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SC1531CSTR Datasheet(PDF) 9 Page - Semtech Corporation |
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SC1531CSTR Datasheet(HTML) 9 Page - Semtech Corporation |
9 / 11 page 9 ã 2000 Semtech Corp. www.semtech.com SC1531(A) POWER MANAGEMENT (Remember that at 125°C, R DS(ON) is generally 1.5x the value at 25°C.) Thermal Considerations When operating from the 5V supply, the power dissipation in the SC1531(A) is approximately equal to the product of the output current and the input to output voltage differential: () OUT OUT IN D I V V P • − ≈ The absolute worst-case dissipation is given by: ( ) ) MAX )( AUX ( Q ) MAX ( ) MAX ( O ) MIN ( ) MAX ( ) MAX ( D I VAUX I VO VIN P • + • − = Note that the VAUX (MAX) x IQ(AUX) term does not apply if VAUX is not available or not connected. Inserting VIN = 5.5V, VO = 3.234V, I O = 200mA, VAUX = 3.6V and I Q(AUX) = 2mA yields: W 46 . 0 P ) MAX ( D = Using this figure, we can calculate the maximum thermal impedance allowable to maintain T J £ 125°C: ( ) () W / C 73 R therefore , W / C 47 R W / C 120 46 . 0 70 125 P T T R ) MAX )( A C ( TH ) MAX )( C J ( TH ) MAX ( D ) MAX ( A ) MAX ( J ) MAX )( A J ( TH ° = ° = ° = − = − = − − − This is readily achievable using pcb copper area to aid in conducting the heat away from the device (see Figure 1 on page 10). Heatsinking the bypass FET is not necessary - its power dissipation is given by: () ) MAX )( ON ( DS 2 ) MAX ( O ) MAX ( D R I P • = For I O = 200mA, and RDS(ON) = 0.6W, PD = 24mW. Layout Considerations While layout for linear devices is generally not as critical as for a switching application, careful attention to detail will ensure reliable operation. See Figure 1 on page 10 for a sample layout. 1) Attaching the part to a larger copper footprint will enable better heat transfer from the device, especially on PCBs where there are internal ground and power planes. 2) Place the bulk and decoupling capacitors close to the device for optimal transient response. 3) If the SENSE lead is being used, route it to the load using a seperate trace from the main VO path. If it is not being used, connect to pin 7 as shown. 4) The external bypass FET is shown close to the device for convenience only. Since it is not being switched, longer gate drive traces can be used without problem. Applications Information (Cont.) |
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