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MAX6470UT33AD3 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX6470UT33AD3 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 20 page 300mA LDO Linear Regulators with Internal Microprocessor Reset Circuit 10 ______________________________________________________________________________________ remotely sense the output voltage of the device. Using FB with an external NPN transistor, the current drive capability can be increased according to the following equation (Figure 2): IOUT(TOTAL) = IOUT ✕ ( β+1) The external NPN pass transistor must meet specifica- tions for current gain, power dissipation, and collector current. The beta influences the maximum output cur- rent the circuit can deliver. The largest guaranteed out- put current is given by ILOAD (max) = 300mA × beta (min). The transistor’s rated power dissipation must exceed the actual power dissipated in the transistor. The power dissipated (PD) equals the maximum load current (ILOAD (max)) times the maximum input-to-out- put voltage differential: PD = ILOAD (max) × (VIN (max) - VOUT). The rated transistor collector current must exceed the maximum load current. Reverse Leakage Protection Reverse OUT to IN Current An internal circuit monitors the MAX6469–MAX6484 input and output voltages. When the output voltage is greater than the input voltage, the internal IN-to-OUT pass transistor and parasitic diode turn off. An external voltage applied to OUT does not reverse charge a bat- tery or power source applied to IN (the leakage path from OUT to IN is 0.01µA typ). When the output voltage exceeds the input voltage, OUT powers the device and shutdown must be logic high (greater than 0.7 ✕ VOUT). RESET asserts until IN exceeds OUT and OUT is above the specified VTHOUT threshold (based on the selected or adjusted regulator OUT nominal voltage). Reverse OUT to Ground Current The MAX6469–MAX6484 maintain a low OUT-to-GND reverse-current flow when the IN power source is removed. When IN floats (input battery removed) and SHDN is pulled up to VOUT (by an external diode), the OUT-to-GND current through the LDO is 40µA (typ). The regulator output can be held up with an external super capacitor or backup battery at OUT until the IN battery is replaced. The RESET output is asserted while the IN bat- tery is removed to place the system in a low-power mode. Volatile memory content is maintained until the super capacitor or battery voltage drops below RAM standby specifications. RESET deasserts when the IN battery has been replaced and OUT exceeds the desired reset threshold. For nonrechargeable backup battery applications, place a reverse diode between OUT and the backup battery (to prevent battery charg- ing). The external diode does not affect the regulator’s dropout voltage because it is not between the LDO out- put and the processor/memory Vcc supply. The diode can be replaced with a current-limiting resistor for rechargeable backup battery applications. Current Limit The MAX6469–MAX6484 include an internal current- limit circuit that monitors and controls the pass transis- tor’s gate voltage, limiting the output current to 450mA (min). The output can be shorted to ground indefinitely without damaging the part. Thermal Shutdown When the junction temperature (TJ) exceeds +180°C (typ), the thermal sensor signals the shutdown logic, turning off the pass transistor and allowing the IC to Figure 3. Battery Backup MAX6469– MAX6484 µP MEMORY OUT IN SHDN 3.0V LITHIUM REMOVABLE LITHIUM ION OR 3-CELL ALKALINE REMOVABLE LITHIUM ION OR 3-CELL ALKALINE MAX6469– MAX6484 µP MEMORY OUT IN SHDN SUPERCAP 3.3 µF 3.3 µF Figure 2. High-Current, External Transistor Application MAX6475/MAX6476 MAX6483/MAX6484 RPULLUP 0.1 µF 3.3 µF 330 Ω µP 5.0V OUT FB RESET GND IN VCC = 3.3V 1A TOTAL CURRENT |
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