LP38691, LP38693

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SNVS321K – JANUARY 2005 – REVISED APRIL 2013

APPLICATION HINTS

EXTERNAL CAPACITORS

Like any low-dropout regulator, external capacitors are required to assure stability. These capacitors must be

correctly selected for proper performance.

INPUT CAPACITOR:

An input capacitor of at least 1µF is required (ceramic recommended). The capacitor must be located not more

than one centimeter from the input pin and returned to a clean analog ground.

OUTPUT CAPACITOR:

An output capacitor is required for loop stability. It must be located less than 1 centimeter from the device and

connected directly to the output and ground pins using traces which have no other currents flowing through them.

The minimum amount of output capacitance that can be used for stable operation is 1µF. Ceramic capacitors are

recommended (the LP38691/3 was designed for use with ultra low ESR capacitors). The LP38691/3 is stable

with any output capacitor ESR between zero and 100 Ohms.

ENABLE PIN (LP38693 only):

The LP38693 has an Enable pin (EN) which allows an external control signal to turn the regulator output On and

Off. The Enable On/Off threshold has no hysteresis. The voltage signal must rise and fall cleanly, and promptly,

through the ON and OFF voltage thresholds. The Enable pin has no internal pull-up or pull-down to establish a

default condition and, as a result, this pin must be terminated either actively or passively. If the Enable pin is

driven from a source that actively pulls high and low, the drive voltage should not be allowed to go below ground

Foldback Current Limiting:

Foldback current limiting is built into the LP38691/3 which reduces the amount of output current the part can

deliver as the output voltage is reduced. The amount of load current is dependent on the differential voltage

SELECTING A CAPACITOR

It is important to note that capacitance tolerance and variation with temperature must be taken into consideration

when selecting a capacitor so that the minimum required amount of capacitance is provided over the full

operating temperature range.

Capacitor Characteristics

CERAMIC

For values of capacitance in the 10 to 100 µF range, ceramics are usually larger and more costly than tantalums

but give superior AC performance for bypassing high frequency noise because of very low ESR (typically less

than 10 m

Ω). However, some dielectric types do not have good capacitance characteristics as a function of

voltage and temperature.

Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or

Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V

also exhibit a severe temperature effect, losing more than 50% of nominal capacitance at high and low limits of

the temperature range.

X7R and X5R dielectric ceramic capacitors are strongly recommended if ceramics are used, as they typically

maintain a capacitance range within ±20% of nominal over full operating ratings of temperature and voltage. Of

course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance.

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Product Folder Links: LP38691 LP38693