Electronic Components Datasheet Search |
|
MAX687ESA Datasheet(PDF) 7 Page - Maxim Integrated Products |
|
MAX687ESA Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 12 page shuts down. Once the MAX687 is shut down, it can only be turned on again when (a) the ON pin is pulsed high, and (b) the conditions that triggered shutdown have changed (e.g., the load has been reduced or the input voltage has increased). In order for the MAX687 to latch into its on state, the ON pulse must remain high until the output voltage has risen above the shutdown threshold. The internal shutdown can be used to pre- vent deep discharge of a battery, for example, to pro- vide “self backup” of CMOS RAM or to protect the battery itself. The RC circuit attached to the ON input in Figure 1 achieves automatic start-up at power-on by delivering a brief pulse whenever the input voltage is suddenly applied. This circuit is not suitable for applications where the input voltage rises slowly. The RC values should be chosen to keep ON high until the output rises above about 3.13V. The values shown in Figure 1 for C4 and R1 (0.1µF and 10k Ω) are suitable for most applications. To ensure start-up when the input voltage is very close to the circuit’s dropout voltage, when the circuit is used to drive a very large capacitive load, or for high-power circuits (ILOAD > 3A), increase the value of R1 to increase the ON pulse width. The 3.3V MAX688 and the 3.0V MAX689 incorporate the same PFO warning whenever VOUT droops. However, the MAX688/MAX689 do not automatically shut down when the output voltage drops even further. Instead, the SHDN input controls external shutdown. As SHDN is pulled low, the chip first enters a low-cur- rent standby state (<25µA). The threshold at which standby mode is entered is precisely controlled (±2%) so the output can be turned off at a well-defined point. 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 resis- tive divider from VIN. When VSHDN is less than 1.2V, the output is off. The device is fully shut down (<1µA) when SHDN is pulled below 200mV. SHDN is not latched, and as SHDN is raised, the MAX688/MAX689 exit shutdown and enter the standby mode. At the higher SHDN threshold, the output is turned on. Figure 1 shows a typical circuit for the MAX687, and Figure 2 shows the same circuit configured for the MAX688/MAX689. The accuracy of the output current limit depends on accurate knowledge of the PNP pass transistor’s cur- rent gain (hFE). With RB = 12 Ω, BLIM limits base cur- rent to 6mA (IBASE = 0.1V / (RB + 5 Ω)). See Base- Current Limiting section. The PFO comparators on all three devices, and the internal shutdown comparator on the MAX687, reject high-speed spikes (<100µs). This reduces the PFO output’s noise sen- sitivity, and stops the MAX687 from being shut down inad- vertently when there is noise on the input supply. _______________Transistor Selection Specifications The PNP pass transistor must meet specifications for • current gain, • power dissipation, and • collector current. The hFE influences the maximum output current the cir- cuit can deliver. The largest guaranteed output current High-Accuracy, Low-Dropout Linear Regulators _______________________________________________________________________________________ 7 MAX687 OUT 3.3V @ 500mA POWER FAIL PFO CC C1 2.2 µF C4 0.1 µF BLIM GND BASE Q1 FZT749 RB 12Ω C3 10nF IN ON +5V INPUT GND R1 10k Ω ON CAN BE DRIVEN DIRECTLY FROM LOGIC WHEN R1 AND C4 ARE NOT USED. C2 68 µF Figure 1. MAX687 Operating Circuit MAX688 MAX689 OUT 3.3V/ 3.0V @ 500mA POWER FAIL PFO CC C1 2.2 µF BLIM GND BASE Q1 FZT749 RB 12Ω C3 10nF IN SHDN +5V INPUT GND R2 100k C2 68 µF ON / OFF Figure 2. MAX688/MAX689 Operating Circuit |
Similar Part No. - MAX687ESA |
|
Similar Description - MAX687ESA |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |