LM1084 参数 Datasheet PDF下载



LM1084

To ensure good transient response with heavy load current changes capacitor values on the order of

100µF are used

in the output of many regulators. To further improve stability and transient response of these devices

larger values of output capacitor can be used.

PROTECTION DIODES

Unlike older regulators, the

LM1084

family does not need any protection

diodes between the adjustment pin and the output and from the output to the input

to prevent over-stressing the die.

Internal resistors are limiting the internal current paths on the

LM1084

adjustment pin, therefore even with capacitors on the adjustment pin no protection

diode is needed to ensure device safety under short-circuit conditions.

Diodes between the input and output are not usually needed.

Microsecond surge currents of 50A to 100A can be handled by the internal diode between the input and output pins of the

device. In normal operations it is difficult to get those values of surge currents even with the use of large output

capacitances. If high value output capacitors are used, such as 1000µF to 5000µF and the input pin is instantaneously

shorted to ground, damage can occur. A diode from output to input is recommended, when a crowbar circuit at the input of

the

LM1084

is used. Normal power supply cycling or even plugging and unplugging in the system will not generate

current large enough to do any damage.

The adjustment pin can be driven on a transient basis ±25V, with respect to the output without any device degradation. As

with any IC regulator, none the protection circuitry will be functional and the internal transistors will break down if the

maximum input to output voltage differential is exceeded.

The ripple rejection values are measured with the adjustment pin bypassed. The impedance of the adjust pin capacitor at the

ripple frequency should be less than the value of R1 (normally 100 to 120) for a proper bypassing and ripple rejection

approaching the values shown. The size of the required adjust pin capacitor is a function of the input ripple frequency. If

R1=100

at 120Hz the adjust pin capacitor should be 25µF. At 10kHz only 0.22µF is needed.

The ripple rejection will be a function of output voltage, in circuits without an adjust pin bypass capacitor. The output ripple

will increase directly as a ratio of the output voltage to the reference voltage (V

OUT

/ V

REF

).

LM1084

RIPPLE REJECTION

OUTPUT VOLTAGE

LM1084

AMS1084M

The

LM1084

series develops a 1.25V reference voltage between the output

and the adjust terminal. Placing a resistor between these two terminals causes a

constant current to flow through R1 and down through R2 to set the overall output

voltage.

This current is normally the specified minimum load current of 10mA. Because I

ADJ

is very small and constant it represents a small error and it can usually be ignored.

I

ADJ=

40µA

LOAD REGULATION

LM1084

AMS1084M

True remote load sensing it is not possible to provide, because the

LM1084

is a three terminal device. The resistance of the wire connecting the regulator to the

load will limit the load regulation.

The data sheet specification for load regulation is measured at the bottom of the

package. Negative side sensing is a true Kelvin connection, with the bottom of the

output divider returned to the negative side of the load.

The best load regulation is obtained when the top of the resistor divider R1 is

connected directly to the case not to the load. If R1 were connected to the load, the

effective resistance between the regulator and the load would be:

Connected as shown Fig.3, R



is not multiplied by the divider ratio. Using 16-gauge wire the parasitic line resistance is about

0.004 per foot, translating to 4mV/ft at 1A load current. It is important to keep the positive lead between regulator and load

as short as possible and use large wire or PC board traces.

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