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MAX8840ELT25+T Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX8840ELT25+T Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 10 page Ultra-Low-Noise, High PSRR, Low-Dropout, 150mA Linear Regulators in µDFN 6 _______________________________________________________________________________________ Detailed Description The MAX8840/MAX8841/MAX8842 are ultra-low-noise, low-dropout, low-quiescent current linear regulators designed for space-restricted applications. The parts are available with preset output voltages ranging from 1.5V to 4.5V. These devices can supply loads up to 150mA. As shown in the Functional Diagram, the MAX8840/MAX8841 consist of an innovative bandgap core and noise bypass circuit, error amplifier, p-chan- nel pass transistor, and internal feedback voltage- divider. The MAX8842 allows for adjustable output with an external feedback network. The 1.225V bandgap reference is connected to the error amplifier’s inverting input. The error amplifier com- pares this reference with the feedback voltage and amplifies the difference. If the feedback voltage is lower than the reference voltage, the pass-transistor gate is pulled low. This allows more current to pass to the output and increases the output voltage. If the feed- back voltage is too high, the pass transistor gate is pulled high, allowing less current to pass to the output. The output voltage is fed back through an internal resis- tor voltage-divider connected to the OUT pin. An external bypass capacitor connected to BP (MAX8840) reduces noise at the output. Additional blocks include a current limiter, thermal sensor, and shutdown logic. Internal p-Channel Pass Transistor The MAX8840/MAX8841/MAX8842 feature a 1 Ω (typ) p-channel MOSFET pass transistor. This provides seve- ral advantages over similar designs using a pnp pass transistor, including longer battery life. The p-channel MOSFET requires no base drive, which considerably reduces quiescent current. pnp-based regulators waste considerable current in dropout when the pass transis- tor saturates. They also use high base-drive current under heavy loads. The MAX8840/MAX8841/MAX8842 do not suffer from these problems and consume only 40µA of quiescent current in light load and 220µA in dropout (see the Typical Operating Characteristics). Output Voltage Selection The MAX8840/MAX8841 are supplied with factory-set output voltages from 1.5V to 4.5V, in 100mV increments (see the Ordering Information). The MAX8842 features a user-adjustable output through an external feedback network (see the Typical Operating Circuits). To set the output of the MAX8842, use the following equation: where R2 is chosen to be less than 240k Ω and VREF = 1.225V. Use 1% or better resistors. Shutdown The MAX8840/MAX8841/MAX8842 feature a low-power shutdown mode that reduces quiescent current less than 1µA. Driving SHDN low disables the voltage refer- ence, error amplifier, gate-drive circuitry, and pass transistor (see the Functional Diagram), and the device output enters a high-impedance state. Connect SHDN to IN for normal operation. Current Limit The MAX8840/MAX8841/MAX8842 include a current limiter, which monitors and controls the pass transis- tor’s gate voltage, limiting the output current to 200mA. RR X V V OUT REF 12 1 = - Pin Description PIN MAX8840 MAX8841 MAX8842 NAME FUNCTION 111 IN Unregulated Input Supply 222 GND Ground 333 SHDN Shutdown. Pull low to disable the regulator. 4— — BP Noise Bypass for Low-Noise Operation. Connect a 10nF capacitor from BP to OUT. It is shorted to OUT in shutdown mode. —— 4FB Adjustable Output Feedback Point 5 4, 5 5 N.C. No Connection. Not internally connected. 666 OUT Regulated Output Voltage. Bypass with a capacitor to GND. See the Capacitor Selection and Regulator Stability section. |
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