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ADP1111AR-5 Scheda tecnica(PDF) 6 Page - Analog Devices |
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ADP1111AR-5 Scheda tecnica(HTML) 6 Page - Analog Devices |
6 / 16 page ADP1111 –6– REV. 0 THEORY OF OPERATION The ADP1111 is a flexible, low-power, switch-mode power supply (SMPS) controller. The regulated output voltage can be greater than the input voltage (boost or step-up mode) or less than the input (buck or step-down mode). This device uses a gated-oscillator technique to provide very high performance with low quiescent current. A functional block diagram of the ADP1111 is shown on the first page of this data sheet. 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 72 kHz oscillator turns on. A driver amplifier provides 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 ADP1111 quiescent current is only 300 µA. The comparator includes a small amount of hysteresis, which ensures loop stability without requiring external compo- nents for frequency compensation. The maximum current in the internal power switch can be set by connecting a resistor between VIN and the ILIM pin. When the maximum current is exceeded, the switch is turned OFF. The current limit circuitry has a time delay of about 1 µs. If an external resistor is not used, connect ILIM to VIN. Further information on ILIM is included in the “APPLICATIONS” section of this data sheet. The ADP1111 internal oscillator provides 7 µs ON and 7 µs OFF times that are ideal for applications where the ratio between VIN and VOUT is roughly a factor of two (such as converting +3 V to + 5 V). However, wider range conversions (such as generating +12 V from a +5 V supply) can easily be accomplished. An uncommitted gain block on the ADP1111 can be connected as a low-battery detector. The inverting input of the gain block is internally connected to the 1.25 V reference. The noninverting input is available at the SET pin. A resistor divider, connected between VIN and GND with the junction connected to the SET pin, causes the AO output to go LOW when the low battery set point is exceeded. The AO output is an open collector NPN transistor that can sink 300 µA. The ADP1111 provides external connections for both the collector and emitter of its internal power switch that permit both step-up and step-down modes of operation. For the step- up mode, the emitter (Pin SW2) is connected to GND, and the collector (Pin SW1) drives the inductor. For step-down mode, the emitter drives the inductor while the collector is connected to VIN. The output voltage of the ADP1111 is set with two external resistors. Three fixed-voltage models are also available: ADP1111–3.3 (+3.3 V), ADP1111–5 (+5 V) and ADP1111–12 (+12 V). The fixed-voltage models are identical to the ADP1111, except that laser-trimmed voltage-setting resistors are included on the chip. On the fixed-voltage models of the ADP1111, simply connect the feedback pin (Pin 8) directly to the output voltage. COMPONENT SELECTION General Notes on Inductor Selection When the ADP1111 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 transferred to the load when the switch turns off. Since both the collector and the emitter of the switch transistor are accessible on the ADP1111, the output voltage can be higher, lower, or of opposite polarity than the input voltage. To specify an inductor for the ADP1111, the proper values of inductance, saturation current and dc resistance must be determined. This process is not difficult, and specific equations for each circuit configuration are provided in this data sheet. In general terms, however, the inductance 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 values. The inductor must also store enough energy to supply the load, without saturating. Finally, the dc resistance of the inductor should be low so that excessive power will not be wasted by heating the windings. For most ADP1111 applications, an inductor of 15 µH to 100 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. 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 three step process: 1. Define the operating parameters: minimum input voltage, maximum input voltage, output voltage and output current. 2. Select the appropriate conversion topology (step-up, step- down, or inverting). 3. Calculate the inductor value using the equations in the following sections. TEMPERATURE – C 350 300 0 –40 85 25 200 150 100 50 250 BIAS CURRENT 070 Figure 14. Set Pin Bias Current vs. Temperature |
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