Low Dropout Regulators—Why the Choice of Bypass Capacitor Matters. Part 2

Selecting Capacitors for LDO Circuits Output Capacitor

Low-dropout regulators (LDOs) from Analog Devices can operate with small, space-saving ceramic capacitors as long as they have low effective series resistance (ESR); the ESR of the output capacitor affects the stability of the LDO control loop. A minimum capacitance of 1 µF with a maximum ESR of 1 Ω is recommended to ensure stability.

The output capacitance also affects the regulator's response to changes in load current. The control loop has finite large-signal bandwidth, so the output capacitor must supply most of the load current for very fast transients. When the load current switches from 1 mA to 200 mA at 500 mA/µs, a 1-µF capacitor, unable to supply enough current, produces a load transient of about 80 mV, as shown in Figure 1. Increasing the capacitance to 10 µF reduces the load transient to about 70 mV, as shown in Figure 2. Increasing the output capacitance further, to 20 µF, allows the regulator control loop to track, actively reducing the load transient as shown in Figure 3. These examples use the ADP151 linear regulator with a 5-V input and a 3.3-V output.

 Transient response with COUT = 1 µF
 Figure 1. Transient response with COUT = 1 µF.

Transient response with COUT = 10 µF
 Figure 2. Transient response with COUT = 10 µF.

Transient response with COUT = 20 µF
 Figure 3. Transient response with COUT = 20 µF.
Input Bypass Capacitor

Connecting a 1 µF capacitor from VIN to GND reduces the circuit's sensitivity to printed circuit board (PCB) layout, especially when long input traces or high source impedance are encountered. Increase the input capacitance to match the output capacitance when more than 1 µF is required on the output.

Input and Output Capacitor Properties

The input and output capacitors must meet the minimum capacitance requirement at the intended operating temperature and working voltage. Ceramic capacitors are available with a variety of dielectrics, each with different behavior vs. temperature and voltage. X5R or X7R dielectrics with a 6.3-V to 10-V voltage rating are recommended for 5-V applications. Y5V and Z5U dielectrics have poor characteristics vs. temperature and dc bias, so they are not suitable for use with LDOs.

Figure 4 shows the capacitance vs. bias voltage characteristic of a 1-µF, 10-V X5R capacitor in a 0402 package. The capacitor's package size and voltage rating strongly influence its voltage stability. In general, a larger package or higher voltage rating will provide better voltage stability. The temperature variation of the X5R dielectric is ±15% over the –40°C to +85°C temperature range and is not a function of package or voltage rating.

Capacitance vs. voltage characteristic
 Figure 4. Capacitance vs. voltage characteristic.

To determine the worst-case capacitance over temperature, component tolerance, and voltage, scale the nominal capacitance by the temperature variation and tolerance, as shown in Equation 1:

  CEFF = CBIAS × (1 – TVAR) × (1 – TOL) (1)

Where

CBIAS is the nominal capacitance at the operating voltage;
TVAR is the worst-case capacitance variation over temperature (as a fraction of 1);
TOL is the worst-case component tolerance (as a fraction of 1).

In this example,

TVAR is 15% from –40°C to +85°C for an X5R dielectric;
TOL is 10%;
CBIAS is 0.94 µF at 1.8 V, as shown in Figure 4.

Using these values in Equation 1 yields:

  CEFF = 0.94 µF × (1 – 0.15) × (1 – 0.1) = 0.719 µF  

The ADP151 specifies a minimum output bypass capacitance of 0.70 µF over the operating voltage and temperature range, so this capacitor meets this requirement.

Conclusion

To guarantee the performance of an LDO, the effects of dc bias, temperature variation, and tolerance of the bypass capacitor must be understood and evaluated. In applications that require low noise, low drift, or high signal integrity, the capacitor technology must also be considered. All capacitors suffer from the effects of nonideal behavior, so the capacitor technology chosen must match the needs of the application.

Appendix

Capacitors commonly used for bypassing power supply rails
 Figure A. Capacitors commonly used for bypassing power supply rails.

Clockwise from top, scale in millimeters:

100-µF/6.3-V polymer solid aluminum capacitor
1-µF/35-V and 10-µF/25-V solid tantalum capacitor
1-µF/25-V, 4.7-µF/16-V, 10-µF/25-V multilayer ceramic capacitor
10-µF/16-V, 22-µF/25-V aluminum electrolytic capacitor

Comparison of Critical Parameters of Various Capacitor Technologies

Capacitor
Technology
Effective
Series
Resistance
Effective
Series
Inductance
Voltage
Stability
Temperature
Stability
Sensitivity
to
Vibration
Capacitance/
Unit
Volume
Aluminum Electrolytic
Highest
Highest
Good
Lowest
Low
Low
Solid Tantalum
Medium
Medium
Best
Good
Low
High
Polymer Solid Aluminum
Low
Low
Best
Good
Low
High
Multilayer Ceramic
Lowest
Lowest
Poor
Good
High
Medium

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