Datasheet

Detailed Description
The MAX1857 is a low-dropout, low-quiescent-current
ripple rejector designed primarily for audio and video
applications. The device supplies loads up to 500mA
and is available with a preset output voltage of 4.75V.
As shown in Figure 1, the MAX1857 consists of a 1.25V
reference, error amplifier, P-channel pass transistor,
and internal feedback voltage-divider.
The 1.25V reference is connected to the error amplifier,
which compares this reference with the feedback volt-
age and amplifies the difference. If the feedback volt-
age is lower than the reference voltage, the
pass-transistor gate is pulled lower, which allows more
current to pass to the output and increases the output
voltage. If the feedback voltage is too high, the pass-
transistor gate is pulled up, allowing less current to
pass to the output.
The output voltage is fed back through either an inter-
nal resistive divider connected to OUT or an external
resistor network connected to SET. The dual-mode
comparator examines V
SET
and selects the feedback
path. If V
SET
is below 50mV, the internal feedback path
is used and the output is regulated to the factory-preset
voltage.
Additional blocks include an output current limiter,
reset comparator, thermal sensor, and shutdown logic.
Internal P-Channel Pass Transistor
The MAX1857 features a 0.25Ω P-channel MOSFET
pass transistor. Unlike similar designs using PNP pass
transistors, 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 MAX1857 does not suf-
fer from these problems and consumes only 175µA of
quiescent current under heavy loads as well as in
dropout.
Output Voltage Selection
The MAX1857’s dual-mode operation allows operation
in either a preset voltage mode or an adjustable mode.
Connect SET to GND to select the preset output volt-
age. The two-digit part number suffix identifies the out-
put voltage (see Selector Guide). For example, the
MAX1857EUA47 has a preset 4.75V output voltage.
The output voltage may also be adjusted by connecting
a voltage-divider from OUT to SET (Figure 2). Select R2
in the 25kΩ to 100kΩ range. Calculate R1 with the fol-
lowing equation:
R1 = R2 [(V
OUT
/ V
SET
) – 1]
where V
SET
= 1.25V, and V
OUT
may range from 1.25V
to 5.0V.
Shutdown
Pull SHDN low to enter shutdown. During shutdown, the
output is disconnected from the input and supply current
drops to 0.1µA. When in shutdown, RST pulls low and
OUT is discharged through an internal 5kΩ resistor. The
capacitance and load at OUT determine the rate at
which V
OUT
decays. SHDN can be pulled as high as 6V,
regardless of the input and output voltage.
Reset Output
The reset output (RST) pulls low when OUT is less than
86% of the nominal regulation voltage. Once OUT
exceeds 86% of the nominal voltage, RST goes high
impedance after 4.5ms. RST is an open-drain N-channel
output. To obtain a voltage at RST, connect a pullup
resistor from RST to OUT. A 100kΩ resistor works well for
most applications. RST can be used as a power-on-reset
(POR) signal to a microcontroller (µC), or drive an exter-
nal LED to indicate power failure. When the MAX1857 is
MAX1857
500mA, Low-Dropout,
Ripple-Rejecting LDO in µMAX
_______________________________________________________________________________________ 7
OUT
OUT
SET
GND
IN
IN
SHDN
RST
C
OUT
3.3
µ
F
V
OUT
V
OUT
C
IN
1
µ
F
ON
OFF
V
IN
= +2.5V TO +5.5V
R
2
R1 = R2
- 1
1.25V
MAX1857
R
1
Figure 2. Adjustable Output Using External Feedback
Resistors