MAQ5280 Просмотр технического описания (PDF) - Micrel

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производитель

MAQ5280

25mA, 120V, Low IQ, High PSRR LDO

Micrel 

Micrel, Inc.

Application Information

MAQ5280 is a linear regulator with a wide operating

range of input voltage 4.5V to 120V. The input can go

down to –24V without any damage to the device. This

feature gives the advantage to use in harsh environment

applications such as automotive load dump and remote

keyless entry as well as telecom applications.

Thermal Protection

MAQ5280 has internal thermal shutdown to protect from

over dissipation and failure. When the junction

temperature exceeds +160°C, an internal thermal sensor

signals the shutdown logic to turn off the pass transistor

and allows the IC to cool. The thermal sensor turns the

pass transistor on again after the junction temperature

cools by 10°C. This results in a cycled output during

continuous thermal overload conditions. The recovery

from thermal shutdown is smooth without any overshoot.

For continuous operation, do not exceed the maximum

junction temperature rating of +125°C.

Current Limit

MAQ5280 has a current limit protection to protect it from

over loads. The recovery from current limit protection is

smooth without any overshoots. The output can be

shorted to GND for an indefinite period of time without

damage to the device. During a short circuit, the power

dissipated across the pass transistor can quickly heat

the device. When the die temperature reaches +160°C,

the MAQ5280 shut down and automatically restart after

the die temperature cools by 10°C. This results in a

pulsed output operation.

CBYP Output when Enable is Low

When the Enable is low and the MAQ5280 is disabled,

while VIN voltage is present, the CBYP pin voltage is

typically 4.5V, even for VIN as high as 120V. The Bypass

regulator although maintaining the Bypass voltage,

requires no current from VIN other than leakage current.

Bypass Capacitor

A 10µF capacitor should be connected from bypass pin

to ground to maintain stability of the bypass regulator.

The voltage rating of the capacitor must be 8V or higher.

Low ESR ceramic capacitors work well. Although a 1µF

capacitor maintains good AC power supply rejection,

higher capacitance values will enhance AC power supply

rejection.

MAQ5280

PSRR

Power Supply Rejection Ratio is the measure of a VOUT

signal caused by a signal on the VIN pin. The ratio of

VIN/VOUT in dB is PSRR. The MAQ5280’s PSRR from

0Hz to 1kHz exceeds 80dB.

Output Voltage Setting

For adjustable regulator, VOUT is programmable from

1.22V to 5.0V. A 2.2µF or larger capacitor must be

connected to VOUT pin-to-ground to maintain stability.

This capacitor value can be increased without bound.

The VOUT capacitor can have an ESR of 0Ω. When VIN is

lower than VOUT, VOUT is not pulled low by VIN. A small

current, typically 4µA, will sink into VOUT pin.

The output voltage is controlled by the feedback

resistors (R1 and R2) and can be calculated as follows:

VOUT

= VREF

⋅ ⎜⎛ R1 + 1⎟⎞

⎝ R2 ⎠

VREF = 1.219V

The evaluation board is initially adjusted to 5V, but can

easily be modified by removing R1 and replacing it with

the value that yields the desired output voltage.

R1

=

R2

⋅

⎜⎜⎝⎛

VOUT

VREF

− 1⎟⎟⎠⎞

Ensure the output voltage selected does not exceed 5V.

Enable Pin

The voltage on enable pin EN of MAQ5280 could vary

from –1V to 120V. So enable pin EN of MAQ5280 can

be used in a different configuration as the application

dictates.

If the MAQ5280 needs to be always enabled then simply

connect the EN pin to CBYP pin, as shown in Figure 1.

Then whenever VIN has a supply voltage output is

developed. Input can swing from –24V to 120V.

One can also connect EN pin through 100kΩ resistor to

input as shown in Figure 2 to achieve the same result.

October 2008

10

M9999-102708-B

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