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LT1576CS8 Datasheet(PDF) 13 Page - Linear Technology |
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LT1576CS8 Datasheet(HTML) 13 Page - Linear Technology |
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13 / 28 page  13 LT1576/LT1576-5 APPLICATIONS INFORMATION V I ESR ESL dI dt RIPPLE = ()( )+() P-P Σ Example: with VIN =10V, VOUT = 5V, L = 30µH, ESR = 0.1Ω, ESL = 10nH: IA dI dt VA mV RIPPLE P-P P-P = () − () () = == = ()( )+ =+= − − − 510 5 10 30 10 200 10 042 10 30 10 033 10 0 42 0 1 10 10 0 33 10 0 042 0 003 45 63 6 6 96 •• . • .• .. • . • .. Σ Output Capacitor Ripple Current (RMS): I VV V Lf V RIPPLE RMS OUT IN OUT IN ( ) = () − () ()( )( ) 029 . Ceramic Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempt- ing for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop stability problems. Solid tantalum capacitor’s ESR generates a loop “zero” at 5kHz to 50kHz that is instrumen- tal in giving acceptable loop phase margin. Ceramic capacitors remain capacitive to beyond 300kHz and usu- ally resonate with their ESL before ESR becomes effective. They are appropriate for input bypassing because of their high ripple current ratings and tolerance of turn-on surges. OUTPUT RIPPLE VOLTAGE Figure 3 shows a typical output ripple voltage waveform for the LT1576. Ripple voltage is determined by the high frequency impedance of the output capacitor, and ripple current through the inductor. Peak-to-peak ripple current through the inductor into the output capacitor is: I VV V VL f P OUT IN OUT IN -P = () − () ()( )( ) For high frequency switchers, the sum of ripple current slew rates may also be relevant and can be calculated from: Σ dI dt V L IN = Peak-to-peak output ripple voltage is the sum of a triwave created by peak-to-peak ripple current times ESR, and a square wave created by parasitic inductance (ESL) and ripple current slew rate. Capacitive reactance is assumed to be small compared to ESR or ESL. 2 µs/DIV 1576 F03 VOUT AT IOUT = 1A INDUCTOR CURRENT AT IOUT = 1A VOUT AT IOUT = 50mA INDUCTOR CURRENT AT IOUT = 50mA 20mV/DIV 200mA/DIV 20mV/DIV 200mA/DIV Figure 3. LT1576 Ripple Voltage Waveform CATCH DIODE The suggested catch diode (D1) is a 1N5818 Schottky, or its Motorola equivalent, MBR130. It is rated at 1A average forward current and 30V reverse voltage. Typical forward voltage is 0.42V at 1A. The diode conducts current only during switch off time. Peak reverse voltage is equal to regulator input voltage. Average forward current in normal operation can be calculated from: I IV V V D AVG OUT IN OUT IN ( )= − () |
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