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SC4524CSETRT Datasheet(PDF) 11 Page - Samtec, Inc |
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SC4524CSETRT Datasheet(HTML) 11 Page - Samtec, Inc |
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11 / 22 page  SC4524C Applications Information (Cont.) To allow for transient headroom, the minimum operating switch on time should be at least 20% to 30% higher than the worst-case minimum on time. 100 120 140 160 180 200 -50 -25 0 25 50 75 100 125 Temperature (OC) VO =1.5V,IO =1A,1MHz Figure 4 — Variation of Minimum On Time with Ambient Temperature Minimum Off Time Limitation The PWM latch in Figure 2 is reset every cycle by the clock. The clock also turns off the power transistor to refresh the bootstrap capacitor. This minimum off time limits the attainable duty cycle of the regulator at a given switching frequency. The measured minimum off time is 00ns typically. If the required duty cycle is higher than the attainable maximum, then the output voltage will not be able to reach its set value in continuous-conduction mode. Inductor Selection The inductor ripple current for a non-synchronous step- down converter in continuous-conduction mode is SW D O L L F ) D ( ) V V ( I x − x + = D where F SW is the switching frequency and L is the inductance. An inductor ripple current between 20% to 50% of the maximum load current gives a good compromise among efficiency, cost and size. Re-arranging the previous equation and assuming 35% inductor ripple current, the inductor is given by SW O D O F I % 35 ) D ( ) V V ( L x x − x + = If the input voltage varies over a wide range, then choose L based on the nominal input voltage. Always verify converter operation at the input voltage extremes. The peak current limit of SC4524C power transistor is at least 2.6A. The maximum deliverable load current for the SC4524C is 2.6A minus one half of the inductor ripple current. Input Decoupling Capacitor The input capacitor should be chosen to handle the RMS ripple current of a buck converter. This value is given by ) D ( D I I O CIN _ RMS − x x = The input capacitance must also be high enough to keep input ripple voltage within specification. This is important in reducing the conductive EMI from the regulator. The input capacitance can be estimated from SW IN O IN F V 4 I C x D x > where DV IN is the allowable input ripple voltage. Multi-layer ceramic capacitors, which have very low ESR (a few mW) and can easily handle high RMS ripple current, are the ideal choice for input filtering. A single 4.7µF X5R ceramic capacitor is adequate for 500kHz or higher switching frequency applications, and 0µF is adequate for 200kHz to 500kHz switching frequency. For high voltage applications, a small ceramic (µF or 2.2µF) can be placed in parallel with a low ESR electrolytic capacitor to satisfy both the ESR and bulk capacitance requirements. |
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