2
MAX687/MAX688/MAX689
High-Accuracy, Low-Dropout
Linear Regulators
6 _______________________________________________________________________________________
______________________________________________________________Pin Description
NAME FUNCTION
1 IN Positive input voltage, 2.7V to 11.0V
2
ON
(MAX687)
ON activates the regulator when pulsed high. In order for the regulator to remain on, ON must remain high
until V
OUT
exceeds the internal shutdown threshold voltage. The MAX687 is shut down when V
OUT
< 2.96V,
and remains latched off until ON is pulsed high. When powered up, the MAX687 does not start up until ON
is pulsed high. Connect to V
IN
if not used.
PIN
SHDN
(MAX688/
MAX689)
SHDN is a three-level input that controls the mode of operation. The device is on when V
SHDN
> 1.25V.
The output is disabled and the supply current is reduced (I
IN
< 25µA, standby mode) when V
SHDN
< 1.2V,
and is fully off (I
IN
< 1µA, shutdown mode) when V
SHDN
< 0.2V. Connect SHDN to IN if the shutdown func-
tion is not used. PFO is low when SHDN is low (below 0.2V).
3 PFO
Power-Fail Output. PFO trips when V
OUT
is 170mV below nominal V
OUT
at I
BASE
= 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.
7 BASE
Base drive for the external PNP transistor. Current limiting is controlled using a resistor (R
B
) connected
between BASE and BLIM. For maximum output current, connect BASE to BLIM.
6 BLIM
Base current limit. A resistor (R
B
) connected between BASE and BLIM sets the maximum base drive to the
PNP transistor: I
BASE
≤ 0.1V / (R
B
+ 5Ω). This limits quiescent current rise during dropout and also pro-
vides 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 (h
FE
) is
controlled. Connect to BASE if not used.
5 OUT Regulator Output. MAX687/MAX688, 3.3V; MAX689, 3.0V.
4 GND Ground
8 CC Compensation Capacitor. Connect a non-polarized capacitor (10nF to 100nF) from CC to GND.
_______________Detailed Description
The MAX687/MAX688/MAX689 are precision low-
dropout linear regulators employing external PNP tran-
sistors 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 volt-
ages), and limits the regulator’s output current. The
dropout voltage is limited only by the PNP transistor’s
V
CE(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).
The MAX687’s output voltage is internally monitored; a
falling V
OUT
is signaled by PFO going low. As the out-
put falls further, the MAX687 automatically enters a low-
power shutdown mode, where the base drive to the
external PNP is cut off. PFO trips at a minimum of
110mV below the nominal V
OUT
, and shutdown occurs
by 2.96V. PFO is guaranteed to trip before the device
Table 1. Device Functions
FUNCTION MAX687 MAX688 MAX689
V
OUT
3.3V 3.3V 3.0V
Shutdown
Control
Automatic at low
V
OUT
. Latched.
Use ON to start.
External
SHDN
External
SHDN
PFO Sources & Sinks Open Drain Open Drain
