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

of R1 to increase the ON pulse width.

The 3.3V MAX688 and the 3.0V MAX689 incorporate

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-

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-

Ω, BLIM limits base cur-

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.

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

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

C3

10nF

IN

SHDN

+5V INPUT

GND

R2

100k

C2

68

µF

ON / OFF

Figure 2. MAX688/MAX689 Operating Circuit