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LTC3406AB Datasheet(PDF) 11 Page - Linear Technology |
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LTC3406AB Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC3406AB 11 3406abfa The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3406AB in dropout at an input voltage of 2.7V, a load current of 600mA and an ambient temperature of 70°C. From the typical per- formance graph of switch resistance, the RDS(ON) of the P-channel switch at 70°C is approximately 0.27Ω. There- fore, power dissipated by the part is: PD = ILOAD2 • RDS(ON) = 97.2mW For the SOT-23 package, the θJA is 250°C/ W. Thus, the junction temperature of the regulator is: TJ = 70°C + (0.0972)(250) = 94.3°C which is below the maximum junction temperature of 125°C. Notethatathighersupplyvoltages,thejunctiontemperature 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 (ΔILOAD • ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or dis- charge 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 monitored 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 (>1μF) supply bypass capacitors. The discharged bypass capacitors are effectively put in paral- lel 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 10μF capacitor charging to 3.3V would require a 250μs 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 LTC3406AB. These items are also illustrated graphically in Figures 3 and 4. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace, the VOUT trace and the VIN trace should be kept short, direct and wide. 2. Does the VFB pin connect directly to the feedback resistors? The resistive divider R1/R2 must be connected between the (+) plate of COUT and ground. 3. Does CIN connect to VIN as closely aspossible? This capacitor provides the AC current to the internal power MOSFETs. 4. Keep the switching node, SW, away from the sensitive VFB node. 5. Keep the (–) plates of CIN and COUT and the IC ground, as close as possible. APPLICATIONS INFORMATION |
Similar Part No. - LTC3406AB_15 |
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Similar Description - LTC3406AB_15 |
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