MAX8940EXKXY-T Просмотр технического описания (PDF) - Maxim Integrated

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MAX8940EXKXY-T

Low-Noise, High PSRR, Low-Dropout, 120mA Linear Regulator

Maxim Integrated 

Low-Noise, High PSRR,

Low-Dropout, 120mA Linear Regulator

the temperature difference between the die junction

and ambient, and the rate of airflow. The power dissi-

pation across the device is:

P = IOUT (VIN - VOUT)

The maximum power dissipation is:

PMAX = (TJ - TA)/(θJC + θCA)

where TJ - TA is the temperature difference between

the MAX8940 die junction and the surrounding air, θJC

is the thermal resistance of the package, and θCA is the

thermal resistance through the PC board, copper

traces, and other materials to the surrounding air.

The GND pin of the MAX8940 performs the dual func-

tion of providing an electrical connection to ground and

channeling heat away. Connect the GND pin to ground

using a large pad or ground plane.

Noise Reduction

For the MAX8940, an external 0.01µF bypass capacitor

between BP and OUT with innovative noise bypass

scheme reduces output noises dramatically, exhibiting

25µVRMS of output voltage noise with CBP = 0.01µF

and COUT = 1µF. Startup time is minimized by a power-

on circuit that precharges the bypass capacitor.

Applications Information

Capacitor Selection

and Regulator Stability

Use a 1µF capacitor on the MAX8940 input and a 1µF

capacitor on the output. Larger input capacitor values

and lower ESRs provide better noise rejection and line-

transient response. Reduce output noise and improve

load-transient response, stability, and power-supply

rejection by using large output capacitors. Note that

some ceramic dielectrics exhibit large capacitance and

ESR variation with temperature. With dielectrics such as

Z5U and Y5V, it may be necessary to use a 2.2µF or

larger output capacitor to ensure stability at tempera-

tures below -10°C. With X7R or X5R dielectrics, 1µF is

sufficient at all operating temperatures. A graph of the

region of stable COUT ESR vs. load current is shown in

the Typical Operating Characteristics.

Use a 0.01µF bypass capacitor at BP for low-output volt-

age noise. The leakage current going into the BP pin

should be less than 10nA. Increasing the capacitance

slightly decreases the output noise. Values above 0.1µF

and below 0.001µF are not recommended.

Noise, PSRR, and Transient Response

The MAX8940 is designed to deliver low noise and high

PSRR, as well as low dropout and low quiescent cur-

rents in battery-powered systems. The MAX8940

power-supply rejection is 65dB at 1kHz.

When operating from sources other than batteries,

improved supply-noise rejection and transient response

can be achieved by increasing the values of the input

and output bypass capacitors, and through passive fil-

tering techniques. The Typical Operating Characteristics

show the MAX8940 line- and load-transient responses.

Dropout Voltage

A regulator’s minimum dropout voltage determines the

lowest usable supply voltage. In battery-powered sys-

tems, this determines the useful end-of-life battery volt-

age. Because the MAX8940 uses a p-channel MOSFET

pass transistor, the dropout voltage is a function of drain-

to-source on-resistance (RDS(ON)) multiplied by the

load current (see the Typical Operating Characteristics).

Chip Information

TRANSISTOR COUNT: 284

PROCESS: BiCMOS

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