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ML4827CS-2 Arkusz danych(PDF) 13 Page - Micro Linear Corporation |
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ML4827CS-2 Arkusz danych(HTML) 13 Page - Micro Linear Corporation |
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13 / 16 page ![]() ML4827 13 The ML4827 should be locally bypassed with a 10nF and a 1 µF ceramic capacitor. In most applications, an electrolytic capacitor of between 100 µF and 330µF is also required across the part, both for filtering and as part of the start-up bootstrap circuitry. LEADING/TRAILING MODULATION Conventional Pulse Width Modulation (PWM) techniques employ trailing edge modulation in which the switch will turn on right after the trailing edge of the system clock. The error amplifier output voltage is then compared with the modulating ramp. When the modulating ramp reaches the level of the error amplifier output voltage, the switch will be turned OFF. When the switch is ON, the inductor current will ramp up. The effective duty cycle of the trailing edge modulation is determined during the ON time of the switch. Figure 5 shows a typical trailing edge control scheme. In the case of leading edge modulation, the switch is turned OFF right at the leading edge of the system clock. When the modulating ramp reaches the level of the error amplifier output voltage, the switch will be turned ON. The effective duty-cycle of the leading edge modulation is determined during the OFF time of the switch. Figure 6 shows a leading edge control scheme. One of the advantages of this control technique is that it requires only one system clock. Switch 1 (SW1) turns off and switch 2 (SW2) turns on at the same instant to minimize the momentary “no-load” period, thus lowering ripple voltage generated by the switching action. With Figure 6. Typical Leading Edge Control Scheme. Figure 5. Typical Trailing Edge Control Scheme. RAMP VEAO TIME VSW1 TIME REF EA – + – + OSC DFF R D Q Q CLK U1 RAMP CLK U4 U3 C1 RL I4 SW2 SW1 + DC I1 I2 I3 VIN L1 U2 REF EA – + – + OSC DFF R D Q Q CLK U1 RAMP CLK U4 U3 C1 RL I4 SW2 SW1 + DC I1 I2 I3 VIN L1 VEAO CMP U2 RAMP VEAO TIME VSW1 TIME FUNCTIONAL DESCRIPTION (Continued) |
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