Data Sheet

ADM7151

Rev. B | Page 17 of 24

BYP Capacitor

The BYP capacitor is necessary to filter the reference buffer. A

1 µF capacitor is typically connected between BYP and GND.

Capacitors as small as 0.1 µF can be used; however, the output

noise voltage of the LDO increases as a result.

In addition, the BYP capacitor can be increased to reduce the

noise below 1 kHz at the expense of increasing the start-up time

noise below 10 Hz. Tantalum capacitors are recommended for

capacitors larger than about 33 µF. A 1 µF ceramic capacitor in

parallel with the larger tantalum capacitor is required to retain

good noise performance at higher frequencies.

FREQUENCY (Hz)

1M

0.1

1

10

100

1k

10k

100k

1

100k

100

1k

10k

10

1000

1

10

100

1

10k

100

1k

10

1Hz

10Hz

100Hz

400Hz

3Hz

30Hz

300Hz

1kHz

Capacitor Properties

Any good quality ceramic capacitors can be used with the

ADM7151 as long as they meet the minimum capacitance

and maximum ESR requirements. Ceramic capacitors are

manufactured with a variety of dielectrics, each with different

behavior over temperature and applied voltage. Capacitors must

have a dielectric adequate to ensure the minimum capacitance

over the necessary temperature range and dc bias conditions.

X5R or X7R dielectrics with a voltage rating of 6.3 V to 50 V

are recommended. However, Y5V and Z5U dielectrics are not

recommended due to their poor temperature and dc bias

characteristics.

Figure 54 depicts the capacitance vs. dc bias voltage of a 1206,

10 µF, 10 V, X5R capacitor. The voltage stability of a capacitor is

strongly influenced by the capacitor size and voltage rating. In

general, a capacitor in a larger package or higher voltage rating

exhibits better 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.

DC BIAS VOLTAGE (V)

10

0

4

8

2

6

0

12

10

8

6

4

2

Figure 54. Capacitance vs. DC Bias Voltage

Use Equation 1 to determine the worst-case capacitance

accounting for capacitor variation over temperature, component

tolerance, and voltage.

(1)

where:

TEMPCO is the worst-case capacitor temperature coefficient.

TOL is the worst-case component tolerance.

In this example, the worst-case temperature coefficient (TEMPCO)

over −40°C to +85°C is assumed to be 15% for an X5R dielectric.

The tolerance of the capacitor (TOL) is assumed to be 10%, and

Substituting these values in Equation 1 yields

Therefore, the capacitor chosen in this example meets the

minimum capacitance requirement of the LDO over

temperature and tolerance at the chosen output voltage.

To guarantee the performance of the ADM7151, it is imperative

that the effects of dc bias, temperature, and tolerances on the

behavior of the capacitors be evaluated for each application.