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CXL180 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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CXL180 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page As the load current decreases, the converters enter a pulse-skip mode in which the PWM comparator is dis- abled. At light loads, efficency is enhanced by a pulse-skip mode in which switching occurs only as needed to service the load. Quiescent current in skip mode is typically 16µA. See the Light-Load Switching Waveforms and Load Transient graphs in the Typical Operating Characteristics. Load-Transient Response/ Voltage Positioning The MAX1556/MAX1556A/MAX1557 match the load regulation to the voltage droop seen during transients. This is sometimes called voltage positioning. The load line used to achieve this behavior is shown in Figures 4 and 5. There is minimal overshoot when the load is removed and minimal voltage drop during a transition from light load to full load. Additionally, the MAX1556, MAX1556A, and MAX1557 use a wide-bandwidth feed- back loop to respond more quickly to a load transient than regulators using conventional integrating feedback loops (see Load Transient in the Typical Operating Characteristics). The MAX1556/MAX1556A/MAX1557 use of a wide-band control loop and voltage positioning allows superior load-transient response by minimizing the amplitude and duration of overshoot and undershoot in response to load transients. Other DC-DC converters, with high gain- control loops, use external compensation to main- tain tight DC load regulation but still allow large voltage droops of 5% or greater for several hundreds of microseconds during transients. For example, if the load is a CPU running at 600MHz, then a dip lasting 100µs corresponds to 60,000 CPU clock cycles. Voltage positioning on the MAX1556/MAX1556A/ MAX1557 allows up to 2.25% (typ) of load-regulation voltage shift but has no further transient droop. Thus, during load transients, the voltage delivered to the CPU remains within spec more effectively than with other regulators that might have tighter initial DC accuracy. In summary, a 2.25% load regulation with no transient droop is much better than a converter with 0.5% load regulation and 5% or more of voltage droop during load transients. Load-transient variation can be seen only with an oscilloscope (see the Typical Operating Characteristics), while DC load regulation read by a voltmeter does not show how the power supply reacts to load transients. Dropout/100% Duty-Cycle Operation The MAX1556/MAX1556A/MAX1557 function with a low input-to-output voltage difference by operating at 100% duty cycle. In this state, the high-side p-channel MOSFET is always on. This is particularly useful in battery-powered applications with a 3.3V output. The sys- tem and load might operate normally down to 3V or less. The MAX1556/MAX1556A/MAX1557 allow the output to follow the input battery voltage as it drops below the regu- lation voltage. The quiescent current in this state rises minimally to only 27µA (typ), which aids in extending bat- tery life. This dropout/100% duty-cycle operation achieves long battery life by taking full advantage of the entire bat- tery range. The input voltage required to maintain regulation is a function of the output voltage and the load. The differ- ence between this minimum input voltage and the out- put voltage is called the dropout voltage. The dropout voltage is therefore a function of the on-resistance of the internal p-channel MOSFET (RDS(ON)P) and the inductor resistance (DCR). 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 8 _______________________________________________________________________________________ -2.5 -1.5 -2.0 -0.5 -1.0 0.5 0 1.0 0 200 400 800 600 1000 1200 LOAD CURRENT (mA) VIN = 3.6V VIN = 5.5V VIN = 2.6V Figure 4. MAX1556 Voltage-Positioning Load Line Figure 5. MAX1557 Voltage-Positioning Load Line 0 200 400 600 LOAD CURRENT (mA) -1.0 -0.4 -0.6 -0.8 -0.2 0 0.2 0.4 0.6 0.8 1.0 VIN = 5.5V VIN = 2.6V VIN = 3.6V |
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Descripción similar - CXL180 |
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