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ADP1110 Datasheet(PDF) 6 Page - Analog Devices |
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ADP1110 Datasheet(HTML) 6 Page - Analog Devices |
6 / 16 page ADP1110 –6– REV. 0 THEORY OF OPERATION The ADP1110 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 ADP1110 is shown on the first page. The internal 220 mV 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 220 mV, the 70 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 220 mV, the oscillator is shut off. While the oscillator is off, the ADP1110 quiescent current is only 300 µA. The comparator includes a small amount of hysteresis, which ensures loop stability without requiring external components 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 800 ns. If an external resistor is not used, connect ILIM to VIN. Further informa- tion on ILIM is included in the “Applications” section of this data sheet. The ADP1110 internal oscillator provides 10 µs ON and 5 µs OFF times, which is ideal for applications where the ratio between VIN and VOUT is roughly a factor of three (such as generating +5 V from a single 1.5 V cell). Wider range conversions, as well as step-down converters, can also be accomplished with a slight loss in the maximum output power that can be obtained. An uncommitted gain block on the ADP1110 can be connected as a low–battery detector. The inverting input of the gain block is internally connected to the 220 mV 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 ADP1110 provides external connections for both the collector and emitter of its internal power switch, which permits 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 ADP1110 is set with two external resistors. Three fixed-voltage models are also available: ADP1110–3.3 (+3.3 V), ADP1110–5 (+5 V) and ADP1110-12 (+12 V). The fixed-voltage models are identical to the ADP1110 except that laser-trimmed voltage-setting resistors are included on the chip. Only three external components are required to form a +3.3 V, +5 V or +12 V converter. On the fixed-voltage models of the ADP1110, simply connect the SENSE pin (Pin 8) directly to the output voltage. COMPONENT SELECTION General Notes on Inductor Selection When the ADP1110 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 transferred to the load when the switch turns off. Because both the collector and the emitter of the switch transistor are accessible on the ADP1110, the output voltage can be higher, lower, or of opposite polarity than the input voltage. To specify an inductor for the ADP1110, 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 ADP1110 applications, an inductor of 15 µH to 100 µH with a saturation current rating of 300 mA to 1A 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. TEMPERATURE – C 400 0 070 25 250 200 150 50 350 300 BIAS CURRENT 100 Figure 14. Set Pin Bias Current vs. Temperature TEMPERATURE – C 220 219 211 070 25 215 214 213 212 217 216 218 REFERENCE VOLTAGE Figure 15. Reference Voltage vs. Temperature |
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