5V/3.3V or Adjustable, Low-Dropout,
Low I
Q
, 500mA Linear Regulators
MAX603/MAX604
IN
SHUTDOWN
MOSFET DRIVER
WITH FOLDBACK
CURRENT LIMIT
REVERSE
CURRENT
PROTECTION
P
ERROR AMP
OFF
SHUTDOWN
LOGIC
OUT
SET
R1
1.20V
REFERENCE
THERMAL
SENSOR
GND
DUAL-MODE
COMPARATOR
80mV
R2
MAX603
MAX604
Figure 2. Functional Diagram
_______________Detailed Description
The MAX603/MAX604 are low-dropout, low-quiescent-
current linear regulators designed primarily for battery-
powered applications. They supply an adjustable 1.25V
to 11V output or a preselected 5V (MAX603) or 3.3V
(MAX604) output for load currents up to 500mA. As
illustrated in Figure 2, they consist of a 1.20V reference,
error amplifier, MOSFET driver, P-channel pass transis-
tor, dual-mode comparator, and internal feedback volt-
age divider.
The 1.20V bandgap reference is connected to the error
amplifier’s inverting input. The error amplifier compares
this reference with the selected feedback voltage and
amplifies the difference. The MOSFET driver reads the
error signal and applies the appropriate drive to the P-
channel pass transistor. If the feedback voltage is lower
than the reference, the pass transistor gate is pulled
lower, allowing more current to pass and increasing 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 internal
resistor voltage divider connected to the OUT pin, or an
external resistor network connected to the SET pin. The
dual-mode comparator examines the SET voltage and
selects the feedback path used. If SET is below 80mV,
internal feedback is used and the output voltage is regulat-
ed to 5V for the MAX603 or 3.3V for the MAX604.
Additional blocks include a foldback current limiter, reverse
current protection, thermal sensor, and shutdown logic.
Internal P-Channel Pass Transistor
The MAX603/MAX604 feature a 500mA P-channel
MOSFET pass transistor. This provides several advan-
tages over similar designs using PNP pass transistors,
including longer battery life.
The P-channel MOSFET requires no base drive, which
reduces quiescent current considerably. PNP based
regulators waste considerable amounts of current in
dropout when the pass transistor saturates. They also
use high base-drive currents under large loads. The
MAX603/MAX604 do not suffer from these problems
and consume only 15µA of quiescent current under
light and heavy loads, as well as in dropout.
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