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UZXYY Folha de dados(PDF) 10 Page - Advanced Analogic Technologies |
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UZXYY Folha de dados(HTML) 10 Page - Advanced Analogic Technologies |
10 / 15 page AAT4620 PCMCIA Current Limit Interface and Capacitor Charger 10 4620.2007.04.1.1 nal resistors connected between the two SET pins and ground. The minimum current limit threshold is specified by I LIM(MIN). If the load condition maintains the device in current limit and the chip temperature reaches a critical point, then an internal power loop will reduce the current to a safe level. Connecting ISETU to ground disables the current limit protec- tion allowing a low impedance path to the host V CC. The load switch is turned off by applying a logic high level to both of the EN pins. When both EN IL and EN IU are selected ISETU will be selected. The EN function has logic level thresholds that allow the AAT4620 to be TTL compatible and may also be controlled by 2.5V to 5.0V CMOS circuits. The voltage level applied to the EN pins should not exceed the input supply level present on the VCC pin. The AAT4620 typically consumes 40µA when operating, when off, the device draws less then 1µA. In the off state, current is prevented from flow- ing between the input and output. Under-voltage lockout circuitry ensures that the V CC supply is high enough for correct operation of the IC. Also included is a System Ready function which will be activated when the capacitor voltage is charged and the load is permitted to take current. For automatic functionality, this pin can be directly connected to the EN IU pin. An external resistor is provided to add hysteresis to this function. Application Information ISETU and ISETL Current Limit Set The AAT4620 current limit is set at two different lev- els. Resistors from ISETL and ISETU set the lower and upper current limit levels respectively. The ISETL and ISETU nodes operate within a win- dow of 0.75V to 1.5V for resistor values ranging from 93.75 Ω to 1.5MΩ. Resistor values outside this range are not recommended. The ISETL and ISETU source current varies with the resistor value used according to Table 1. To determine the resulting cur- rent limit multiply the R SETU or RSETL node voltage times the gain in Table 1. Note that the voltage at the node varies from 0.75V to 1.5V and the current limit gain varies depending on the resistor value used. If the set pin is open circuit or allowed to exceed 2V, all power devices are disabled and the input is disconnected from the output. Shorting the set pin to GND enables all power devices and shorts the V CC to the output pin with no current limit. To activate the upper current level (R SETU) pull EN IU low. To activate the lower current level (R SETL) pull EN IL low. If both EN IU and EN IL are pulled low the EN IU current limit level (R SETU) will be selected. V ISET = RSET · IISET = 0.75V to 1.5V Table 1: R SET Table. R SET Range I ISET (µA) I LIM/VISET (A/V) Current Limit Range (A) Current Limit 1.5Meg - 750k 1 0.8 1.2-0.6 R SET*1µA*0.8 750k - 375k 2 0.4 0.6-0.3 R SET*2µA*0.4 375k - 187.5k 4 0.2 0.3-0.15 R SET*4µA*0.2 187.5 - 93.75k 8 0.1 0.15-0.075 R SET*8µA*0.1 |
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