Datasheet

LP3961, LP3964
SNVS056H –MAY 2000–REVISED APRIL 2013 www.ti.com
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: The LP3961/4 requires a low source impedance to maintain regulator stability because the
internal bias circuitry is connected directly to V
IN
. The input capacitor must be located less than 1 cm from the
LP3961/4 device and connected directly to the input and ground pins using traces which have no other currents
flowing through them (see PCB Layout).
The minimum allowable input capacitance for a given application depends on the type of the capacitor and ESR
(equivalent series resistance). A lower ESR capacitor allows the use of less capacitance, while higher ESR types
(like aluminum electrolytics) require more capacitance.
The lowest value of input capacitance that can be used for stable full-load operation is 68 µF (assuming it is a
ceramic or low-ESR Tantalum with ESR less than 100 mΩ).
To determine the minimum input capacitance amount and ESR value, an approximation which should be used is:
C
IN
ESR (mΩ) / C
IN
(µF) ≤ 1.5
This shows that input capacitors with higher ESR values can be used if sufficient total capacitance is provided.
Capacitor types (aluminum, ceramic, and tantalum) can be mixed in parallel, but the total equivalent input
capacitance/ESR must be defined as above to assure stable operation.
IMPORTANT: The input capacitor must maintain its ESR and capacitance in the "stable range" over the entire
temperature range of the application to assure stability (see Capacitor Characteristics).
OUTPUT CAPACITOR: An output capacitor is also required for loop stability. It must be located less than 1 cm
from the LP3961/4 device and connected directly to the output and ground pins using traces which have no other
currents flowing through them (see PCB Layout).
The minimum value of the output capacitance that can be used for stable full-load operation is 33 µF, but it may
be increased without limit. The output capacitor's ESR is critical because it forms a zero to provide phase lead
which is required for loop stability. The ESR must fall within the specified range:
0.2Ω ≤ C
OUT
ESR ≤ 5Ω
The lower limit of 200 mΩ means that ceramic capacitors are not suitable for use as LP3961/4 output capacitors
(but can be used on the input). Some ceramic capacitance can be used on the output if the total equivalent ESR
is in the stable range: when using a 100 µF Tantalum as the output capacitor, approximately 3 µF of ceramic
capacitance can be applied before stability becomes marginal.
IMPORTANT: The output capacitor must meet the requirements for minimum amount of capacitance and also
have an appropriate ESR value over the full temperature range of the application to assure stability (see
Capacitor Characteristics).
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. In general, a good Tantalum capacitor will show very little capacitance variation
with temperature, but a ceramic may not be as good (depending on dielectric type). Aluminum electrolytics also
typically have large temperature variation of capacitance value.
Equally important to consider is a capacitor's ESR change with temperature: this is not an issue with ceramics,
as their ESR is extremely low. However, it is very important in Tantalum and aluminum electrolytic capacitors.
Both show increasing ESR at colder temperatures, but the increase in aluminum electrolytic capacitors is so
severe they may not be feasible for some applications (see Capacitor Characteristics).
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