Rainbow Electronics MAX8842 - Manual (Page 6)

MAX8840/MAX8841/MAX8842
Ultra-Low-Noise, High PSRR, Low-Dropout,
150mA Linear Regulators in µDFN
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Detailed Description
The MAX8840/MAX8841/MAX8842 are ultra-low-noise,
low-dropout, low-quiescent current linear regulators
designed for space-restricted applications. The parts
are available with preset output voltages ranging from
1.5V to 4.5V. These devices can supply loads up to
150mA. As shown in the Functional Diagram, the
MAX8840/MAX8841 consist of an innovative bandgap
core and noise bypass circuit, error amplifier, p-chan-
nel pass transistor, and internal feedback voltage-
divider. The MAX8842 allows for adjustable output with
an external feedback network.
The 1.225V bandgap reference is connected to the
error amplifier’s inverting input. The error amplifier com-
pares this reference with the feedback voltage and
amplifies the difference. If the feedback voltage is
lower than the reference voltage, the pass-transistor
gate is pulled low. This allows more current to pass to
the output and increases the output voltage. If the feed-
back voltage is too high, the pass transistor gate is
pulled high, allowing less current to pass to the output.
The output voltage is fed back through an internal resis-
tor voltage-divider connected to the OUT pin.
An external bypass capacitor connected to BP
(MAX8840) reduces noise at the output. Additional
blocks include a current limiter, thermal sensor, and
shutdown logic.
Internal p-Channel Pass Transistor
The MAX8840/MAX8841/MAX8842 feature a 1 Ω (typ)
p-channel MOSFET pass transistor. This provides seve-
ral advantages over similar designs using a pnp pass
transistor, including longer battery life. The p-channel
MOSFET requires no base drive, which considerably
reduces quiescent current. PNP-based regulators waste
considerable current in dropout when the pass transis-
tor saturates. They also use high base-drive current
under heavy loads. The MAX8840/MAX8841/MAX8842
do not suffer from these problems and consume only
40µA of quiescent current in light load and 220µA in
dropout (see the Typical Operating Characteristics).
Output Voltage Selection
The MAX8840/MAX8841 are supplied with factory-set
output voltages from 1.5V to 4.5V, in 100mV increments
(see the Ordering Information). The MAX8842 features
a user-adjustable output through an external feedback
network (see the Typical Operating Circuits).
To set the output of the MAX8842, use the following
equation:
where R2 is chosen to be less than 240k Ω and V
REF =
1.225V. Use 1% or better resistors.
Shutdown
The MAX8840/MAX8841/MAX8842 feature a low-power
shutdown mode that reduces quiescent current less
than 1µA. Driving SHDN low disables the voltage refer-
ence, error amplifier, gate-drive circuitry, and pass
transistor (see the Functional Diagram), and the device
output enters a high-impedance state. Connect SHDN
to IN for normal operation.
Current Limit
The MAX8840/MAX8841/MAX8842 include a current
limiter, which monitors and controls the pass transis-
tor’s gate voltage, limiting the output current to 200mA.
R R X
V
V
OUT
REF
1 2 1 =
⎛
⎝⎜ ⎞
⎠⎟
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Pin Description
PIN
MAX8840 MAX8841 MAX8842
NAME FUNCTION
1 1 1 IN Unregulated Input Supply
2 2 2 GND Ground
3 3 3 SHDN Shutdown. Pull low to disable the regulator.
4 — — BP
Noise Bypass for Low-Noise Operation. Connect a 10nF capacitor
from BP to OUT. It is shorted to OUT in shutdown mode.
— — 4 FB Adjustable Output Feedback Point
5 4, 5 5 N.C. No Connection. Not internally connected.
6 6 6 OUT
Regulated Output Voltage. Bypass with a capacitor to GND. See the
Capacitor Selection and Regulator Stability section.