Datasheet

LP38691-ADJ, LP38693-ADJ
www.ti.com SNVS324I –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-ADJ was designed for use with ultra low ESR capacitors). The LP38691/3-ADJ is
stable with any output capacitor ESR between zero and 100 Ohms.
SETTING THE OUTPUT VOLTAGE:
The output voltage is set using the external resistors R1 and R2 (see Typical Application Circuits). The output
voltage will be given by the equation:
V
OUT
= V
ADJ
x (1 + ( R1 / R2 ) ) (1)
Because the part has a minimum load current requirement of 100 µA, it is recommended that R2 always be 12k
Ohms or less to provide adequate loading. Even if a minimum load is always provided by other means, it is not
recommended that very high value resistors be used for R1 and R2 because it can make the ADJ node
susceptible to noise pickup. A maximum Ohmic value of 100k is recommended for R2 to prevent this from
occurring.
ENABLE PIN (LP38693-ADJ only):
The LP38693–ADJ 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-ADJ 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.
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