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ADP1109AAR-5 Folha de dados(PDF) 6 Page - Analog Devices |
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ADP1109AAR-5 Folha de dados(HTML) 6 Page - Analog Devices |
6 / 8 page ADP1109A –6– REV. 0 APPLICATION INFORMATION THEORY OF OPERATION The ADP1109A is a flexible, low power switch-mode power supply (SMPS) controller for step-up dc/dc converter applica- tions. This device uses a gated-oscillator technique to provide very high performance with low quiescent current. For example, more than 2 W of output power can be generated from a +5 V source, while quiescent current is only 360 µA. A functional block diagram of the ADP1109A is shown on the front page. The internal 1.25 V reference is connected to one input of the comparator, while the other input is externally connected (via the FB pin) to a feedback network connected to the regulated output. When the voltage at the FB pin falls below 1.25 V, the 120 kHz oscillator turns on. A driver amplifier pro- vides base drive to the internal power switch, and the switching action raises the output voltage. When the voltage at the FB pin exceeds 1.25 V, the oscillator is shut off. While the oscillator is off, the ADP1109A quiescent current is only 460 µA. The com- parator includes a small amount of hysteresis, which ensures loop stability without requiring external components for fre- quency compensation. A shutdown feature permits the oscillator to be shut off. Hold- ing SHUTDOWN low will disable the oscillator, and the ADP1109A’s quiescent current will remain 460 µA. The output voltage of the ADP1109A is set with two external resistors. Three fixed-voltage models are also available: the ADP1109A-3.3 (+3.3 V), ADP1109A-5 (+5 V) and ADP1109A-12 (+12 V). The fixed-voltage models are identical to the ADP1109A, except that laser-trimmed voltage-setting resistors are included on the chip. On the fixed-voltage models of the ADP1109A, simply connect the SENSE pin (Pin 8) directly to the output voltage. COMPONENT SELECTION General Notes on Inductor Selection When the ADP1109A internal power switch turns on, current begins to flow in the inductor. Energy is stored in the inductor core while the switch is on, and this stored energy is then trans- ferred to the load when the switch turns off. To specify an inductor for the ADP1109A, the proper values of inductance, saturation current and dc resistance must be deter- mined. This process is not difficult, and specific equations are provided in this data sheet. In general terms, however, the induc- tance value must be low enough to store the required amount of energy (when both input voltage and switch ON time are at a minimum) but high enough that the inductor will not saturate when both VIN and switch ON time are at their maximum val- ues. The inductor must also store enough energy to supply the load, without saturating. Finally, the dc resistance of the induc- tor should be low, so that excessive power will not be wasted by heating the windings. For most ADP1109A applications, an inductor of 10 µH to 47 µH, with a saturation current rating of 300 mA to 1 A and dc resistance <0.4 Ω is suitable. Ferrite core inductors that meet these specifications are available in small, surface-mount packages. Air-core inductors, as well as RF chokes, are unsuitable because of their low peak current ratings. The ADP1109A is designed for applications where the input voltage is fairly stable, such as generating +12 V from a +5 V logic supply. The ADP1109A does not have an internal switch current limiting circuit, so the inductor may saturate if the input voltage is too high. The ADP1111 or ADP3000 should be considered for battery powered and similar applications where the input voltage varies. To minimize Electro-Magnetic Interference (EMI), a toroid or pot core type inductor is recommended. Rod core inductors are a lower-cost alternative if EMI is not a problem. Calculating the Inductor Value Selecting the proper inductor value is a simple two step process: 1. Define the operating parameters: minimum input voltage, maximum input voltage, output voltage and output current. 2. Calculate the inductor value, using the equations in the fol- lowing section. Inductor Selection In a step-up, or boost, converter (Figure 1), the inductor must store enough power to make up the difference between the input voltage and the output voltage. The inductor power is calculated from the equation: P L = VOUT +V D − VIN MIN () ()× I OUT () (1) where VD is the diode forward voltage ( 0.5 V for a 1N5818 Schottky). Energy is only stored in the inductor while the ADP1109A switch is ON, so the energy stored in the inductor on each switching cycle must be must be equal to or greater than: P L f OSC (2) in order for the ADP1109A to regulate the output voltage. When the internal power switch turns ON, current flow in the inductor increases at the rate of: IL t () = VIN R' 1 − e −R't L (3) where L is in Henrys and R' is the sum of the switch equivalent resistance (typically 0.8 Ω at +25°C) and the dc resistance of the inductor. In most applications, the voltage drop across the switch is small compared to VIN so a simpler equation can be used: I L t () = VIN L t (4) Replacing t in the above equation with the ON time of the ADP1109A (5.5 µs, typical) will define the peak current for a given inductor value and input voltage. At this point, the induc- tor energy can be calculated as follows: EL = 1 2 L × I 2 peak (5) As previously mentioned, EL must be greater than PL/fOSC so that the ADP1109A can deliver the necessary power to the load. For best efficiency, peak current should be limited to 1 A or less. Higher switch currents will reduce efficiency because of increased saturation voltage in the switch. High peak current also increases output ripple. As a general rule, keep peak current as low as possible to minimize losses in the switch, inductor and diode. |
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