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UZXYY 데이터시트(PDF) 11 Page - Advanced Analogic Technologies |
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UZXYY 데이터시트(HTML) 11 Page - Advanced Analogic Technologies |
11 / 15 page AAT4620 PCMCIA Current Limit Interface and Capacitor Charger 4620.2007.04.1.1 11 Figure 1: Calculated Current Limit R SET Characteristic. System Ready Hysteresis (RHYS) and System Ready (RDY) In internal comparator senses the output voltage and delivers a ready signal to the external micro controller when the output voltage reaches 98% of the final value (V CC). The comparator hysteresis is programmed by a resistor from the RHYS pin to ground. The RHYS voltage determines the hys- teresis voltage and is equal to the RHYS source current (1µA) times the RHYS resistance. The system ready pin (RDY) is an open drain out- put that switches low when the output voltage reaches 98% of the input voltage. It requires an external pull up resistor tied to the input voltage with a typical value of 100k Ω. Power Loop The AAT4620 power loop limits the load current in the event that the device power dissipation becomes excessive. The AAT4620 power loop regulates the die temperature to 100°C, decreasing the load cur- rent in increments of 1/32 of the current limit set point when the die temperature reaches 100°C. When the die temperature exceeds 100°C it decreases the load current by 1/32 of the current limit set point. If die temperature is less than 100°C, it increases the load current in increments of 1/32 of the current limit set point until the set current limit point is reached or the die temperature exceeds 100°C. The delay time between die temperature measure- ments varies depending on the load current limit set point. The delay ranges from 1ms for a 75mA current limit set point to 8ms for a 1.5A current limit set point. For the condition where there is no current limit set point (shorting R SETU or RSETL to ground) the soft start increments the current limit is 1/32 steps of 1.2A until the current reaches 1.2A, at which point the power MOSFET turns on fully. At this point, the current is limited by the R DS(ON) of the power MOS- FET and other series resistance. The Power Loop and Over-Temperature Loop then takes control to limit the current until the output voltage has fully charged. Over-Temperature Protection If the die temperature rate of rise is fast enough to exceed the power loop regulated temperature, over-temperature shutdown disables the device. The over-temperature threshold is 145°C. After over-temperature shutdown, soft start is initiated once the die temperature drops to 130°C. Short Circuit Protection The series pass power MOSFET turns off com- pletely after the output has charged to within 18mV of the input voltage. This protects the device in the event of a short circuit applied to the output. Applying a short circuit or heavy load to the output initiates the digital soft start where the current ramps from zero to the final programmed value. The power loop will be activated once the digital soft start begins. Under-Voltage Shutdown Under-voltage lockout enables the device at 2.7V typical with 100mV of hysteresis. The maximum UVLO level is 3.0V. I LOAD vs. RSETU and RSETL RSETU and RSETL (M Ω Ω) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.10 0.30 0.50 0.70 0.90 1.10 1.30 1.50 V HYST = RHYS · 1μA = 200k Ω · 1μA = 0.2V |
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