Pin Functions

Output (Pin 5): Reference Output.

Input (Pin 4):Positive Supply.

Ground (Pin 2):Negative Supply or Ground Connection.

Enable (Pin 3):Pulled to input for normal operation. Forcing

this pin to ground will turn-off the output.

REF (Pin 1):REF Pin. This pin should be left unconnected.

Application Hints

The standard application circuit for the LM4120 is shown in

Figure 1. It is designed to be stable with ceramic output ca-

pacitors in the range of 0.022µF to 0.047µF. Note that

0.022µF is the minimum required output capacitor. These ca-

pacitors typically have an ESR of about 0.1 to 0.5

Ω. Smaller

ESR can be tolerated, however larger ESR can not. The out-

put capacitor can be increased to improve load transient re-

sponse, up to about 1µF. However, values above 0.047µF

must be tantalum. With tantalum capacitors, in the 1µF

range, a small capacitor between the output and the refer-

ence pin is required. This capacitor will typically be in the

50pF range. Care must be taken when using output capaci-

tors of 1µF or larger. These application must be thoroughly

tested over temperature, line and load.

An input capacitor is typically not required. However, a 0.1µF

ceramic can be used to help prevent line transients from en-

tering the LM4120. Larger input capacitors should be tanta-

lum or aluminium.

The reference pin is sensitive to noise, and capacitive load-

ing. Therefore, the PCB layout should isolate this pin as

much as possible.

The enable pin is an analog input with very little hysteresis.

About 6µA into this pin is required to turn the part on, and it

must be taken close to GND to turn the part off (see spec.

table for thresholds). There is a

minimum slew rate on this

pin of about 0.003V/µS to prevent glitches on the output. All

of these conditions can easily be met with ordinary CMOS or

TTL logic. If the shutdown feature is not required, then this

pin can safely be connected directly to the input supply.

Floating this pin is not recommended.

Input Capacitor

Noise on the power-supply input can effect the output noise,

but can be reduced by using an optional bypass capacitor

between the input pin and the ground.

Printed Circuit Board Layout Consideration

The mechanical stress due to PC board mounting can cause

the output voltage to shift from its initial value. References in

SOT packages are generally less prone to assembly stress

than devices in Small Outline (SOIC) package.

To reduce the stress-related output voltage shifts, mount the

reference on the low flex areas of the PC board such as near

to the edge or the corner of the PC board.

DS101047-32

FIGURE 1.

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