Datasheet

MAX1793
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:
V
OUT
= V
SET
[1 + (R1/R2)]
where V
SET
= +1.25V. To simplify resistor selection:
R1 = R2 [(V
OUT
/V
SET
) – 1]
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 125kΩ is acceptable for R2.
In preset voltage mode, the impedance from SET to
GND should be less than 10kΩ. 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 5kΩ internal resistance.
RST Comparator
The MAX1793 features a low V
OUT
indicator that asserts
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
100kΩ 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
exceeds T
J
= +170°C, a thermal sensor turns off the
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.
Low-Dropout, Low I
Q
, 1A Linear Regulator
8 _______________________________________________________________________________________
NC
MAX1793
IN
IN
IN
IN
IN
OFF
ON
C
IN
= 4.7µF
SHDN
OUT
OUT
OUT
OUT
RST
SET
GND
C
OUT
= 6.8µF
OUT
RESET
OUTPUT
Figure 2. Typical Operating Circuit with Preset Output Voltage
NC
MAX1793
IN
IN
IN
IN
IN
OFF
ON
C
IN
= 4.7µF
SHDN
OUT
OUT
OUT
OUT
RST
SET
GND
R2
R1
C
OUT
= 6.8µF
OUT
RESET
OUTPUT
Figure 3. Typical Operating Circuit with Adjustable Output
Voltage