LP38691SD-3.3 Datasheet, PDF (11/27 Pages) Texas Instruments – LP38693 500mA Low Dropout CMOS Linear Regulators Stable with Ceramic Output Capacitors

LP38691, LP38693
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APPLICATION HINTS
SNVS321K – JANUARY 2005 – REVISED APRIL 2013
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
potential or higher than V_IN. If the application does not require the Enable function, the pin should be connected
directly to the V_IN pin.
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
between V_IN and V_OUT. Typically, when this differential voltage exceeds 5V, the load current will limit at about 350
mA. When the V_IN - V_OUT differential is reduced below 4V, load current is limited to about 850 mA.
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 mΩ). 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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