350mA, 16.5V Input,
Low-Dropout Linear Regulators
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.
ogy of a typical circuit operating in adjustable mode.
The output voltage is set by the following equation:
R1
V
OUT
=
V
SET
1
+
R2
where V
SET
= 1.21V. Solving for R1 yields:
V
R1 = R2
OUT
−
1
V
SET
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 500kΩ
in most applications.
MAX1658/MAX1659
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-
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.
ON
OFF
2
SHDN
OUT
4, 5
OUTPUT VOLTAGE
3.3V/350mA
(5V/350mA)
INPUT
VOLTAGE
3, 6, 7
IN
MAX1658
MAX1659
OUT
4, 5
OUTPUT
VOLTAGE
INPUT
VOLTAGE
3, 6, 7
MAX1658
(MAX1659)
IN
10µF
0.1µF
GND
8
SET
1
2
SHDN
SET
1
R1
C
OUT
10µF
R2
GND
8
0.1µF
V
OUT
= V
SET
1 + R1
R2
V
SET
= 1.21V
(
)
Figure 2. Preset Output Configuration
Figure 3. Adjustable Output Configuration Using External
Feedback Resistors
7
_______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
