MAX689CSA

19-0329; Rev 0; 12/94 NUAL KIT MA ATION U EET L H A S V A E T WS DA FOLLO High-Accuracy, Low-Dropout Linear Regulators ____________________________Features ♦ 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) ♦ Low Supply Current: 150µA Operating 1.25V. The output is disabled and the supply current is reduced (IIN < 25µA, standby mode) when V SHDN < 1.2V, and is fully off (IIN < 1µA, shutdown mode) when V SHDN < 0.2V. Connect SHDN to IN if the shutdown function is not used. PFO is low when SHDN is low (below 0.2V). 3 PFO Power-Fail Output. PFO trips when VOUT is 170mV below nominal VOUT at IBASE = 1mA. PFO sources and sinks current in the MAX687, but is an open drain in the MAX688/MAX689. When shut down, PFO is always low and sinks current. Leave PFO open if not used. 4 GND Ground 5 OUT Regulator Output. MAX687/MAX688, 3.3V; MAX689, 3.0V. 6 BLIM Base current limit. A resistor (RB) connected between BASE and BLIM sets the maximum base drive to the PNP transistor: IBASE ≤ 0.1V / (RB + 5Ω). This limits quiescent current rise during dropout and also provides current limiting without using a current-sense resistor in the collector of the external PNP, thus not impacting dropout. Output current limiting accuracy depends on how well the external PNP beta (hFE) is controlled. Connect to BASE if not used. 7 BASE Base drive for the external PNP transistor. Current limiting is controlled using a resistor (RB) connected between BASE and BLIM. For maximum output current, connect BASE to BLIM. 8 CC 2 Compensation Capacitor. Connect a non-polarized capacitor (10nF to 100nF) from CC to GND. _______________Detailed Description The MAX687/MAX688/MAX689 are precision lowdropout linear regulators employing external PNP transistors to achieve a wide range of output currents at voltages of 3.0V or 3.3V. The maximum base current for the PNP can be limited using a resistor. Limiting the base drive keeps high currents from being wasted when the device is in dropout (e.g., at low input voltages), and limits the regulator’s output current. The dropout voltage is limited only by the PNP transistor’s VCE(SAT). The Power-Fail Output (PFO) goes low when the output voltage drops 170mV below the nominal level. The three devices differ in their output voltages, in their shutdown-control functions, and in PFO’s output drive (see Table 1). 6 Table 1. Device Functions FUNCTION MAX687 MAX688 MAX689 VOUT 3.3V 3.3V 3.0V Shutdown Control Automatic at low VOUT. Latched. Use ON to start. External SHDN External SHDN PFO Sources & Sinks Open Drain Open Drain The MAX687’s output voltage is internally monitored; a falling VOUT is signaled by PFO going low. As the output falls further, the MAX687 automatically enters a lowpower shutdown mode, where the base drive to the external PNP is cut off. PFO trips at a minimum of 110mV below the nominal VOUT, and shutdown occurs by 2.96V. PFO is guaranteed to trip before the device _______________________________________________________________________________________ High-Accuracy, Low-Dropout Linear Regulators Q1 FZT749 +5V INPUT 3.3V @ 500mA 70mV of hysteresis between the standby and on states prevents chatter between the two modes. The voltage applied to the SHDN pin can be derived from a resistive divider from VIN. When VSHDN is less than 1.2V, the output is off. The device is fully shut down (