Internal p-Channel Pass Transistor

The MAX1793 features a 1A p-channel MOSFET pass

transistor. Unlike similar designs using pnp pass tran-

sistors, p-channel MOSFETs require no base drive,

which reduces quiescent current. pnp-based regula-

tors also waste considerable current in dropout when

the pass transistor saturates and use high base-drive

currents under large loads. The MAX1793 does not suf-

fer from these problems and consumes only 225µA of

quiescent current under heavy loads, as well as in

dropout.

Output Voltage Selection

The MAX1793 features Dual Mode operation. In preset

voltage mode, the output of the MAX1793 is set to an

internally trimmed voltage (see the

Ordering

Information). Select this mode by connecting SET to

GND (Figure 2).

In adjustable mode, an output is selected between

+1.25V and +5V using two external resistors connected

as a voltage-divider to SET (Figure 3). The output volt-

age is determined by the following equation:

Since the input bias current at SET is less than 100nA,

large resistance values can be used for R1 and R2 to

minimize power consumption without losing efficiency.

Up to 125k

Ω is acceptable for R2.

In preset voltage mode, the impedance from SET to

GND should be less than 10k

Ω. Otherwise, spurious

conditions could cause the voltage at SET to exceed

the 50mV dual-mode threshold.

Shutdown

A logic-low on SHDN disables the MAX1793. In shut-

down, the pass transistor, control circuitry, reference,

and all internal circuits are turned off, reducing supply

current to typically 0.1µA. Connect SHDN to IN for nor-

mal operation. In shutdown, RST goes low and OUT is

discharged to GND with a 5k

Ω internal resistance.

RST Comparator

when the output voltage falls out of regulation. The

open-drain RST output goes low when OUT falls 6%

below its nominal output voltage. RST remains low for

4ms after OUT has returned to its nominal value. A

100k

Ω pullup resistor from RST to a suitable logic sup-

ply voltage (typically OUT) provides a logic control sig-

nal. RST can be used as a power-on-reset (POR) signal

to a microcontroller or can drive an external LED to indi-

cate power failure. RST is low during shutdown.

Current Limiting

The MAX1793 features a current limit that monitors the

pass transistor, typically limiting short-circuit output

current to 1.8A. The current limit is increased to

approximately 2.8A when the output voltage is in regu-

lation, improving performance with large transient

loads. The output can be shorted to ground for an

indefinite period of time without damaging the device.

Thermal-Overload Protection

Thermal-overload protection limits total power dissipa-

tion in the MAX1793. When the junction temperature

pass transistor, allowing the IC to cool. The thermal sen-

sor turns the pass transistor on once the IC’s junction

temperature drops approximately 20°C. Continuous

short-circuit conditions will result in a pulsed output.

8

_______________________________________________________________________________________

NC

MAX1793

IN

IN

IN

IN

IN

OFF

ON

µF

SHDN

OUT

OUT

OUT

OUT

RST

SET

GND

µF

OUT

RESET

OUTPUT

Figure 2. Typical Operating Circuit with Preset Output Voltage

NC

MAX1793

IN

IN

IN

IN

IN

OFF

ON

µF

SHDN

OUT

OUT

OUT

OUT

RST

SET

GND

R2

R1

µF

OUT

RESET

OUTPUT

Figure 3. Typical Operating Circuit with Adjustable Output

Voltage