LP3961_15 Datasheet, PDF (15/31 Pages) Texas Instruments – LP396x 800-mA Fast Ultra-Low-Dropout Linear Regulators

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Typical Applications (continued)
8.2.1 Design Requirements
DESIGN PARAMETERS
Input voltage
Output voltage
Output current
Input capacitor
Output capacitor
ERROR pullup resistor (LP3964 only)
LP3961, LP3964
SNVS056J – MAY 2000 – REVISED JUNE 2015
VALUE
5.3 V, ±10%
3.3 V, ±3%
800 mA (maximum)
68 uF (minimum)
33 uF (minimum)
10 kΩ
8.2.2 Detailed Design Procedure
8.2.2.1 External Capacitors
Like any low-dropout regulator, external capacitors are required to assure stability. these capacitors must be
correctly selected for proper performance.
8.2.2.1.1 Input Capacitor
The LP3961 or LP3964 requires a low source impedance to maintain regulator stability because the internal bias
circuitry is connected directly to the IN pin. The input capacitor must be located less than 1 cm from the LP3961
or LP3964 device and connected directly to the IN and GND pins using traces which have no other currents
flowing through them (see the Layout Guidelines section).
The minimum allowable input capacitance for a given application depends on the type of the capacitor and
equivalent series resistance (ESR). 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 is used:
C_IN ESR (mΩ) / C_IN (µF) ≤ 1.5
(1)
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 the Capacitor Characteristics section).
8.2.2.1.2 Output Capacitor
An output capacitor is also required for loop stability. It must be located less than 1 cm from the LP3961 or
LP3964 device and connected directly to the OUT and GND pins using traces which have no other currents
flowing through them (see the Layout Guidelines section).
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 ESR of the output capacitor 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:
0.2 Ω ≤ C_OUT ESR ≤ 5 Ω
(2)
The lower limit of 200 mΩ means that ceramic capacitors are not suitable for use as LP3961 or LP3964 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 the
Capacitor Characteristics section).
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