MAX8940EXKXY-T Просмотр технического описания (PDF) - Maxim Integrated
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MAX8940EXKXY-T Datasheet PDF : 9 Pages
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Low-Noise, High PSRR,
Low-Dropout, 120mA Linear Regulator
Pin Description
PIN
NAME
FUNCTION
1
IN
Unregulated Input Supply
2
GND
Ground
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 short to OUT
in shutdown mode.
5
OUT
Regulated Output Voltage. Bypass with a capacitor to GND. See the Capacitor Selection and Regulator
Stability section for more details.
Detailed Description
The MAX8940 is a low-noise, low-dropout, low-quies-
cent current linear regulator designed for space-
restricted applications. The part is available with preset
output voltages 2.8V or 3.0V. This device can supply
loads up to 120mA. As shown in the Functional
Diagram, the MAX8940 consist of an innovative
bandgap core and noise bypass circuit, error amplifier,
p-channel pass transistor, and internal feedback volt-
age-divider.
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 reduces
noise at the output. Additional blocks include a current
limiter, thermal sensor, and shutdown logic.
Internal P-Channel Pass Transistor
The MAX8940 features a 1Ω (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 cur-
rent. PNP-based regulators waste considerable current
in dropout when the pass transistor saturates. They
also use high base-drive current under heavy loads.
The MAX8940 does not suffer from these problems and
consume only 40µA of quiescent current in light load
and 220µA in dropout (see the Typical Operating
Characteristics).
Shutdown
The MAX8940 features a low-power shutdown mode that
reduces quiescent current less than 1µA. Driving SHDN
low disables the voltage reference, error amplifier, gate-
drive circuitry, and pass transistor (see the Functional
Diagram), and the device output enters a high-imped-
ance state. Connect SHDN to IN for normal operation.
Current Limit
The MAX8940 includes a current limiter, which monitors
and controls the pass transistor’s gate voltage, limiting
the output current to 200mA. For design purposes, con-
sider the current limit to be 130mA (min) to 300mA (max).
The output can be shorted to ground for an indefinite
amount of time without damaging the part.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation
in the MAX8940. When the junction temperature exceeds
TJ = +160°C, the thermal sensor signals the shutdown
logic, turning off the pass transistor and allowing the IC
to cool down. The thermal sensor turns the pass transis-
tor on again after the IC’s junction temperature drops by
10°C, resulting in a pulsed output during continuous
thermal-overload conditions.
Thermal-overload protection is designed to protect the
MAX8940 in the event of a fault condition. For continual
operation, do not exceed the absolute maximum junc-
tion temperature rating of TJ = +150°C.
Operating Region and Power Dissipation
The MAX8940 maximum power dissipation depends on
the thermal resistance of the case and circuit board,
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