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AAT3223IGU-2.7-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT3223IGU-2.7-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 17 page  AAT3223 250mA NanoPower™ LDO Linear Regulator with Power-OK 3223.2005.04.1.4 11 Ceramic Capacitor Materials: Ceramic capaci- tors less than 0.1µF are typically made from NPO or C0G materials. NPO and C0G materials are typically tight tolerance and are very stable over temperature. Larger capacitor values are typically composed of X7R, X5R, Z5U, and Y5V dielectric materials. Large ceramic capacitors, typically greater than 2.2µF, are often available in the low- cost Y5V and Z5U dielectrics. These two material types are not recommended for use with LDO reg- ulators since the capacitor tolerance can vary more than ±50% over the operating temperature range of the device. A 2.2µF Y5V capacitor could be reduced to 1µF over the full operating temperature range. This can cause problems for circuit opera- tion and stability. X7R and X5R dielectrics are much more desirable. The temperature tolerance of X7R dielectric is better than ±15%. Capacitor area is another contributor to ESR. Capacitors that are physically large in size will have a lower ESR when compared to a smaller sized capacitor of equivalent material and capacitance value. These larger devices can also improve cir- cuit transient response when compared to an equal value capacitor in a smaller package size. Consult capacitor vendor data sheets carefully when selecting capacitors for use with LDO regulators. Enable Function The AAT3223 features an LDO regulator enable / disable function. This pin (EN) is compatible with CMOS logic. For a logic high signal, the EN control level must be greater than 2.0 volts. A logic low signal is asserted when the voltage on the EN pin falls below 0.5 volts. For example, the active high version AAT3223 will turn on when a logic high is applied to the EN pin. If the enable function is not needed in a specific application, it may be tied to the respective voltage level to keep the LDO regu- lator in a continuously on state; e.g., the active high version AAT3223 will tie V IN to EN to remain on. Power-OK Function The Power-OK (POK) function is a very useful basic active low error flag. When the AAT3223 output voltage level is within regulation limits, the POK output pin is a high impedance and should be tied high to the LDO output through a high value resistor (100k Ω is a good resistor value for this pur- pose). An internal comparator has a reference threshold set to trigger at 10% of the nominal AAT3223 output voltage. If the output voltage level drops below this preset threshold, the POK func- tion will become active and turn on an open-drain N-channel MOSFET to pull the POK output pin to ground. There is a fixed 1ms delay circuit between the POK comparator output and the N-channel MOSFET gate. The purpose of the delay is to pre- vent a false triggering of the POK output during device turn-on or during very short duration load transient events. If necessary, additional POK flag delay can be added by placing a capacitor in paral- lel with the POK pull-up resistor. The additional delay time will be set by the RC time constant, the pull-up resistor, and parallel capacitor values. When the AAT3223 is in the shutdown state with the EN pin low, the POK pin becomes low imped- ance. The LDO output will be discharged through the high value POK pull-up resistor. When entering the shutdown state, there is no delay associated with the POK output; the open-drain device turns on immediately. This offers the added advantage of having a hard application turn-off when the LDO regulator is turned off. This additional function has no adverse effect on regulator turn-on time. Short-Circuit and Thermal Protection The AAT3223 is protected by both current limit and over-temperature protection circuitry. The internal short-circuit current limit is designed to activate when the output load demand exceeds the maxi- mum rated output. If a short-circuit condition were to continually draw more than the current limit threshold, the LDO regulator's output voltage will drop to a level necessary to supply the current demanded by the load. Under short-circuit or other over-current operating conditions, the output volt- age will drop and the AAT3223's die temperature will rapidly increase. Once the regulator's power dissipation capacity has been exceeded and the internal die temperature reaches approximately 140°C, the system thermal protection circuit will become active. The internal thermal protection cir- cuit will actively turn off the LDO regulator output pass device to prevent the possibility of over-tem- perature damage. The LDO regulator output will |
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