PNP transistor is turned on fully as regulation is lost.
Even with a load current of a few microamperes, the
base current will be driven above 5mA. Figure 8 shows
how this base current may be significant.
Consequently, a mostly discharged battery can be fur-
ther discharged at end-of-life.
Figure 6 shows how this condition can be modified by
connecting DD to SHDN with a 47kΩ resistor, R1, paral-
leled with a 0.1µF capacitor to GND. This modification
reduces the no-load quiescent current to approximately
160µA when dropout is reached (Figure 9), but increas-
es the dropout voltage by about 0.1V. The output volt-
age drops to approximately 3V once DD begins to
activate SHDN, but it does not fall to zero because
SHDN is only partially activated.
A second alternate connection (Figure 7) further
reduces quiescent current near the dropout voltage,
compared to the circuit in Figure 6. The output must be
set with external resistors (R1, R2), so DD lowers the
output voltage as the input voltage falls by sourcing
current into SET via R3. Quiescent current remains low
for inputs down to 3.5V, and peaks before falling to 0 at
low input voltages. Although the current peak is higher
than with the connection in Figure 6, this may be more
useful because the quiescent current peaks at an input
voltage well below the useful range of most batteries
(Figure 9). Also, as IN falls below 5V, OUT tracks IN
minus the dropout voltage. This connection still allows
separate use of the SHDN input.
Power Dissipation
The MAX667 can regulate currents as high as 250mA
and withstand input-output differential voltages as high
as 15.2V, but not simultaneously. The maximum power
dissipation is dependent on the package and the tem-
perature (see Absolute Maximum Ratings). Figure 10
shows the maximum output current at various input-out-
put differential voltages for the plastic DIP and SO
packages. The MAX667 can withstand short-circuit
loads up to 1 second.
Operation from AC Sources
The MAX667 is a micropower CMOS regulator intended
principally for battery operation. When operating from
AC sources, consider power-supply ripple rejection.
The MAX667’s error amplifier produces very low gain
bandwidth, and the input power-supply rejection ratio
MAX667
+5V/Programmable Low-Dropout
Voltage Regulator
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MAX667
+5V/Programmable Low-Dropout
Voltage Regulator
