20 40 60 80 100 120 140 Ambient Temperature ( °C) Single Layer PCB SOT-23-5, TSOT-23-5 Packages |
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