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CB2012T2R2M Datasheet(PDF) 11 Page - Linear Technology |
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CB2012T2R2M Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LTC3410 3410fb APPLICATIO S I FOR ATIO the junction temperature reaches approximately 150 °C, both power switches will be turned off and the SW node will become high impedance. To avoid the LTC3410 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The tempera- ture rise is given by: TR = (PD)(θJA) where PD is the power dissipated by the regulator and θJAis the thermal resistance from the junction of the die to the ambient temperature. The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3410 in dropout at an input voltage of 2.7V, a load current of 300mA and an ambient temperature of 70 °C. From the typical perfor- mance graph of switch resistance, the RDS(ON) of the P-channel switch at 70 °C is approximately 1.0Ω. Therefore, power dissipated by the part is: PD = ILOAD2 • RDS(ON) = 90mW For the SC70 package, the θJA is 250°C/W. Thus, the junction temperature of the regulator is: TJ = 70°C + (0.09)(250) = 92.5°C which is well below the maximum junction temperature of 125 °C. Note that at higher supply voltages, the junction tempera- ture is lower due to reduced switch resistance (RDS(ON)). 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, which generates a feedback error signal. The regulator loop then acts to return VOUT to its steady- state value. During this recovery time VOUT can be moni- tored for overshoot or ringing that would indicate a stability problem. For a detailed explanation of switching control loop theory, see Application Note 76. A second, more severe transient is caused by switching in loads with large (>1 µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 • CLOAD). Thus, a 10 µF capacitor charging to 3.3V would require a 250 µs rise time, limiting the charging current to about 130mA. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3410. These items are also illustrated graphically in Figures 4 and 5. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace and the VIN trace should be kept short, direct and wide. |
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