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ML4770CS Datasheet(PDF) 5 Page - Micro Linear Corporation |
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ML4770CS Datasheet(HTML) 5 Page - Micro Linear Corporation |
5 / 8 page ML4770 5 FUNCTIONAL DESCRIPTION The ML4770 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 P-channel MOSFET to reduce losses, eliminate an external component, and provide the means for load disconnect. Also included on-chip are an N-channel MOSFET main switch and current sense resistor. REGULATOR OPERATION The ML4770 is a variable frequency, current mode switching regulator. Its unique control scheme converts efficiently over more than three decades of load current. A block diagram of the boost converter including the key external components is shown in Figure 2. Error amp A3 converts deviations in the desired output voltage to a small current, ISET. The inductor current is measured through a current sense resistor (RSENSE) 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 1.3A. 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 a second. When 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 VL2 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 point 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 ML4770 output can be shut down by pulling the SHDN pin high (to VIN). When SHDN is high, the regulator stops switching, the control circuitry is powered down, and the body diode of the PMOS synchronous rectifier is disconnected from the output. By switching Q1, Q2, and Q3 off, the load is isolated from the input. This allows the output voltage to be independent of the input while in shutdown. DESIGN CONSIDERATIONS OUTPUT CURRENT CAPABILITY The maximum current available at the output of the regulator is related to the maximum inductor current by the ratio of the input to output voltage and the conversion efficiency. The maximum inductor current is limited by the boost controller to about 1.0A. The conversion efficiency is determined mainly by the internal switches as well as the external components, but can be estimated at about 80%. The maximum output current can be determined by using the typical performance curves shown in Figures 4 and 5, or by calculation using the following equation: I V V VA OUT MAX IN MIN OUT OUT () () .. . = + ! " $## - 0 0972 0 486 0 144 16 27 (1) Since the maximum output current is based on when the inductor current goes into current limit, it is not recommended to operate the ML4770 at the maximum output current continuously. Applications that have high transient load currents should be evaluated under worst case conditions to determine suitability. INDUCTOR SELECTION The ML4770 is able to operate over a wide range of inductor values. A value of 10µH is a good choice, but any value between 5µH and 33µH is acceptable. As the inductor value changes, the control circuitry will automatically adjust to keep the inductor current under control. Choosing an inductance value of less than 10µH will reduce the component’s footprint, but the efficiency and maximum output current may drop. It is important to use an inductor that is rated to handle 1.5A peak currents without saturating. Also look for an inductor with low winding resistance. A good rule of thumb is to allow 5 to 10m W of resistance for each 1µH of inductance. The final selection of the inductor will be based on trade- offs between size, cost and efficiency. Inductor tolerance, core and copper loss will vary with the type of inductor selected and should be evaluated with a ML4770 under worst case conditions to determine its suitability. Several manufacturers supply standard inductance values in surface mount packages: Coilcraft (847) 639-6400 Coiltronics (561) 241-7876 Dale (605) 665-9301 Sumida (847) 956-0666 |
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