MAX687-MAX689 MAXIM [Maxim Integrated Products], MAX687-MAX689 Datasheet - Page 7

• Manufacturer Part Number: MAX687-MAX689
• Description: High-Accuracy, Low-Dropout Linear Regulators
• Manufacturer: MAXIM [Maxim Integrated Products]

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 10kΩ) 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 (I
of R1 to increase the ON pulse width.
The 3.3V MAX688 and the 3.0V MAX689 incorporate
the same PFO warning whenever V
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.
Figure 1. MAX687 Operating Circuit
ON CAN BE DRIVEN DIRECTLY FROM LOGIC WHEN R1 AND C4 ARE NOT USED.
0.1 F
+5V INPUT
GND
C4
2.2 F
R1
10k
C1
Q1 FZT749
_______________________________________________________________________________________
IN
ON
BASE
MAX687
R
B
LOAD
12
GND
BLIM
OUT
PFO
> 3A), increase the value
CC
68 F
C3
10nF
C2
OUT
3.3V @ 500mA
POWER FAIL
droops.
High-Accuracy, Low-Dropout
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 V
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 (h
rent to 6mA (I
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.
The PNP pass transistor must meet specifications for
The h
cuit can deliver. The largest guaranteed output current
Figure 2. MAX688/MAX689 Operating Circuit
_______________Transistor Selection
+5V INPUT
FE
ON / OFF
current gain,
power dissipation, and
collector current.
influences the maximum output current the cir-
GND
2.2 F
C1
FE
Linear Regulators
). With R
BASE
Q1 FZT749
IN
IN
SHDN
BASE
. When V
= 0.1V / (R
GND
B
MAX688
MAX689
R
B
= 12Ω, BLIM limits base cur-
12
BLIM
CC
SHDN
OUT
PFO
B
C3
10nF
+ 5Ω)). See Base-
3.3V/ 3.0V @ 500mA
Specifications
is less than 1.2V,
C2
68 F
POWER FAIL
R2
100k
7