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ADM1070 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADM1070 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page REV. 0 –10– ADM1070 2. If a voltage between the SENSE and VEE Pins increases to 100 mV (the analog current limit voltage) during tLIMITON, then the ADM1070 takes action to reduce this current to a safer level. The internal analog current limit loop dynami- cally adjusts the gate drive, keeping the load current at the 100 mV/RSENSE level. The FET now acts as a current source, limiting the load current to the level set by the value of the sense resistor. The sense voltage is also above the circuit breaker limit voltage, so the limited consecutive retry function is still operational. If the current fault is not cleared (sense resistor voltage brought below 79 mV) after seven consecutive faults, then the device is latched off. 3. If a serious short circuit occurs on the load side, the –48 V supply can cause massive currents to flow very quickly. Because of this, the gate voltage must be reduced quickly to prevent a catastrophic failure. If the ADM1070 detects a voltage greater than the fast current limit voltage (126 mV) across the sense resistor, it is apparent that a serious short circuit is present and the load current must be reduced as quickly as possible. The fast current limit loop takes over and pulls gate low much faster than in the previous case. SENSE RESISTOR The ADM1070’s current limiting function can operate at differ- ent current levels. The sense resistor is inserted between the VEE and sense pins, and a current fault occurs whenever the voltage across the sense resistor is greater than 100 mV for longer than the on time, tLIMITON. The current limit is determined by selec- tion of the sense resistor, RSENSE. Table I shows how the maximum allowable load current (ILOAD(MAX)) and the mini- mum and maximum in-rush currents (ILIMIT(MIN)) and ILIMIT(MAX)) are related to the value of RSENSE. RSENSE ILOAD(MAX) ILIMIT(MIN) ILIMIT(MAX) (m ) (A) (A) (A) 5 12.0 18.0 22.0 10 6.0 9.0 11.0 15 4.0 6.0 7.3 18 3.3 5.0 6.1 22 2.7 4.1 5.0 33 1.8 2.7 3.3 47 1.3 1.9 2.3 51 1.2 1.7 2.2 68 0.9 1.3 1.6 75 0.8 1.2 1.5 90 0.7 1.0 1.2 SHUNT REGULATOR A shunt regulator shunts the ADM1070 VIN Pin. Power is derived from the –48 V supply through the combination of an internal Zener diode and an external shunt resistor, RDROP. Table II shows the operational voltage range and power dissipa- tion for different values of RDROP. Note that 16 k Ω is the default value for RDROP. Min Allowable Max Allowable PDROP VDD Voltage VDD Voltage @ 48 V RDROP (V) (V) (W) 10 k Ω (0.25 W) 26 61 0.13 12 k Ω 28.6 65.8 0.11 14 k Ω 31.2 70.2 0.09 16 k Ω 33.8 74.2 0.08 18 k Ω 36.4 78.1 0.07 20 k Ω 39 81.7 0.065 22 k Ω 41.6 85.2 0.06 10 k Ω (0.5 W) 26 81.7 0.13 12 k Ω 28.6 88.5 0.11 14 k Ω 31.2 94.7 0.09 16 k Ω 33.8 100.4 0.08 18 k Ω 36.4 105.9 0.07 20 k Ω 39 111 0.065 22 k Ω 41.6 115.9 0.06 INTERNAL UNDERVOLTAGE LOCKOUT The VIN Pin is monitored for undervoltage lockout. When the voltage at VIN is above 8.5 V (VLKO), the device is enabled. If this voltage drops below 8.5 V, the device is disabled and gate is pulled low. Note that this is unrelated to the undervoltage and overvoltage functions performed at the UV/OV Pin. TIMER The TIMER Pin on the ADM1070 gives the user control over the timing functions on the part. By connecting an external capacitor between the TIMER Pin and VEE, the user can set the UV/OV glitch filter time, tFLT, the power-on reset delay time, tPOR, the maximum current on time, tON, the current limit time out, tOFF, and the continuous short circuit time before latched shutdown, tSHORT (see Table III). Note that all times are scaled relative to each other and cannot be altered individually (without changing the other times). The default values for these times are selected by tying the TIMER Pin directly to VEE. Table III. Timer Capacitor Values and Timing Values CTIMER tFLT tPOR tLIMITON tPWMOFF tSHORT (pF) (ms) (ms) (ms) (ms) (ms) 220 0.58 0.58 4.8 150 1000 330 0.85 0.85 7.1 230 1400 470 1.21 1.21 9.9 320 2000 Tied to VEE 1.55 1.55 12.8 410 2600 680 1.74 1.74 14.2 450 2800 1000 2.54 2.54 20.8 660 4100 2200 5.64 5.64 46.2 1465 9113 Table I. ILOAD(MAX), ILIMIT(MIN), and ILIMIT(MAX) for Different Values of RSENSE Table II. Minimum and Maximum Allowable Operating Voltages for Different Values of RDROP |
Podobny numer części - ADM1070_15 |
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Podobny opis - ADM1070_15 |
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