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MAX1793EUE-33 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1793EUE-33 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 10 page Internal P-Channel Pass Transistor The MAX1793 features a 1A P-channel MOSFET pass transistor. Unlike similar designs using PNP pass tran- sistors, P-channel MOSFETs require no base drive, which reduces quiescent current. PNP-based regula- tors also waste considerable current in dropout when the pass transistor saturates and use high base-drive currents under large loads. The MAX1793 does not suf- fer from these problems and consumes only 225µA of quiescent current under heavy loads, as well as in dropout. Output Voltage Selection The MAX1793 features Dual Mode operation. In preset voltage mode, the output of the MAX1793 is set to an internally trimmed voltage (see Ordering Information). Select this mode by connecting SET to GND (Figure 2). In adjustable mode, an output is selected between +1.25V and +5V using two external resistors connected as a voltage divider to SET (Figure 3). The output volt- age is determined by the following equation: VOUT = VSET [1 + (R1 / R2)] where VSET = +1.25V. To simplify resistor selection: R1 = R2 [(VOUT / VSET) – 1] Since the input bias current at SET is less than 100nA, large resistance values can be used for R1 and R2 to minimize power consumption without losing efficiency. Up to 125k Ω is acceptable for R2. In preset voltage mode, the impedance from SET to GND should be less than 10k Ω. Otherwise, spurious conditions could cause the voltage at SET to exceed the 50mV dual-mode threshold. Shutdown A logic low on SHDN disables the MAX1793. In shut- down, the pass transistor, control circuitry, reference, and all internal circuits are turned off, reducing supply current to typically 0.1µA. Connect SHDN to IN for nor- mal operation. In shutdown, RST goes low and OUT is discharged to GND with a 5k Ω internal resistance. RST Comparator The MAX1793 features a low VOUT indicator that asserts when the output voltage falls out of regulation. The open-drain RST output goes low when OUT falls 6% below its nominal output voltage. RST remains low for 4ms after OUT has returned to its nominal value. A 100k Ω pull-up resistor from RST to a suitable logic sup- ply voltage (typically OUT) provides a logic control sig- nal. RST can be used as a power-on-reset (POR) signal to a microcontroller or can drive an external LED to indi- cate power failure. RST is low during shutdown. Current Limiting The MAX1793 features a current limit that monitors the pass transistor, typically limiting short-circuit output current to 1.8A. The current limit is increased to approximately 2.8A when the output voltage is in regu- lation, improving performance with large transient loads. The output can be shorted to ground for an indefinite period of time without damaging the device. Thermal Overload Protection Thermal overload protection limits total power dissipa- tion in the MAX1793. When the junction temperature exceeds TJ = +170°C, a thermal sensor turns off the pass transistor, allowing the IC to cool. The thermal sen- sor turns the pass transistor on once the IC’s junction Low-Dropout, Low IQ, 1A Linear Regulator 8 _______________________________________________________________________________________ NC MAX1793 IN IN IN IN IN OFF ON CIN = 4.7 µF SHDN OUT OUT OUT OUT RST SET GND COUT = 6.8 µF OUT RESET OUTPUT Figure 2. Typical Operating Circuit with Preset Output Voltage NC MAX1793 IN IN IN IN IN OFF ON CIN = 4.7 µF SHDN OUT OUT OUT OUT RST SET GND R2 R1 COUT = 6.8 µF OUT RESET OUTPUT Figure 3. Typical Operating Circuit with Adjustable Output Voltage |
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