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 (1.25V option only). This pin should be

left unconnected for 1.25V option.

Adj (Pin 1):V

cation Hints section.

Application Hints

The standard application circuit for the LM4121 is shown in

Figure 1. The output voltage is set with the two feedback re-

sistors, according to the following formula:

V

Values for R1 and R2 should be chosen to be less than 1

M

Ω.I

and I

table. For best accuracy, be sure to take into account the

variation of V

The LM4121 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 (Adj) pin is required. This capacitor will typically be in

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

pacitors of 1µF or larger. These application must be thor-

oughly tested over temperature, line and load. Also, when

the LM4121 is used as a controller, with external active com-

ponents, each application must be carefully tested to ensure

a stable design. The adjust pin is sensitive to noise and ca-

pacitive loading. The trace to this pin must be as short as

possible and the feedback resistiors should be close to this

pin. Also, a single point ground to the LM4121 will help en-

sure good accuracy at high load currents.

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

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

tering the LM4121. Larger input capacitors should be tanta-

lum or aluminium.

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.

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.

DS101291-32

DS101291-43

FIGURE 1. Standard Application Circuit

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