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 47k

Ω resistor, R1, par-

alleled with a 0.1µF capacitor to GND. This modifica-

tion reduces the no-load quiescent current to

approximately 160µA when dropout is reached (Figure

9), but increases the dropout voltage by about 0.1V.

The output voltage 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-

output 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 intend-

ed principally for battery operation. When operating

from AC sources, consider power-supply ripple rejec-

tion. The MAX667’s error amplifier produces very low

gain bandwidth, and the input power-supply rejection

+5V/Programmable Low-Dropout

Voltage Regulator

_______________________________________________________________________________________

7

+5V/Programmable Low-Dropout

Voltage Regulator

VIN (V)

800

0

12

4

6

200

600

35

400

CIRCUIT OF

FIGURE 7

CIRCUIT OF

FIGURE 6

Figure 9. Quiescent Current Below Dropout with Connections

of Figures 6 and 7

VIN-VOUT (V)

400

0

015

100

300

200

10

5

GUARANTEED 250mA

DIP PACKAGE

DISSIPATION LIMIT

SO PACKAGE

DISSIPATION LIMIT

Figure 10. MAX667 Load Current vs. Input-Output Differential

Voltage

Figure 11. Output Response to +4V/100

µs Input Step

+10V

1ms/div

+6V

INPUT

+2V/div

+5V OUTPUT

+0.2V/div