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LM9070 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM9070 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 13 page Application Information (Continued) An output bypass capacitor of at least 10 µF is required for stability (47 µF is recommended). The ESR of this capacitor should be less than 3 Ω. An input capacitor of 1 µF or larger is recommended to improve line transient and noise perfor- mance. With the Keep-Alive input, a system microprocessor has the ability to keep the regulator ON (with a logic “0” on Keep-Alive) after the ON/OFF input has been commanded OFF. A power-down sequence, when system variables are typically stored in programmable memory, can be executed and take as much time as necessary. At the end of the op- eration the micro then pulls Keep-Alive high and the regula- tor and system turn OFF and revert to the low quiescent cur- rent standby mode. For additional system reliability, consideration has been made for the possibility of a short circuited load at the output of the regulator. When the regulator is switched ON, conven- tional current limiting and thermal shutdown protect the regu- lator. When the regulator is switched OFF however, a grounded V CC supply to the micro (due to the shorted regu- lator output) will force the Keep-Alive input to be low and thus try to maintain the Keep-Alive mode of operation. With a shorted load, the drain on the battery could be as high as 1.5A. A separate internal circuit monitors the output voltage of the regulator. If V OUT is less than 4V, as would be the case with a shorted load, the Keep-Alive function is logically dis- abled to ensure that the regulator turns OFF and reverts to only a 50 µA load on the battery. Conventional load dump protection is built in to withstand up to +60V and −50V transients. A 1 k Ω resistor in series with the ON/OFF and Keep-Alive inputs are recommended to provide the same level of transient protection for these pins if required. Protection against reverse polarity battery con- nections is also built in. With a reversed battery the output of the LM9070 will not go more negative than one diode drop below ground. This will prevent damage to any of the 5V load circuits. For applications where the control logic is not required the logic pins should be configured as shown in Figure 2.A separate device, called the LM9071, can be used. The LM9071 is available in a 5-lead TO-220 package and does not provide control logic functions, but still retains all of the protection features of the LM9070. RESET FLAG Excessive loading of the output to the point where the output voltage drops by 300 mV to 500 mV will signal a reset flag to the micro. This will warn of a V CC supply that may produce unpredictable operation of the system. On power-up and re- covery from a fault condition the delay capacitor is used to hold the micro in a reset condition for a programmable time interval to allow the system operating voltages and clock to stabilize before executing code. The typical delay time interval can be estimated using the following equation: INPUT STABILITY Low dropout voltage regulators which utilize a PNP power transistor usually exhibit a large increase in current when in dropout (V IN < 5.5V). This increase is caused by the satura- tion characteristics ( β reduction) of the PNP transistor. To significantly minimize this increase in current the LM9070 detects when the PNP enters saturation and reduces the op- erating current. This reduction in input current can create a stability problem in applications with higher load current (> 100 mA) where the input voltage is applied through a long length of wire, which in effect adds a significant amount of inductance in se- ries with the input. The drop in input current may create a positive input voltage transient which may take the PNP out of saturation. If the input voltage is held constant at the threshold where the PNP is going in and out of saturation, an oscillation may be created. This is only observed where significant series inductance is present in the input supply line and when the rise and fall time of the input supply is very slow. If the application and re- moval of the input voltage changes at a rate greater than 500 mV/µs, the input voltage moves through the dropout region of operation (V IN of 3V to 5.5V) too quickly for an oscillation to be established. MICROPROCESSOR SYSTEM REGULATOR WITH KEEP-ALIVE INTERVAL AT TURN-OFF The following circuit illustrates a system application utilizing both of the logic control inputs of the LM9070. Closing the ON/OFF switch powers ON the system. Once powered, the system controller sets the Keep-Alive line low. The NPN transistor is used only to signal the controller that the ON/ OFF switch has been opened and the system is to be turned OFF. Upon detecting this high level at the ON/OFF Sense in- put line, the controller can then perform a power down rou- tine. The system will remain fully powered until the controller commands total shut down by taking the Keep-Alive line high. The system then shuts OFF and reverts to a very low current drain standby condition until switched back on. DS012831-24 FIGURE 2. Control Logic Not Used www.national.com 8 |
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