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ML4865ES-2 Datasheet(PDF) 5 Page - Micro Linear Corporation |
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ML4865ES-2 Datasheet(HTML) 5 Page - Micro Linear Corporation |
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5 / 10 page  ML4865 5 FUNCTIONAL DESCRIPTION The ML4865 combines a unique form of current mode control with a synchronous rectifier to create a boost converter that can deliver high currents while maintaining high efficiency. Current mode control allows the use of a very small, high frequency inductor and output capacitor. Synchronous rectification replaces the conventional external Schottky diode with an on-chip PMOS FET to reduce losses, eliminate an external component, and allows for load disconnect. Also included on-chip are an NMOS switch and current sense resistor, further reducing the number of external components, which makes the ML4865 very easy to use. REGULATOR OPERATION The ML4865 is a variable frequency, current mode switching regulator. Its unique control scheme converts efficiently over more than three decades of load current. A detailed block diagram of the boost converter is shown in Figure 2. Error amplifier A3 converts deviations in the desired output voltage to a small current, ISET. The inductor current is measured through a 150m W resistor which is amplified by A1. The boost control block matches the average inductor current to a multiple of the ISET current by switching Q1 on and off. The peak inductor current is limited by the controller to about 1.2A. At light loads, ISET will momentarily reach zero after an inductor discharge cycle causing Q1 to stop switching. Depending on the load, this idle time can extend to tenths of seconds. While the circuit is not switching, only 25µA of supply current is drawn from the output. This allows the part to remain efficient even when the load current drops below 250µA. Amplifier A2 and the PMOS transistor Q2 work together to form a low drop diode. When transistor Q1 turns off, the current flowing in the inductor causes pin 6 to go high. As the voltage on VL2 rises above VOUT, amplifier A2 allows the PMOS transistor Q2 to turn on. In discontinuous operation, (where IL always returns to zero), A2 uses the resistive drop across the PMOS switch Q2 to sense zero inductor current and turns the PMOS switch off. In continuous operation, the PMOS turn off is independent of A2 and is determined by the boost control circuitry. Typical inductor current and voltage waveforms are shown in Figure 3. SHUTDOWN The SHDN pin should be held low for normal operation. Raising the shutdown voltage above the threshold level will disable the synchronous rectifier, Q2 and Q3, and force ISET to zero. This prevents switching from occurring and disconnects the body diode of Q2 from the output. As a result, the output voltage is allowed to drop below the input voltage and current is prevented from flowing from the input to the output. FEEDBACK The SENSE pin should be left open or bypassed to ground for normal operation. The addition of the resistor divider R1 and R2 causes the input of error amplifier A3 to reach the threshold voltage before the internal resistors do. This allows the ML4865 to provide output voltages lower than the preset 12V if desired. |
Similar Part No. - ML4865ES-2 |
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Similar Description - ML4865ES-2 |
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