350mA, 16.5V Input,

Low-Dropout Linear Regulators

_______________________________________________________________________________________

7

P-Channel Pass Transistor

The MAX1658/MAX1659 feature an internal P-channel

MOSFET pass transistor. Using a MOSFET provides

several advantages over similar PNP designs, including

lower dropout voltage and extended battery life. Unlike

bipolar transistors, MOSFETs reduce quiescent current,

because they require no base current, particularly at

heavy loads and in dropout. As a result, the

MAX1658/MAX1659 operate at a low quiescent current

even in dropout.

Output Voltage Selection

Dual Mode operation allows the MAX1658/MAX1659 to

operate at either a preset or a user-adjustable output

voltage. The device compares the SET pin voltage with

an internal 65mV reference. If the voltage is lower than

65mV (typically achieved by grounding SET), the

device switches to an internal resistor-divider feedback

network that sets the output voltage. The MAX1658’s

preset output voltage is 3.3V and the MAX1659’s is 5V

(Figure 2).

If the SET pin is not below 65mV, the device switches to

external feedback and SET becomes a feedback input.

The feedback network can be configured to produce

an output between 16V and the voltage reference

(nominally 1.21V). Under regulation, the feedback

mechanism adjusts the error signal such that the volt-

age at the SET pin equals the reference voltage.

Therefore, to achieve the minimum output, connect SET

directly to OUT. For other voltages, a resistive voltage-

divider network is necessary. Figure 3 shows the topol-

ogy of a typical circuit operating in adjustable mode.

The output voltage is set by the following equation:

The input leakage current of the SET input is less than

25nA. This allows the use of large resistors in the feed-

back network to minimize output current loss without

compromising accuracy. R2 can be as high as 500k

Ω

in most applications.

Shutdown

A logic low on the SHDN pin places the MAX1658/

MAX1659 in shutdown. This mode deactivates all func-

tions, including the pass transistor. The device con-

sumes less than 1µA of supply current in shutdown,

and its output becomes high impedance. The

MAX1658/MAX1659 exit shutdown in 100µs.

Output Current Limit

The MAX1658/MAX1659 include current-limiting circuit-

ry that monitors and controls the pass transistor and

limits output current to around 900mA. The output can

be shorted to ground indefinitely without damaging the

device.

R1 = R2

V

V

1

OUT

SET

−









V

V

R

R

OUT

SET

=

+









1

1

2

MAX1658

(MAX1659)

SHDN

2

4, 5

8

1

3, 6, 7

IN

INPUT

VOLTAGE

ON

OFF

OUTPUT VOLTAGE

3.3V/350mA

(5V/350mA)

10

µF

0.1

µF

GND

SET

OUT

Figure 2. Preset Output Configuration

MAX1658

MAX1659

IN

3, 6, 7

4, 5

1

8

2

SHDN

OUTPUT

VOLTAGE

10

µF

R1

R2

0.1

µF

INPUT

VOLTAGE

GND

OUT

SET

R2

Figure 3. Adjustable Output Configuration Using External

Feedback Resistors