LM4121

SNVS073C – APRIL 2000 – REVISED APRIL 2013

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PIN DESCRIPTIONS

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)

APPLICATION HINTS

The standard application circuit for the LM4121 is shown in Figure 34. The output voltage is set with the two

feedback resistors, according to the following formula:

(1)

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

output voltage.

The LM4121 is designed to be stable with ceramic output capacitors in the range of 0.022µF to 0.047µF. Note

that 0.022µF is the minimum required output capacitor. These capacitors typically have an ESR of about 0.1 to

0.5

Ω. Smaller ESR can be tolerated, however larger ESR can not. The output capacitor can be increased to

improve load transient response, 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 reference (Adj) pin is

required. This capacitor will typically be in the 50pF range. Care must be taken when using output capacitors of

1µF or larger. These application must be thoroughly tested over temperature, line and load. Also, when the

LM4121 is used as a controller, with external active components, each application must be carefully tested to

ensure a stable design. The adjust pin is sensitive to noise and capacitive loading. The trace to this pin must be

as short as possible and the feedback resistors should be close to this pin. Also, a single point ground to the

LM4121 will help ensure 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 entering the LM4121. Larger input capacitors should be tantalum 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 Electrical Characteristics - LM4121-1.250V and

Electrical Characteristics - LM4121-ADJ tables 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.

Figure 34. Standard Application Circuit

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