The MAX687/MAX688/MAX689 low-dropout linear regulators operate with an input-to-output voltage differential limited only by an external PNP transistor. Outputs are fixed at 3.3V (MAX687/MAX688) or 3.0V (MAX689). The only external components required are a PNP pass transistor and output, compensation, and bypass capacitors. Base drive to the external transistor is at least 10mA, permitting output currents to exceed 1A when using high-gain transistors ( > 100). Output current limiting is implemented by limiting the external transistor's base current. Output voltage monitoring and shutdown functions are included.
The 3.3V MAX687 automatically shuts down whenever the output voltage drops below 2.96V. An internal power-fail comparator also monitors the output and provides an early warning of low output voltage before the device shuts down. When shut down, the output is latched off until the ON input is pulsed. Turning off the power supply in this way prevents battery damage due to excessive discharge or cell-reversal. Typical applications include portable telephones and other battery-powered equipment where the power supply must be disabled when the battery voltage is low.
The MAX688 and MAX689 do not have an automatic shutdown function, and are identical except for their output voltages. Each device has an active-low shutdown-control input, used to turn its output on or off at any time. As SHDN-bar falls, the device enters a standby mode before fully shutting down. When in standby, the reference and comparators are fully operational, permitting the transition from normal mode to standby mode to occur at a precise voltage level on SHDN-bar.
Key Features
Applications/Uses
Fixed Outputs
3.3V (MAX687/MAX688)
3.0V (MAX689)
Directly Drives External PNP Transistor
10mA Min Base-Current Drive for >1A Output
Low Dropout Voltage:
100mV Dropout at 650mA Output (FZT749)
40mV Dropout at 200mA Output (FZT749)
0.8V Dropout at 4A Output
Power-Fail Output Monitors the Output Voltage
Automatic, Latched Shutdown when Output Falls Out of Regulation (MAX687)
Precision Threshold Shutdown Control (MAX688/MAX689)
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> 100). Output current limiting is implemented by limiting the external transistor's base current. Output voltage monitoring and shutdown functions are included.
