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MAX1744AUB+ Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1744AUB+ Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 15 page High-Voltage, Step-Down DC-DC Controllers in µMAX 8 _______________________________________________________________________________________ 100% Duty Cycle and Dropout The MAX1744/MAX1745 operate with a duty cycle up to 100%. This feature extends the input voltage range by turning the MOSFET on continuously when the supply voltage approaches the output voltage. This services the load when conventional switching regulators with less than 100% duty cycle would fail. Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. Dropout depends on the MOSFET drain-to-source on-resistance, current-sense resistor, and inductor series resistance, and is propor- tional to the load current: Regulation Control Scheme The MAX1744/MAX1755 have a unique operating scheme that allows PWM operation at medium and high current, with automatic switching to pulse-skipping mode at lower currents to improve light-load efficiency. Figure 2 shows the simplified block diagram. Under medium- and heavy-load operation, the inductor current is continuous and the part operates in PWM mode. In this mode, the switching frequency is set by either the 1µs minimum on-time or the 2µs minimum off- time, depending on the duty cycle. The duty cycle is approximately the output voltage divided by the input voltage. If the duty cycle is less than 33%, the minimum on-time controls the frequency; and the frequency is approximately f ≈ 1MHz D, where D is the duty cycle. If the duty cycle is greater than 33%, the off-time sets the frequency; and the frequency is approximately f ≈ 500kHz (1 - D). In both cases, the voltage is regulated by the error comparator. For low duty cycles (<33%), the MOSFET is turned on for the minimum on-time, causing fixed-on- time operation. During the MOSFET on-time, the output voltage rises. Once the MOSFET is turned off, the volt- age drops to the regulation threshold (set by the inter- nal voltage-divider for the MAX1745 and by the external voltage-divider for the MAX1744), at which time another cycle is initiated. For high duty cycles (>33%), the MOSFET remains off for the minimum off-time, causing fixed-off-time operation. In this case, the MOSFET remains on until the output voltage rises to the regula- tion threshold. Then the MOSFET turns off for the mini- mum off-time, initiating another cycle. By switching between fixed-on-time and fixed-off-time operation, the MAX1744/MAX1745 can operate at high input-output ratios, yet still operate up to 100% duty cycle for low dropout. Note that when transitioning from fixed-on-time to fixed-off-time operation, the output volt- age drops slightly due to the output ripple voltage. In fixed-on-time operation, the minimum output voltage is regulated, but in fixed-off-time operation, the maximum output voltage is regulated. Thus, as the input voltage drops below approximately three times the output volt- age, a decrease in line regulation can be expected. The drop in voltage is approximately VDROP ≈ VRIPPLE / 2. At light output loads, the inductor current is discontinu- ous, causing the MAX1744/MAX1745 to operate at Figure 1. Typical Application Circuit GND REF VL D2 IN MAX1744 VH 3/5 5V 3.3V ON OFF SHDN EXT INPUT 4.5V TO 36V P D1 NIHON EC2IQ506 M1 FAIRCHILD NDS9407 OUT CS OUT 3.3V OR 5V 2A C1 220 μF L1 22 μH RSENSE 40m Ω C3 4.7 μF C2 4.7 μF LOW ESR 0.47 μF Dropout voltage= I x R + R + R OUT DS(ON) SENSE INDUCTOR [] |
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