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MAX1658ESA Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX1658ESA Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 12 page 350mA, 16.5V Input, Low-Dropout Linear Regulators _______________________________________________________________________________________ 7 P-Channel Pass Transistor The MAX1658/MAX1659 feature an internal P-channel MOSFET pass transistor. Using a MOSFET provides several advantages over similar PNP designs, including lower dropout voltage and extended battery life. Unlike bipolar transistors, MOSFETs reduce quiescent current, because they require no base current, particularly at heavy loads and in dropout. As a result, the MAX1658/MAX1659 operate at a low quiescent current even in dropout. Output Voltage Selection Dual Mode operation allows the MAX1658/MAX1659 to operate at either a preset or a user-adjustable output voltage. The device compares the SET pin voltage with an internal 65mV reference. If the voltage is lower than 65mV (typically achieved by grounding SET), the device switches to an internal resistor-divider feedback network that sets the output voltage. The MAX1658’s preset output voltage is 3.3V and the MAX1659’s is 5V (Figure 2). If the SET pin is not below 65mV, the device switches to external feedback and SET becomes a feedback input. The feedback network can be configured to produce an output between 16V and the voltage reference (nominally 1.21V). Under regulation, the feedback mechanism adjusts the error signal such that the volt- age at the SET pin equals the reference voltage. Therefore, to achieve the minimum output, connect SET directly to OUT. For other voltages, a resistive voltage- divider network is necessary. Figure 3 shows the topol- ogy of a typical circuit operating in adjustable mode. The output voltage is set by the following equation: where VSET = 1.21V. Solving for R1 yields: The input leakage current of the SET input is less than 25nA. This allows the use of large resistors in the feed- back network to minimize output current loss without compromising accuracy. R2 can be as high as 500k Ω in most applications. Shutdown A logic low on the SHDN pin places the MAX1658/ MAX1659 in shutdown. This mode deactivates all func- tions, including the pass transistor. The device con- sumes less than 1µA of supply current in shutdown, and its output becomes high impedance. The MAX1658/MAX1659 exit shutdown in 100µs. Output Current Limit The MAX1658/MAX1659 include current-limiting circuit- ry that monitors and controls the pass transistor and limits output current to around 900mA. The output can be shorted to ground indefinitely without damaging the device. R1 = R2 V V 1 OUT SET − V V R R OUT SET = + 1 1 2 MAX1658 (MAX1659) SHDN 2 4, 5 8 1 3, 6, 7 IN INPUT VOLTAGE ON OFF OUTPUT VOLTAGE 3.3V/350mA (5V/350mA) 10 µF 0.1 µF GND SET OUT Figure 2. Preset Output Configuration MAX1658 MAX1659 IN 3, 6, 7 4, 5 1 8 2 SHDN OUTPUT VOLTAGE COUT 10 µF R1 R2 0.1 µF INPUT VOLTAGE GND OUT SET VOUT = VSET (1 + R1 ) R2 VSET = 1.21V Figure 3. Adjustable Output Configuration Using External Feedback Resistors |
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