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MAX1992 Datasheet(PDF) 32 Page - Maxim Integrated Products |
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MAX1992 Datasheet(HTML) 32 Page - Maxim Integrated Products |
32 / 36 page Calculating again with h = 1 and the typical K-factor value (K = 3.3µs) gives the absolute limit of dropout: Therefore, VIN must be greater than 3.06V, even with very large output capacitance, and a practical input voltage with reasonable output capacitance would be 3.47V. Multiple Output Voltage Settings (MAX1993 Only) While the MAX1993 is optimized to work with applica- tions that require two dynamic output voltages, it can produce three or more output voltages if required by using discrete logic or a DAC. Figure 11 shows an application circuit providing four voltage levels using discrete logic. Switching resistors in and out of the resistor network changes the voltage at REFIN. An edge detection circuit is added to gener- ate a 1µs pulse on GATE to trigger the fault-blanking and forced-PWM operation. When using PWM mode (SKIP = VCC), the edge detection circuit is only required if fault blanking is enabled. Otherwise, leave OD unconnected. Active Bus Termination (MAX1993 Only) Active bus termination power supplies generate a volt- age rail that tracks a set reference. They are required to source and sink current. DDR memory architecture requires active bus termination. In DDR memory archi- tecture, the termination voltage is set at exactly half the memory supply voltage. Configure the MAX1993 to generate the termination voltage using a resistor- divider at REFIN. In such an application, the MAX1993 must be kept in PWM mode (SKIP = VCC) in order for it to source and sink current. Figure 12 shows the MAX1993 configured as a DDR termination regulator. Connect GATE and FBLANK to GND when unused. Voltage Positioning In applications where fast-load transients occur, the output voltage changes instantly by ESRCOUT x ∆ILOAD. Voltage positioning allows the use of fewer out- put capacitors for such applications, and maximizes the output voltage AC and DC tolerance window in tight tolerance applications. Figure 13 shows the connection of OUT and FB in a voltage-positioned circuit. In nonvoltage-positioned cir- cuits, the MAX1992/MAX1993 regulate at the output capacitor. In voltage-positioned circuits, the MAX1992/ MAX1993 regulate on the inductor side of the current- sense resistor. VOUT is reduced to: VOUT(VPS) = VOUT(NO LOAD) - RSENSEILOAD V VV ns s VV V IN MIN () .. . . .. . = + − × += − 25 01 1 1 5 500 33 01 01 306 µ Quick-PWM Step-Down Controllers with Inductor Saturation Protection and Dynamic Output Voltages 32 ______________________________________________________________________________________ MAX1993 REF REFIN GND GATE R4 R3 A C1 R2 R1 B 1000pF 1k Ω 1000pF 1k Ω Figure 11. Multiple Output Voltage Settings |
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