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ML4861ES-3 Datasheet(PDF) 7 Page - Micro Linear Corporation |
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ML4861ES-3 Datasheet(HTML) 7 Page - Micro Linear Corporation |
7 / 11 page 7 ML4861 70% 13 2 75% 80% 85% 90% 65% 0 L = 10µH L = 15µH L = 27µH L = 56µH VIN ML4861-3.3 95% 75% 1.0 3.0 4.0 2.0 80% 85% 90% 95% 70% 0 L = 10µH L = 15µH L = 27µH L = 57µH VIN (V) ML4861-5.0 75% 1.0 3.0 4.0 2.0 80% 85% 70% 0 L = 10µH L = 15µH L = 27µH L = 57µH VIN (V) ML4861-6.0 5.0 6.0 90% 95% OUTPUT CAPACITOR The choice of output capacitor is also important, as it controls the output ripple and optimizes the efficiency of the circuit. Output ripple is influenced by three capacitor parameters: capacitance, ESR, and ESL. The contribution due to capacitance can be determined by looking at the change in capacitor voltage required to store the energy delivered by the inductor in a single charge-discharge cycle, as determined by the following formula: ∆V TV LC V V OUT ON IN OUT IN = × ×× × − 22 2( ) (4) For a 2.4V input, and 5V output, a 27µH inductor, and a 47µF capacitor, the expected output ripple due to capacitor value is 87mV. Capacitor Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL), also contribute to the output ripple due to the inductor discharge current waveform. Just after the NMOS transistor turns off, the output current ramps quickly to match the peak inductor current. This fast change in current through the output capacitor’s ESL causes a high frequency (5ns) spike that can be over 1V in magnitude. After the ESL spike settles, the output voltage still has a ripple component equal to the inductor discharge current times the ESR. This component will have a sawtooth shape and a peak value equal to the peak inductor current times the ESR. ESR also has a negative effect on efficiency by contributing I-squared R losses during the discharge cycle. An output capacitor with a capacitance of 100µF, an ESR of less than 0.1ý, and an ESL of less than 5nH is a good general purpose choice. Tantalum capacitors which meet these requirements can be obtained from the following suppliers: AVX (207) 282-5111 Sprague (207) 324-4140 If ESL spikes are causing output noise problems, an EMI filter can be added in series with the output. Figure 5. Typical Efficiency as a Function of VIN. |
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