LP38691-ADJ_16 Datasheet, PDF (14/27 Pages) Texas Instruments – 500-mA Low-Dropout CMOS Linear Regulators Stable

LP38691-ADJ, LP38693-ADJ, LP38691-ADJ-Q1, LP38693-ADJ-Q1
SNVS324K – JANUARY 2005 – REVISED JANUARY 2016
www.ti.com
8.2.2 Detailed Design Procedure
8.2.2.1 Setting the Output Voltage
The output voltage is set using the external resistors R1 and R2 (see Typical Applications . The output voltage
will be given by Equation 1:
V_OUT = V_ADJ × (1 + ( R1 / R2 ))
(1)
Because the part has a minimum load current requirement of 100 μA, it is recommended that R2 always be 12
kΩ 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 100 kΩ is recommended for R2 to prevent this from
occurring.
8.2.2.2 External Capacitors
In common with most regulators, the LP3869x-ADJ devices require an external capacitors for regulator stability.
The devices are specifically designed for portable applications requiring minimum board space and smallest
components. These capacitors must be correctly selected for good performance.
8.2.2.3 Input Capacitor
An input capacitor of at least 1 μF 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.
8.2.2.4 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 1 μF. Ceramic capacitors
are recommended; the LP3869x-ADJ devices were designed for use with ultra-low equivalent series resistance
(ESR) capacitors. The LP3869x-ADJ is stable with any output capacitor ESR between 5 mΩ to 500 mΩ.
8.2.2.5 Capacitor Characteristics
It is important that capacitance tolerance and variation with temperature be taken into consideration when
selecting a capacitor so that the minimum required amount of capacitance is provided over the full operating
temperature range.
8.2.2.5.1 Ceramic Capacitors
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.
The LP3869x-ADJ is designed to work with ceramic capacitors on the output to take advantage of the benefits
they offer. For capacitance values in the range of 0.47 µF to 4.7 µF, ceramic capacitors are the smallest, least
expensive and have the lowest ESR values, thus making them best for eliminating high frequency noise. The
ESR of a typical 1-µF ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which easily meets the ESR
requirement for stability for the LP3869x.
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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