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MAX1557 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1557 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/MAX1557 match the load regulation to the voltage droop seen during transients. This is some- times 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 min- imal voltage drop during a transition from light load to full load. Additionally, the MAX1556 and MAX1557 use a wide-bandwidth feedback 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/MAX1557 use of a wide-band control loop and voltage positioning allows superior load-tran- sient response by minimizing the amplitude and dura- tion of overshoot and undershoot in response to load transients. Other DC-DC converters, with high gain- control loops, use external compensation to maintain 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/MAX1557 allows up to 2.25% (typ) of load-regulation voltage shift but has no further transient droop. Thus, during load tran- sients, 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 oscil- loscope (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/MAX1557 function with a low input-to-out- put 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 appli- cations with a 3.3V output. The system and load might operate normally down to 3V or less. The MAX1556/ MAX1557 allow the output to follow the input battery voltage as it drops below the regulation voltage. The qui- escent current in this state rises minimally to only 27µA (typ), which aids in extending battery life. This dropout/100% duty-cycle operation achieves long battery life by taking full advantage of the entire battery 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). VDROPOUT = IOUT x (RDS(ON)P + 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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